Viral Disease

Course No. PP-202 Module No. 2

Viruses can be defined as “small obligate intracellular parasites, which contain either an RNA or DNA genome surrounded by a protective, virus-coded protein coat”.

Rod-shaped Flexuous thread

Isometric virus Bacilliform

In 1886, Dutchman Adolph Mayer injected juice obtained from tobacco plant leaves showing various patterns of greenish yellow mosaic into healthy tobacco plants and the latter then developed similar mosaic patterns.

History

Earliest Record of Plant Virus

Yellow Vein Net Disease of Eupatorium was described in a Japanese poem, written in the eighth century (Saunders et al., 2003)

Tobacco Mosaic Virus (TMV) ssRNA

Known to occur in all the tobacco growing countries of the world

55% reduction in yield has been reported Sub stranded produce fetch low price in

the market

Symptoms Leaves show mild

veins clearing Mottle and

mosaic pattern Light

discoloration along the veins of young leaves

Wrinkling crinkling twisting margins and narrowing of leaf lets

Dwarfing of entire plant

Mark reduction in yield

Epidemiology and Disease cycle

Survive in herbaceous and woody plants Tomato, Potato, pepper grown indoor are

source of infection Virus present in plant debris, soil, cloths

and even manufactured tobacco cigarette and cigar

Contact with the wounded plants No insect vector, Mechanical transmission Wide host range of 500 species of flowering

plants Temp. 28-32 ºC, RH 44-56%

Management Use clean and healthy seed in the nursery All workers should disinfect their hands Rouging Crop rotation maize and wheat, avoiding

tomato and pepper Healthy leaves harvest first than infected

one Field sanitation practices

Potato leaf roll diseasePLRV

ssRNA Occur world wide Most destructive virus of potato

Symptoms Leaf mottling or yellowing, leaf deformation,

necrotic leaf spots or rings, veinal necrosis, Necrotic stem-streaking, leaf drop and

premature death of stems. The tubers remain smaller in size Mild strains and tolerant cultivars may

develop milder foliage symptoms without any necrosis, leaf drop or premature death of shoots.

Secondary symptoms includes dwarfing and wrinkling of leaves

Disease cycle More than 10 species of aphids act as

vector (Myzus persicae) Infected tubers are primary source infection Aphid feed for several hrs (48-54 hrs) Alternate hosts are datura stramonium,

physalis floridana

Epidemiology 15-20 0c temperature 40-65% Relative Humidity

Sugarcane Mosaic Virus DiseaseSCMV ssRNA

First recorded in Java in 1892, in Punjab in 1926-27

Occur in every sugarcane growing country.

Estimated yield losses vary greatly depending on the time period and sugarcane growing area involved.

SYMPTOMS

Contrasting shades of green, islands of normal green or yellowish chlorotic areas on the leaf blade

Infection accompanied by varying degrees of leaf reddening or necrosis.

Chlorotic areas most evident at base of the leaf.

Chlorotic areas may also be present on the leaf sheath, but rarely on the stalk. Young, rapidly growing plants are more susceptible to infection

SPREAD AND EPIDEMIOLOGY

There are three principal modes of spread of SCMV:

(1) by aphid vectors, (2) by infected seed cane and (3) by mechanical inoculation. Only aphid vectors and infected seed cane

are important in the field. Temp. 35-38 ºC, RH 68-78%

At least 12 species of aphids transmit SCMV The spread of mosaic is most rapid when vector populations are high, susceptible

sugarcane varieties are grown, and SCMV-infected plants are plentiful.

Mosaic is primarily spread by planting infected seed cane.

Management Resistant varieties is the most effective

method of mosaic control. Periodic surveys of SCMV strains are

necessary to tested against prevalent strains.

Management practices targeting insect vectors and control methods aimed at eradication have not been very effective.

Citrus Tristeza Virus (CTV)

ssRNA Biggest threats to citrus production in

globe. More than 30 virus, virus-like diseases of

citrus known in the world, of which CTV is most destructive.

In 1981, the total world loss attributable to this disease was estimated at 50 million trees.

Symptoms Quick decline Dieback of the

phloem in the sour orange rootstock below the bud union.

slow decline' where trees decline in a period of years.

A third common symptom is stunting where the virus does not kill the tree, but the tree does not grow.

Sweet orange cv. Valencia grafted on sour orange rootstock affected by Citrus tristeza closterovirus (CTV).

Either deep pits are present under depressed areas of the bark, or more severe strains may cause a more general distribution of the stem pitting, with a thickening of the bark and many fine pits.

Stem pitting induced by CTV on a grapefruit trunk

Vein clearing induced by CTV ininoculated Mexican lime (C. aurantiifolia) seedlings incubated in a glasshouse at 18-25°C.

Disease cycle Vectors of CTV are black citrus aphid

(Toxoptera citricida), brown citrus aphid (Toxoptera citricida)

Due to the aphid introduction, CTV rate of spread has increased dramatically

Additionally, growers have aided in the spread of the virus with the propagation of trees using infected bud-wood.

Management

Exclude any introduction of severe strains, Need for mild strain protection in the

future. Strict control by quarantine must be

continued Rootstocks that offer resistance to tristeza

COTTON LEAF CURL DISEASE(CLCuV)

ssDNA

First reported in Nigeria in 1912

In 1959 from Philippines

In 1967 in Pakistan near Multan

Symptoms

Thickening of veins Color become dark green Upward or downward curling of leaves Leaf enation Twisted leaf petioles Stunted growth

Disease Cycle Not seed-borne or soil-borne Over winter on alternate hosts (tomato,

tobacco, beans, lehli, okra, datura etc Transmitted by whitefly (Bemisia tabaci) 30 Min. feeding on infected plants 24 hours latent period 30 Min. inoculation feeding period Retention period: through out life

Epidemiology Minimum temp. 25-30 c Maximum temp. 33-45 c Optimum temp. 32 c Relative humidity: 56-60%

Management Resistant varieties, NIBGE-2, NIAB-111, PB-

899, CIM-446, MNH-786 Destruction of alternate hosts Control insect vector (Imidacloprid) Uproot & burry first observed infected

plants