review: potato virus y (pvy) and potato leaf roll virus
TRANSCRIPT
University of Pretoria Faculty of Natural and Agricultural SciencesDepartment of Zoology and Entomology Pretoria 0002 Tel (012) 420-3233 Fax (012) 362-5242
Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV):
L i t e ra tu re Rev iew fo r
Po ta toes South A f r i ca
Prepared by
M. Warren, K. Krüger & A.S. Schoeman
April 2005
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
Table of Contents
1. Introduction ..................................................................................................................3 2. Potato virus Y (PVY) ...................................................................................................4
2.1. The disease and virus ............................................................................................4 2.2. Geographic distribution ........................................................................................4 2.3. Host range .............................................................................................................5 2.4. Virus transmission ................................................................................................6 2.5. Disease management .............................................................................................8
2.5.1. Crop management and tuber storage .........................................................8 2.5.2. Chemical control .....................................................................................11 2.5.3. Host plant resistance to the virus ............................................................13 2.5.4. Host plant resistance to the vector ..........................................................14
2.6. Recommendations for seed producing growers: PVY ........................................14 2.6.1. Reduction of virus inoculum within the crop .........................................14 2.6.2. Reduction of virus inoculum outside the crop ........................................15 2.6.3. Monitoring aphid flight to minimize transmission .................................15 2.6.4. Isolation ...................................................................................................15 2.6.5. Barriers ....................................................................................................15 2.6.6. Insecticides ..............................................................................................16
3. Potato leaf roll virus (PLRV) ....................................................................................16 3.1. The disease and virus ..........................................................................................16 3.2. Geographic distribution ......................................................................................16 3.3. Host range ...........................................................................................................17 3.4. Virus transmission ..............................................................................................18 3.5. Disease management ...........................................................................................19
3.5.1. Crop management and tuber storage .......................................................19 3.5.2. Chemical control of vectors ....................................................................21 3.5.3. Host plant resistance to the virus ............................................................22 3.5.4. Host plant resistance to the vector ..........................................................22
3.6. Recommendations for seed producing growers: PLRV .....................................22 3.6.1. Reduction of virus inoculum within the crop .........................................22 3.6.2. Reduction of virus inoculum outside the crop ........................................22 3.6.3. Monitoring aphid flight to minimize transmission .................................23 3.6.4. Isolation ...................................................................................................23 3.6.5. Barriers ....................................................................................................23 3.6.6. Insecticides ..............................................................................................23 3.6.7. Cultivars ..................................................................................................23
4. References ...................................................................................................................28
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
1. Introduction
Aphid-transmitted potato viruses, such as potato virus Y (PVY) and potato leaf roll virus
(PLRV), have long been recognized as a major problem in the profitable production of
seed potatoes. Planting of PVY- and PLRV-infected seed may result in total yield loss
(Radcliffe, 1982; Thomas et al., 1997).
Potato virus Y (PVY) and potato leaf roll virus (PLRV) are the two most important
viruses of potato globally (Radcliffe, 1982). Primary infection (current season inoculation
with virus) of tubers by PVY increases the number of undersized tubers, while infection
with PLRV induces net necrosis in tubers (darkening of the vascular bundle) (Radcliffe &
Ragsdale, 2002). This results in losses with respect to yield and quality for commercial
producers (Ragsdale et al., 1994). Infection with these viruses also leads to a downgrading
of seed lots because of the low tolerances (0-1 %) required by seed certification
programmes for high quality seed (Radcliffe & Ragsdale, 2002; South African Seed
Certification Programme). In South Africa, Daiber (1965) showed that the incidence of
PLRV increased from 0.4 % to 97 % after four plantings. Although tubers with higher
virus infection levels (1-4%) are unsuitable as seed potatoes, such tubers may still be used
to plant the commercial crop (Ragsdale et al., 2001).
Potato virus management strategies are preventative (reduction of virus inoculum) or
therapeutic (reduction of vector (aphid) numbers) (Radcliffe & Ragsdale, 2002). These
include spatial and temporal isolation of seed potato crops, mechanical barriers and weed
control (Ragsdale et al., 2001). The effective control of potato viruses requires a
combination of both management strategies and risk assessment with regard to other pests
to establish the best option in a region-specific manner (see Mowry, 1994; Caldiz et al.,
2002). For example, the need for virus-free seed potatoes requires a much higher level of
management than would be required for processed potatoes (Mowry, 1994).
This review focuses on potato virus Y and potato leaf roll virus, their insect vectors and
existing management strategies to control the spread of the viruses in potato crops. Local
and international research findings are reviewed and evaluated with specific reference to
potato production in a South African context. Areas requiring further research to
understand the epidemiology of PVY and PLRV in South Africa are identified.
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
2. Potato virus Y (PVY)
Potato virus Y (PVY) causes serious damage in potato (10-100 % yield losses) and other
solanaceous crops worldwide. The severity of the disease depends on the PVY strain
involved, host tolerance, time of infection and environmental factors. In potato, PVY is
transmitted to the new crop via seed tubers or through aphid vectors. Primary symptoms
caused by PVY may be mild or hardly detectable, which causes problems particularly in
potato seed production.
2.1. The disease and virus
Potato virus Y is a Potyvirus (family Potyviridae), and thus belongs to a taxon that
comprises the largest and economically most important group of plant viruses (de Bokx &
Huttinga, 1981; Büchen-Osmond, 1987). PVY is a single stranded RNA virus. Strains of
PVY include PVYO (common strain), PVYN (tobacco veinal necrosis strain), and PVYC
(stipple-streak strain, including potato virus C). The main diseases caused by PVY include
mild to severe leaf mottling, streak or ‘leaf-drop streak’ (PVYO) with necrosis along the
veins of the underside of leaflets (PVYN) and ‘stipple-streak’ (PVYC) (de Bokx &
Huttinga, 1981; Harrison, 1984).
2.2. Geographic distribution
The distribution of PVY is global, although some virus strains are restricted to certain
continents (de Bokx & Huttinga, 1981; Harrison, 1984). PVYO strains occur worldwide,
PVYN strains occur in Europe, parts of Africa and South America, and PVYC strains have
been reported from Australia, India, and Europe. The two major strains identified in South
Africa are PVYO (common strain) and PVYN (tobacco veinal necrosis strain) (Thompson,
1997). PVY-81 (a strain similar to PVYO) appears to be common in potatoes grown in the
Northern Cape region (Thompson et al., 1987), while other strains have been identified in
tobacco (Vorster et al., 1990).
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
2.3. Host range
The host range of PVY is broad, comprising many crop species in the Solanaceae
(including tomato, pepper, and tobacco) and some species in the families, Asteraceae,
Brassicaceae, Chenopodiaceae (Amaranthaceae), Commelinaceae, and Fabaceae (Tables
1, 2; de Bokx & Huttinga, 1981; Büchen-Osmond, 1987, Fletcher, 2001). Weed species
that are suitable hosts of PVY may be important virus reservoirs (Latorre, 1983).
Table 1. Examples of host plants of potato virus Y (PVY)
Family Species Plant recorded from South Africa
Asteraceae Cotula australis (Sieber ex Spreng.) Hook. F. X
Asteraceae Senecio vulgaris L. X
Brassicaceae Capsella bursa-pastoris (L.) Medik. X
Chenopodiaceae (Amaranthaceae)
Chenopodium quinoa Willd.
Chenopodiaceae (Amaranthaceae)
Chenopodium giganteum D. Don. (tree spinach) X
Commelinaceae Tinantia erecta (Jacq.) Schltdl.
Solanaceae Capsicum annuum L.
Solanaceae Capsicum frutescens L. (hot pepper, chili pepper)
X
Solanaceae Lycium sp.
Solanaceae Lycopersicon esculentum Mill. (tomato) X
Solanaceae Nicotiana glutinosa L.
Solanaceae Nicotiana tabacum L. (tobacco)
Solanaceae Physalis floridana L. (strawberry-tomato)
Solanaceae Solanum atropurpureum Schrank
Solanaceae Solanum chacoense Bitter
Solanaceae Solanum demissum Lindl.
Solanaceae Solanum tuberosum L. (potato) X
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
Table 2. Insusceptible host of potato virus Y (PVY)
Family Species
Solanaceae Datura stramonium L.
2.4. Virus transmission
The virus can be transmitted by sap inoculation, grafting and aphids in a non-persistent
manner (stylet-borne). Although PVY can be transmitted mechanically, this mode of
transmission is usually of minor importance in the field (Ragsdale et al., 2001).
Non-persistent viruses have a loose association with vectors, i.e. virus specificity is not
marked and many vectors can transmit the virus. In the case of PVY more than 50 aphid
species have been shown to be able to transmit the virus, although some species are more
efficient than others (Table 5; Büchen-Osmond, 1987; Ragsdale et al., 2001). The most
efficient vector appears to be the cosmopolitan peach-potato aphid Myzus persicae (Sulz.)
(Hemiptera: Aphididae) (de Bokx & Huttinga, 1981), which is also known as the green
peach or peach-potato aphid.
Viruses that are non-persistently transmitted can be acquired and transferred during very
short acquisition and inoculation feeding times. Infective aphids are capable of acquiring
PVY within seconds to minutes and able to transmit PVY within seconds of probing an
uninfected plant (Harrison, 1984). There is no latent period involved, i.e. the virus can be
transmitted immediately after acquisition. As the name suggests, non-persistent viruses do
not persist or replicate in the vector, and aphids lose the ability to transmit PVY after
having probed one or two uninfected plants following acquisition, unless they feed again
on an infected plant (Bradley & Rideout, 1953).
Table 5 lists the aphid species that have been identified as being present near potato fields
by researchers of the ARC-Vegetable and Ornamental Plant Institute (ARC-VOPI) in a
survey conducted across South Africa (1994-1997) (Thompson, 1997), as well as the
aphid species that are known to feed on potato and occur in South Africa. Of the known
PVY aphid vectors, 25 species have been recorded in South Africa (Thompson, 1997), 12
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
of which have been reported to feed on potato (I. Millar, pers. comm.). The relative
efficiencies with which aphid vectors transmit viruses have been determined for some of
the species (Table 5; see Ragsdale et al., 2001).
Aphids recorded can be divided into colonizing (reproduce on potato) and non-
colonizingpotato (reproduce on plants other than, and do not establish on, potato) species.
Usually colonizing aphid species are more efficient vectors that non-colonizing species.
Most of the species listed in Table 5 do not colonize potato. However, when searching for
a host, aphids take sap samples from the epidermis cells of plants. These sap sampling
probes are sufficient to transmit PVY. Although non-colonizing aphid species may be not
efficient vectors, the population density of these aphid species in a potato field can be
high, thus compensating for their lower transmission efficiency (Radcliffe, 1982; Robert
et al., 2000). Therefore, all species that occur throughout the potato-growing season and
that occur consistently across years may be important virus vectors and require
monitoring to apply the correct control measures (see DiFonzo et al., 1997).
A recent review by Radcliffe & Ragsdale (2002) highlighted the importance of vector
biology when attempting to understand and control the spread of potato viruses. For both
PVY and PLRV, the virus is moved from an inoculum source outside the potato field into
the field almost exclusively by winged aphids (Boiteau, 1997). Apterae (wingless aphids)
have not been shown to be significant in within-field spread of PVY (Ragsdale et al.,
1994).
In cooler, temperate climates most aphids of potato crops overwinter either as eggs on a
primary host or as viviparae (wingless aphids bearing living young instead of eggs) in
protected sites (Radcliffe, 1982; Radcliffe & Ragsdale, 2002). Primary hosts are usually
fruit trees (e.g. Prunus spp.). Secondary hosts, mainly weed species, allow aphid numbers
to increase before potato plants are available for colonization and provide a source of
migrants to crops (Radcliffe & Ragsdale, 2002). In milder climates, as is the case in South
Africa, parthenogenic (asexual) reproduction is likely to be year-round on weeds,
available crops (e.g. cabbage) and garden plants, providing an immediate aphid source
when potatoes are planted (Daiber & Schöll, 1959; Radcliffe, 1982; Annecke & Moran,
1982).
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
Although large populations of aphids are able to damage potato plants directly, the
economic importance of aphids results mainly from their role as virus vectors (Radcliffe
& Ragsdale, 2002; Radcliffe, 1982). To actively manage the economic impact of the
aphid vectors, it is essential to know which aphid species occur in a region, the efficiency
with which they are likely to transmit the viruses to potato plants and to understand vector
biology (Hanafi et al., 1995; Radcliffe & Ragsdale, 2002; Radcliffe & Ragsdale, 1998).
For example, in South Africa, peak M. persicae flight activity occurs in October -
November and January - February but fluctuates for each locality and across years (see
Daiber, 1965).
2.5. Disease management
Many aphid species that have been reported as vectors of PVY are incapable of
establishing on potato crops (i.e. they are non-colonising aphid species). However, both
colonizing and non-colonizing aphid species play an important role as vectors of PVY, the
former because they are usually efficient vectors, the latter because they may occur in
high numbers, thus compensating for their usually lower PVY transmission efficiency.
Therefore, all species that occur throughout the potato-growing season and that occur
consistently across years may be important virus vectors and require monitoring for
gaining an understanding of the spread of PVY in potato fields (Radcliffe, 1982) and to
apply the correct control measures (see DiFonzo et al., 1997). The importance of insect
pests also depends on climate, plant age (young plants are more susceptible to net
necrosis) and cropping practices (Caldiz et al., 2002; Radcliffe & Ragsdale, 2002). Thus,
an understanding of all the factors likely to affect potato production is required to
formulate an adequate integrated pest management (IPM) plan for potato seed producers
(see e.g. Marsh et al., 1998; Jones, 2004).
2.5.1. Crop management and tuber storage a) Adjustment of planting date and avoidance of vector (spatial and temporal
segregation)
Spatial and temporal separation of seed stocks from other crops (e.g. potatoes) that are
likely sources of virus inoculum remains one of the most effective preventative
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
management strategies (Radcliffe & Ragsdale, 2002). The same authors state that a
minimum separation distance should be established according to an acceptable level of
risk of spread from virus sources to potato fields. Separation distances of 400 m to 5 km
have been effective under different management scenarios for seed potato cultivation
(Radcliffe & Ragsdale, 2002). However, although maiden flights of Myzus persicae alatae
seldom exceed 100 m, wind-supported flight may cause aphids to disperse up to several
hundred kilometers (Radcliffe & Ragsdale, 2002). Therefore, viruliferous aphids from
distant localities may still occasionally colonize isolated potato fields.
Windswept areas along the coast may be almost aphid-free and provide good sites for the
location of seed potato fields. Growing all or most seed in a single region, or a group of
regions, spatially separated from other crops is practiced in many countries, e.g. Golan
Heights in Israel.
Isolation may also include modified planting and harvesting dates (Ragsdale et al., 2001).
Planting early if vector species colonise the crop late is a useful strategy. Older plants are
less likely to become infected than young plants (mature plant resistance) (Roosen et al.,
1997). Although early planting of seed potatoes is an effective strategy in cooler climates
where aphids overwinter or abundances are substantially reduced during the winter, the
strategy is unlikely to be effective across South Africa. Warmer winters in, for example,
the highveld are unlikely to lead to a reduction in aphid numbers. Year-round crops also
provide the aphids with a continual food supply so that abundances remain high. Delayed
(late) planting is another strategy that has been shown to reduce virus spread by 50 %
compared to early plantings (Ragsdale et al., 2001). Tuber yield will, however, be
compromised (Boiteau, 1984). Late plantings reduce colonization of the crop by M.
persicae (early colonizer) but has no effect on Macrosiphum euphorbiae (potato aphid)
abundances in South Africa.
Growing crops in periods in which aphid vectors are absent or aphid population density is
low is another effective strategy to reduce the spread of virus inoculum (de Bokx &
Huttinga, 1981). Early planting together with ‘vine kill’ (roguing/haulm destruction) has
been effective in The Netherlands; however, such strategies cannot be applied where the
growing season is short (Radcliffe & Ragsdale, 2002).
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
b) Roguing
During rouging, haulms of seed-potato crops are destroyed before maturity so as to
restrict virus spread at the end of the growing season. Roguing is effective only when
virus incidence is low (~1 %) and when fields are small as each plant requires multiple
inspections throughout the growing season (Radcliffe & Ragsdale, 2002). A disadvantage
of roguing is that it creates gaps, making the plants more apparent to the aphids (Ragsdale
at al., 2001). To be more effective at finding all infected plants, seed pieces cut from the
same tuber should be planted together but this is labour intensive and extends the time
taken to complete the planting of a field (Ragsdale et al., 2001). Because roguing requires
identifying infected plants through symptom expression, cultivars expressing symptoms
should preferably be grown instead of symptom-free carriers (Ragsdale et al., 2001). All
infective plants need to be removed prior to the arrival of winged aphids (Radcliffe &
Ragsdale, 2002).
c) Crop mulching, covers and barriers
Although reflective mulches and sticky yellow sheets may reduce PVY spread (51-80 %
in peppers), they are expensive, effective for a short time only (before the canopy closes)
and require disposal after use (W. Pett, pers. comm.; Ragsdale et al., 2001). Crop covers
may be relatively effective at reducing PVY spread but increased temperatures under the
covers negatively impact plant and tuber development, especially where daytime
temperatures are high (Ragsdale et al., 2001). Also, infestations under crop covers may go
undetected for extended periods of time (Ragsdale et al., 2001). Barrier crops are more
effective at reducing PVY spread than insecticides (Radcliffe & Ragsdale, 2002; DiFonzo
et al., 1996). If PVY-infected aphids feed on the barrier first, the probability of retaining
virus inoculum for transmission to potatoes is low (DiFonzo et al., 1996). Soybean or
sorghum barriers (~ 1 m wide) are just as effective at reducing PVY spread as taller crops
and should be planted with no gap between the barrier and potato crop (Radcliffe &
Ragsdale, 2002; DiFonzo et al., 1996). Barrier crops should have a fallow border to the
outside to be apparent to winged aphids (DiFonzo et al., 1996). Soybean is not a host for
the virus and it is therefore particularly suitable as a barrier crop, although potato barriers
have also been used. These need to be discarded from the seed lots on harvesting.
Standard barrier widths have not been established but it appears that barriers of a few
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
metres wide are effective (DiFonzo et al., 1996). Spraying the barriers with insecticides
has no influence on reducing the spread of PVY and should be avoided as the natural
enemies of the aphids will be negatively affected.
d) Elimination of virus sources
Weeds (e.g. other solanaceous plants) and volunteer potatoes may be sources of virus
inoculum and viruliferous aphids (Thomas & Richards, 2004). Virus inoculum must be
reduced by eliminating weeds and volunteer potatoes (Hanafi et al., 1995). Many South
African plant species that occur naturally may be sources of virus inoculum. However,
when commercial (ware) potato growers plant seed with acceptable levels of virus (1-5 %
according to international certification standards) in neighbouring areas, the potato crop
itself becomes a greater source of inoculum in the area than weeds (Ragsdale et al., 2001).
Virus levels vary depending on the generation and the end use of the crop. In South
Africa, these values are zero PVY infection levels for the first two generations and both
inspections used for the production of seed potatoes (South African Seed Certification
Programme). As is the international standard, much higher levels of virus are acceptable
for commercial production (South African Seed Certification Programme). Therefore,
potato fields are likely to be the greatest source of PVY inoculum in South Africa.
Spraying in wind breaks where aphids numbers build up has also been suggested as an
additional management strategy to reduce sources of virus inoculum.
2.5.2. Chemical control
Chemical control of aphid species such as M. persicae on overwintering host plants,
weeds and within potato and other crops has been shown to be largely ineffectual in
controlling the spread of potato viruses (de Bokx & Huttinga, 1981; Radcliffe &
Ragsdale, 2002). This is due to the non-persistent nature of PVY transmission, i.e. the
virus may be transmitted within the first few seconds of probing an uninfected plant
(Bradley, 1954). PVY may therefore be transmitted before the insecticide has time to kill
the vector (Radcliffe & Ragsdale, 2002).
However, pesticide usage may be required to curb aphid outbreaks resulting from
insecticide applications targeting other insect pests (Radcliffe & Ragsdale, 2002). The
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
following chemicals have been used to control aphid populations either as in-furrow
applications on seed potato plantings (imidacloprid; provides approximately 60-90 days of
aphid protection, but has no effect on reducing PVY spread, Boiteau & Singh (1999)) or
foliage applications: imidacloprid, methamidophos and thiamethoxam. Most insecticide
classes registered for usage on potatoes, such as organophosphates, carbamates and
pyrethroids, may cause outbreaks of potato-colonising aphids (ffrench-Constant et al.,
1988; Harrington et al., 1989). Myzus persicae is resistant to most insecticide classes,
making chemical control of this species particularly problematic (Radcliffe & Ragsdale,
2002). A new insecticide, pymetrozine, affects aphid stylet penetration and is likely to be
used substantially by the industry in the future (Radcliffe & Ragsdale, 2002). Pyrethroids
may cause outbreaks in aphid populations, increasing virus spread (Ragsdale et al., 2001).
In addition, some insecticides are irritating to aphids and increase PVY spread. Extreme
care must be taken when applying insecticides in PVY infected areas (Ragsdale et al.,
2001).
Where action thresholds of aphid abundances for the prevention of PVY spread in seed
potato crops have been established, it has become clear that these vary across regions
(PVY spread is not reduced when aphid densities are maintained at the lowest levels
possible) (Ragsdale et al., 1994). It is therefore essential to monitor aphid populations
within the crop and to establish an action threshold relevant to a particular locality. An
important point to remember in aphid control is that of cumulative vector intensity
(Basky, 2002; Halbert et al., 2003). This is a function of both the abundance of the aphid
species within a field and their efficiency at transmitting the virus (Basky, 2002).
Therefore, a rare (low abundance) aphid species may be a serious threat to crops if its
transmission efficiency is high, or a highly abundant, less efficient vector may pose a
problem regarding PVY spread (Halbert et al., 2003). Also, non-colonising alate (winged)
aphids are important vectors of PVY (Radcliffe & Ragsdale, 2002).
The disadvantages of chemical control include the build-up of aphid genotypes that are
resistant to chemicals (as mentioned above, Myzus persicae is resistant to many classes of
insecticides) and environmental degradation (e.g. pollution of water sources (Radcliffe &
Ragsdale, 2002)).
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
By reducing insecticide usage on leaf litter under the plants and on surrounding natural
vegetation, predator numbers may be enhanced, thereby reducing aphid numbers.
Certain mineral oils are known to reduce aphid colonization on plants, and thus the
transmission of virus diseases (Simons & Zitter, 1980; Martín et al., 2004). Mechanical
control with mineral oils reduces the incidence of PVY in tubers (Marco, 1986). Use of
mineral oils has resulted in reducing virus spread by more than 50 % in Minnesota, USA,
without severely affecting natural enemies or the environment (Ragsdale et al., 2001).
However, phytotoxicity of mineral oils applied in-field needs to be established (e.g. oils
may be phytotoxic when temperatures are high) (see Ragsdale et al., 2001; Martín et al.,
2004).
Although whitewash sprays also aid in PVY control, these may be less efficient than net
covers and may attract, rather than repel, some aphid species (Marco, 1986).
2.5.3. Host plant resistance to the virus
Numerous potato cultivars are available, some of which are symptom-free carriers
(infected with PVY and/or PLRV but few symptoms are visible), while a few cultivars
appear to be immune or almost immune to PLRV net necrosis (Mowry, 1994). Symptom-
free carriers are useful for table and processing potatoes. However, these cultivars are
often the bane of seed potato producers as infected plants and tubers are not visibly
discernable from uninfected plants, making roguing difficult (Radcliffe & Ragsdale,
2002). The planting of virus resistant potato cultivars is an important avenue of potential
control of the viruses but these are costly to develop and need to be durable sources of
resistance (Mowry, 1994; Lecoq et al., 2004; Shubert et al., 2004). In addition, the
benefits gained (e.g. yield, firmness) from established cultivars also need to be
incorporated into the new cultivars (Mowry, 1994).
Enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) tests
are the only certain means of ascertaining the infection status of potato tubers, especially
symptomless carriers (Robert et al., 2000). ELISA is used in seed certification
programmes to process large amounts of samples rapidly and ensures that infection levels
in seed potatoes are low (Robert et al., 2000).
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
2.5.4. Host plant resistance to the vector
Although the planting of aphid resistant cultivars was suggested as a potential virus
control strategy (Adams, 1975), only wild potato species are truly resistant to aphids
(Radcliffe & Ragsdale, 2002). A study by Raman (1988) indicated that when resistant and
susceptible cultivars were exposed to insecticides commonly used in the potato industry,
no difference in M. persicae mortality was observable.
2.6. Recommendations for seed producing growers: PVY
Potato virus management problems are regional and effective control may only be
maintained through co-operation on a regional level between growers (Radcliffe &
Ragsdale, 2002). As stated by Ragsdale et al. (2001), a reduction of virus inoculum
(preventative) and reduction in virus vectors (therapeutic) is required to manage potato
viruses. When seed certification programmes (including multiple inspections, roguing and
testing of tubers for viruses) are successful, virus inoculum is all but eliminated (Radcliffe
& Ragsdale, 2002). This is the case in the US where, in general, PLRV is no longer a
major threat to potato growers (pers. comm. Dr W. Pett, Michigan State University).
However, on occasions, disease incidence may increase and other measures need to be
implemented to reduce disease incidence (Radcliffe & Ragsdale, 2002). Different
management strategies are required depending on which virus (PVY or PLRV) is causing
the major problem.
2.6.1. Reduction of virus inoculum within the crop
Immediate roguing of plants expressing secondary infection symptoms reduce PVY
sources within the crop. Infected plants should be removed before winged aphids arrive.
Arrival dates may differ for each region in South Africa. For example, peak M. persicae
flight activity occurs in October - November and January - February. An additional peak
occurs in June/July in Pretoria (see Daiber, 1965). In addition, different species appear to
colonise the crop at different times (Daiber, 1962).
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M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
2.6.2. Reduction of virus inoculum outside the crop
Potential host plants should be identified and volunteer potato plants eliminated. If virus
incidence in the crop exceeds 5 %, then the crop is likely to be the greatest source of virus
inoculum. Commerical fields located close to seed potato fields are likely to serve as PVY
inoculum sources for seed fields.
2.6.3. Monitoring aphid flight to minimize transmission
Alate aphids should be monitored in traps or apterae should be counted on potato foliage
to provide an early warning system to growers to initiate mineral oil spraying or early
harvest.
2.6.4. Isolation
An effective strategy to curb the spread of PVY is to grow seed potatoes in isolation from
commercial potatoes (Radcliffe & Ragsdale, 2002). The separation distance between seed
and commercial potato fields will depend on the acceptable level of risk, weather
conditions, time when aphids first colonise the crop, crop age at first aphid flight and
cultivar, i.e. it varies between localities (Ragsdale et al., 2001; Radcliffe & Ragsdale,
2002). Thresholds to determine when haulm destruction must occur are region-specific
and no recommendations can be made in this regard without knowledge of these regional
factors.
2.6.5. Barriers
Mechanical barriers (e.g. polypropylene sheets) have been shown to reduce PVY spread
(Ragsdale et al., 2001). Barrier crops, planted with a fallow border to the outside and
without a gap between it and the crop, will likely reduce PVY spread.
15
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
2.6.6. Insecticides
Insecticides do not act quickly enough to reduce PVY spread as aphids usually transmit
the viruses before they die. Mineral oils may be effective.
3. Potato leaf roll virus (PLRV)
Potato leaf roll virus (PLRV) is one of the most damaging viruses of seed, processing and
fresh market potatoes globally. The virus is transmitted naturally through aphid vectors.
Infection of potato with PLRV may cause yield losses through stunting of plants and a
reduction in tuber number and size. In addition, infection can lead to internal net necrosis,
resulting in tubers being unsuitable for processing. Furthermore, PLRV infection in seed
potato crops leads to rejection from certification schemes if the rate of infection exceeds
tolerance levels (Ragsdale et al., 2001; Radcliffe & Ragsdale, 2002).
3.1. The disease and virus
Potato leafroll virus (genus Polerovirus, family Luteoviridae) is a single-stranded RNA
virus (Harrison, 1984). Strains from potato have been determined based on the severity of
symptoms induced in Solanum tuberosum (potato), Physalis floridana and Montia
perfoliata, as well as by ease of transmission through M. persicae (Harrison, 1984).
However, these strains did not differ antigenetically (Tamada et al., 1984).
Symptoms of PLRV through primary infection (current season inoculation) in potato
plants include pallor or reddening of leaf tips, which may roll and become erect.
Secondary symptoms in plants grown from infected potato tubers include stunting of
shoots and leaflets rolling upwards, starting with the oldest leaves. In plants with primary
infection the virus is transmitted through a variable portion of tubers, whereas all tubers
on plants with secondary infection are viruliferous (Harrison, 1984).
3.2. Geographic distribution
The distribution of PLRV is global (Harrison, 1984).
16
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
3.3. Host range
The majority of known hosts of PLRV belong to the Solanaceae. Non-solanaceous hosts
have been reported from a few species belonging to nine plant families. These include the
Amarathaceae (Chenopodiaceae), Brassicaceae, Malvaceae, Asteraceae, Cucurbitaceae,
Lamiaceae, Nolanaceae, and Portulacraceae (1). (Harrison, 1984; Thomas, 1984; Natti et
al., 1953; Tamada et al., 1984) (Table 3). Examples of non-host plants are given in Table
4.
Table 3. Examples of host plants of potato leafroll virus (PVLR)
Family Species Plant recorded from South Africa
Amaranthaceae (Chenopodiaceae)
Amaranthus caudatus L. X
Amaranthaceae (Chenopodiaceae)
Celosia argentea L. X
Amaranthaceae (Chenopodiaceae)
Gomphrena globosa L. X
Brassicaceae Capsella bursa-pastoris (L.) Medik. X
Brassicaceae Sisymbrium altissimum L.
Nolanaceae Nolana lanceolata
Portulacaceae Montia perfoliata Donn ex Willd.
Solanaceae Atropa spp.
Solanaceae Capsicum spp.
Solanaceae Datura stramonium L. X
Solanaceae Hyoscyamus spp.
Solanaceae Lycopersicon esculentum Mill. (tomato) X
Solanaceae Nicandra physalodes (L.) Gaertn. X
Solanaceae Nicotiana clevelandii A. Gray
Solanaceae Nicotiana spp.
Solanaceae Physalis floridana L. (husk-tomato)
Solanaceae Solanum tuberosum L. subsp. andigenum (Juz. & Bukasov) Hawkes
Solanaceae Solanum tuberosum L. subsp. tuberosum (potato) X
17
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
Table 4. Insusceptible hosts of potato leafroll virus (PVLR)
Family Species
Brassicaceae Brassica rapa L. subsp. campestris (L.) A. R. Clapham
Brassicaceae Brassica oleracea L. var. capitata L. (cabbage)
Brassicaceae Brassica rapa L. subsp. pekinensis (Lour.) Hanelt (Chinese cabbage)
Fabaceae Pisum sativum L. (pea)
Brassicaceae Raphanus sativus L. (radish)
Asteraceae Senecio vulgaris L.
Fabaceae Vicia faba L. (broad bean)
3.4. Virus transmission
The virus is not transmitted by manual inoculation with sap nor through true seed but is
transmissible by vector aphids and by grafting. Because PLRV is not seed-transmitted,
annual weeds are not thought to be an important virus source in spring in regions where
winters are cold (Thomas, 1983). However in warmer regions the situation may differ and
perennial and tuber-forming weeds can harbour PLRV through winter (Natti et al., 1953;
Fox et al., 1993).
PLRV is transmitted by aphids in a persistent circulative non-propagative manner
(Thomas, 1987; Harrison, 1984). As is the case generally with persistently transmitted
viruses, the longer the acquisition and inoculation times, the higher is the rate of
transmission of PLRV. Persistently transmitted viruses usually have a narrow range of
vectors, are passed on when the vectors moult and need a latent period. PLRV does not
replicate in the vector (circulative-non propagative) virus.
Vectors must feed on the phloem tissue of potato plants. PLRV is therefore only
transmitted by aphids colonizing potatoes and, once acquired, the aphid remains infective
for life (Radcliffe & Ragsdale, 2002; Harrison, 1984). Aphids colonizing the potato crop
require 1.6 – 15 minutes to acquire PLRV from an infected plant and become infective
within 8 – 72 hours (latent period) (Radcliffe, 1982; Radcliffe & Ragsdale, 2002).
18
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
The efficiency of virus transmission by aphids varies and depends on the species, and
within a species on the biotype, morph or instar, e.g. nymphs or adults of winged or
apterous viviparous (wingless aphids bearing living young instead of eggs) species, on
viviparous morphs producing sexual forms (gynoparae) and on sexual morphs themselves
(oviparae and males) (Table 5).
Myzus persicae is the most efficient vector of PLRV and is able to acquire more of the
virus and transmit it faster when feeding at comparatively higher temperatures (Syller,
1994; Bradley & Rideout, 1953). The role of Macrosiphum euphorbiae in PLRV
transmission is variable; many populations are poor vectors while the species has been
reported as an important vector in the early-season spread of the virus at some localities
(see Radcliffe & Ragsdale, 2002). Generally, monitoring of aphid populations in the field
for PLRV control focuses on M. persicae (Ragsdale et al., 1994). In South Africa, nine
aphid species known to transmit PLRV have been recorded in the vicinity of potato fields
(Table 5). Unlike PVY, within-field spread of PLRV is usually by apterae (wingless
aphids) (Boiteau, 1997; Mowry, 2001). However, Radcliffe & Ragsdale (2002) suggest
that within-field spread of PLRV may yet be associated with alatae and not apterae
because maiden flights of alate M. persicae seldom exceed 100 m.
3.5. Disease management
3.5.1. Crop management and tuber storage
a) Adjustment of planting date and avoidance of vector (spatial and temporal
segregation)
As for PVY, isolation of seed and commercial potatoes is an effective strategy to curb the
spread of PLRV (Radcliffe & Ragsdale, 2002). The separation distance varies between
localities but is likely to be > 30 km (Ragsdale et al., 2001; Radcliffe & Ragsdale, 2002).
Changes in the cropping practices in a region must be noted as these may reduce the
effective isolation distance.
19
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
Isolation may also be via modified planting and harvesting dates (Ragsdale et al., 2001).
Delayed planting (planting after the colonizing flight of M. persicae) has been suggested
as a potential PLRV control strategy (Hanafi et al., 1995; Boiteau, 1984). Yield and tuber
size are reduced with delayed plantings.
b) Roguing
In the Pacific Northwest, USA, twelve plants surrounding the infective PLRV-infected
plant need to be removed as neighbouring plants may be infective but not symptomatic
(Mowry, 1994). This is unlikely to occur in the field where symptoms are mild and/or
fields are large (Mowry, 1994).
c) Crop mulching, covers and barriers
Whitewashes substantially reduce PLRV incidence in tubers, but are less efficient than
white net covers (Marco, 1986). However, crop covers reduce yield (Marco, 1986) and
where infective aphids penetrate the covers, virus spread is often dramatic because of
delayed control measures. This risk of increased virus spread under covers and the cost of
crop covers make this management strategy unlikely to be adopted at large scales.
Mechanical barriers (sticky polyethylene sheets) may reduce PLRV spread by 38 %
(Marco, 1981).
d) Elimination of virus sources
Weeds and volunteer potatoes may be important sources of viruliferous aphids and PLRV
inoculum (e.g. Hanafi et al., 1995; Thomas & Richards, 2004). Many South African plant
species that occur naturally may be sources of virus inoculum. However, when
commercial potato growers plant seed with acceptable levels of virus (1-4 % according to
US certification standards) in neighbouring areas, the potato crop itself becomes a greater
source of inoculum in the area than weeds (Ragsdale et al., 2001). In South Africa, these
values are zero for PLRV levels for the first two generations and both inspections used for
the production of seed potatoes (South African Seed Certification Programme). In line
with international standards, higher levels are virus are acceptable for commercial
20
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
production in South Africa (South African Seed Certification Programme). Therefore,
potato fields are the likely to be the greatest source of PLRV inoculum in South Africa.
Crop rotation is an important mechanism used to reduce the quantity of volunteer potato
plants in the Pacific Northwest but farms need to be separated by large distances and three
to four year rotation cycles need to be maintained (Mowry, 1994).
e) Tuber storage management
The probability of net necrosis in tubers increases to a maximum at 90 days of storage
(Marsh et al., 1998).
3.5.2. Chemical control of vectors
Systemic insecticides and/or accurately timed foliar insecticide applications are useful to
reduce within-field spread of PLRV, especially if colonizing aphids are virus-free on
arrival. Some insecticides (e.g. the carbamate aldicarb) control within-field spread of
PLRV by apterae but do not prevent transmission by alatae (see Ragsdale et al., 2001).
The chloronicotinyl class of insecticides (imidacloprid) and the new insecticide
pymetrozine appear effective at controlling within-field spread of PLRV (Ragsdale et al.,
2001). Imidacloprid applied systemically reduces PLRV spread (Boiteau & Singh, 1999).
Recommendations for the timing of insecticide applications for PLRV control in seed
potatoes are affected by aphid biology and arrival time relative to crop age (Ragsdale et
al., 2001). Vector control is usually focused primarily on M. persicae, as it is the most
efficient vector species colonizing potatoes (Ragsdale et al., 1994). For PLRV, action
thresholds of aphid abundances for seed potato crops are 10 M. persicae apterae per 100
leaves, but such thresholds are region-specific (Mowry, 1994; Ragsdale et al., 1994).
Weekly monitoring may occur at too coarse a scale to provide rapid results to apply
control measures (aphid populations expand rapidly and exponentially (Radcliffe &
Ragsdale, 2002). A 3-4 day aphid monitoring programme, instead of 7 days, has been
recommended by some authors (Mowry, 2001). This provides sufficient time to apply
foliage sprays before aphids have exponentially spread the virus to plants throughout the
fields (Mowry, 2001). A zero-tolerance policy has been advocated to prevent the spread
of PLRV in seed potatoes in Idaho (Mowry, 2001). However, such intensive insecticide
usage is likely to contribute to insecticide resistance in the future (Mowry, 1994). The
21
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
negative consequences of the uncontrolled use of chemicals have led to the development
of economic risk assessments where the end-use of the potatoes is taken into
consideration in an attempt to minimize insecticide applications (Marsh et al., 1998).
Antifeedants and repellants such as neem (azadirachtin) have also been used with varying
results to slow the spread of PLRV (van den Heuvel, 1998; Nisbet et al., 1996). These
may be applied as oil formulations (Cranshaw & Baxendale, 2005).
3.5.3. Host plant resistance to the virus
Commercial potato cultivars do not show significant resistance (immunity) to PLRV but
some (e.g. ‘Norgold Russet’) are resistant to tuber net necrosis (Mowry, 1994; Thomas et
al., 2000). This review is not concerned with establishing to which degree the potato
cultivars currently grown in South Africa are susceptible to PLRV.
3.5.4 Host plant resistance to the vector
Genetically modified potato cultivars may decrease PLRV spread between plants by
Myzus persicae (Thomas et al., 2000).
A summary of distribution, host range, symptomatology, vectors, mode of transmission
and control of PVY and PLRV is provided in Table 6.
3.6 Recommendations for seed producing growers: PLRV
3.6.1. Reduction of virus inoculum within the crop
Plants expressing secondary infection symptoms should be removed to reduce within crop
virus inoculum. Symptom-free carriers should not be planted in seed potato regions,
thereby reducing the ease of roguing.
3.6.2. Reduction of virus inoculum outside the crop
Virus inoculum must be reduced by eliminating weeds and volunteer potatoes (Hanafi et
al., 1995).
22
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
3.6.3. Monitoring aphid flight to minimize transmission
Alate aphids should be monitored in traps or apterae counted on potato foliage to provide
an early warning system for growers to initiate mineral oil spraying or early harvest.
3.6.4. Isolation
Isolate seed and commercial potato production. Effective isolation distances need to be
established after determining cropping practices in the region.
3.6.5. Barriers
Mechanical barriers (sticky polyethylene sheets) may reduce PLRV spread by 38%
(Marco, 1981). Crop barriers may also slow PLRV spread.
3.6.6. Insecticides
Action thresholds based on counts of wingless aphids need to be developed for each
region in South Africa where seed potatoes are grown (Ragsdale et al., 1994; Ragsdale et
al., 2001). Clear guidelines need to be formulated to prevent the over-use of chemicals
that will likely lead to the development of increased resistance in aphid populations and to
further outbreaks. Minimizing damage to the environment is also a concern.
3.6.7. Cultivars
Cultivars should be planted that are not susceptible or are less susceptible to net necrosis
but that are still symptomatic (or else roguing is affected).
The incidence of PLRV and PVY in tubers is presently high in certain growing areas (Ben
Pieterse, pers comm.). It will potentially take a number of years before the levels of virus
inoculum will decline and stabilize to lower levels than are currently experienced and for
the epidemic to end once additional control measures are implemented (Radcliffe &
Ragsdale, 2002). Over this time, it will be essential for growers to follow the guidelines
provided by their managing organization (Potatoes South Africa) and the Seed
Certification Programme.
23
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
Table 5. Aphid vectors of potato virus Y (PVY) and potato leaf roll virus (PLRV) recorded in the vicinity of seed potato fields in South Africa and aphid species known to colonize potato Species PVY vector Percent
transmission efficiency of PVY
a
c
PLRV vectora Percenttransmission efficiency of PLRVc
Known to colonize potato b
Acyrthosiphon pisum (Harris) X 3.8 - 14
Aphis fabae (Scopoli) X 7.6 - 24 X not reported X
Aphis gossypii (Glover) X 12 - 31 X 4-74 X
Aphis nasturtii Kaltenbach X 19.0 - 50 X 20 X
Aphis spiraecola (Patch) X
Aulacorthum circumflexum (Buckton) X 75 X
Aulacorthum solani (Kaltenbach) X 5.0 X not reported X
Brachycaudus helichrysi (Kaltenbach) X 4.8 - 12.5
Brachycaudus rumexicolens (Patch) X not reported
Brevicoryne brassicae (L.) X
Capitophorus hippophaes (Walker) X 3.0 - 3.1
Diuraphis noxia (Mordvilko) X 4.0 - 7.0
Dysaphis (Pomaphis) spp. X
Hyalopterus pruni (Geoffroy) X 13.9
Hyperomyzus lactucae (L.) X 0.4 - 17.4
24
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
Table 5 continued
Species PVY vector Percenttransmission efficiency of PVY
a
c
PLRV vectora Percenttransmission efficiency of PLRVc
Known to colonize potato plantsb
Lipaphis erysimi (Kaltenbach) X 10
Macrosiphum euphorbiae (Thomas) X 4.0 - 29.0 X 0 - 25 X
Metopolophium dirhodum (Walker) X 0.5 – 10
Myzus ornatus Laing X not reported X
Myzus persicae (Sulzer) X 4.7 - 71.1 X 2.4 - 83.8 X
Rhopalosiphoninus latysiphon (Davidson) X X not reported X
Rhopalosiphum maidis (Fitch)d X not reported
Rhapalosiphum rufiabdominalis (Sasaki) X
Rhopalosiphum padi (L.) X 0.5 - 11.5
Schizaphis graminum (Rondani) X not reported
Sitobion sp. X
Sitobion avenae (F.) X 0.06 - 1.8
Sitobion fragariae (Walker) X 0.09 – 10
Smynthurodes betae Westwood X
Uroleucon sonchi (L.) X a Thompson, 1997; Ragsdale et al., 2001 b I. Millar (pers. comm., ARC-PPRI. Although all of the species marked are known to colonize potato, some have not been collected on potato in South Africa, which could be a reflection of collecting effort). c Ragsdale et al., 2001; d This species has not been recorded from potato
25
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
Table 6. Tabulated summary of distribution, host range, symptomatology, vectors, mode of transmission and control of i) Potato virus Y
and ii) potato leafroll virus
Distribution Host range Symptoms Identified vectors Transmission Control6
i) Potato Y potyvirus (PVY)
Worldwide, in potato and outdoor crops of pepper, tobacco and tomato in warmer countries (de Bokx and Huttinga 1981)
Mainly Solanaceae (Solanum tuberosum (potato), Capsicum spp. (peppers), Nicotiana spp. (e.g. tobacco), Lycopersicon esculentum (tomato)), but also some members of Chenopodiaceae (Amaranthaceae), Compositae, Leguminosae are susceptible
a) leaf mottling, b) ‘leaf drop streak’ with vein necrosis, c)‘stipple-streak’1 primary symptoms mild to very mild mottle, may appear late in growing season; secondary symptoms more obvious, may appear early in growing season depending on climatic conditions; differences often indistinct because of diversity of potato cultivars and virus strains (de Bokx and Huttinga 1981), necrosis may occur in tubers
Myzus persicae (most efficient), Aphis fabae, Macrosiphum euphorbiae, Myzus (Nectarosiphon) certus, Myzus (Phorodon) humuli, Rhopalosiphum insertum2
-Non-persistent-Mechanical inoculation (25 spp of aphids) -grafting -no helper virus required
Insecticides ineffective i) avoid infection ii) avoid growing crops near established crop of same species iii) roguing iv) spray with mineral oils to reduce transmission3
v) breed for resistance vi) reflective surfaces and sticky yellow sheets4
26
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
Table 6 continued
Distribution Host range Symptoms Identified vectors Transmission Control6
ii) Potato leafroll virus (PLRV)
Worldwide in most potato crops; tomato yellow top diseases reported in some countries but not determined that all caused by PLRV
Potato (Solanum tuberosum) Tomato 20 spp (most Solanaceae); confined mainly to potato
Potato: Primary: pallor/reddening of leaf tips, may become rolled; Secondary (plants grown from tubers): shoot stunting, upward rolling of leaflets esp. lower leaves, may develop marginal necrosis; internal net necrosis may develop in tubers with primary/secondary infection Tomato: stunting, marginal yellowing, curling of leaflets, death of flowerbuds7
Several spp reported to transmit virus2; Myzus persicae (most efficient), Macrosiphum euphorbiae (good vector of Australian yellow top isolates)
-not transmitted by manual inoculation -persistent -transmissible by aphid vectors and grafting
i) select tubers from symptom-free clones of mother plants ii) heat treatment of tubers5 iii) grow seed-potato crops where vectors are few or arrive late in growing season or unfavourable weather conditions for aphid activity iv) roguing v) apply insecticides to minimize aphid activity vi) harvest/destroy haulms (early ‘vine kill’) before virus pass from shoots to tubers vii) isolate healthy seed-potato crops from infected viii) Plant field-resistant cultivars ix) Assess tuber health after harvest (serological tests esp. ELISA)
1Beemster and Rozendaal 1972; 2Kennedy et al. 1962, Van Hoof 1980; 3Bradeley et al. 1966, Vanderveken 1977; 4Loebenstein and Raccah 1980; 5Kassanis 1949; 6de Bokx and Huttinga 1981; Harrison 1984; 7 leaf drop streak = necrosis, starts as spots or rings on leaflets, may cause leaves to collapse and drop from plant or remain hanging on stem stipple streak = necrotic lesions and streaks on leaves and variable streaking on petiole and stems
27
M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective
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