Distribution of Nothanguina phyllobia and Its Potential as a Biological Control Agent for Silver-leaf Nightshade

A. F. ROBINSON, C. C. ORR and J. R. ABERNATHY ~

Abstract: The nematode Nolhanguitza phyllobia Thor ne was found within large foliar galls on the perennial weed Sola~ltm elaeag~i[olium Cav. in west Texas. A two-year survey of a fr400- sq-km area in west Texas showed extensive distr ibut ion of the nematode. No hosls other than S. elaeagni[olium were observed. Densities of juvenile nematodes in the soil were high. N. p]lyllobia spread rapidly after small n tnnbers of infective juveniles were applied in ,a foliar spray 1o an S. elaeagnifolium populat ion. T h e host plant declined in vigor and frequently died. Artificial inoculation of an S. elaeagni[oliltnt populat ion with large numbers of the nematodes by broadcasting infected plant tissue restdted in high infection incidence. Key Words: leaf gall nematode, hiological weed control.

Solanum elaeagniIolium Cav. (silver-leaf nightshade) is an erect herbaceous perennial with deep running rootstocks (1). It is a troublesome weed in every county of the Texas southern high plains, infesting more than 1.2 mill ion hectares of crop land. Weed scientists consider it one of the most important weeds in the area's agriculture. Smith (5) found silver-leaf nightshade infestations to be associated with yield reductions in cotton and grain sorghum of 5-14% and 4-10%, respectively. Orr (4) suggested that Nothanguina phyllobia Thorne , a nematode which causes large foliar galls on silver-leaf nightshade (6), should be evaluated as a biological control agent for the weed. This research investi- gated the distribution, virulence, and biology of N. phyllobia.

M A T E R I A L S AND M E T H O D S

Geographic distribution of N. phyllobia: A 6400-sq-km area surrounding Lubbock, Texas, was scouted for 2 consecutive years to ascertain the distribution of N. phyllobia. Six 80-kin transects were traveled during August 18-20 in 1975 and 1976. Scouts stopped every 8 km to record dominant weeds and the severity of nematode galling on silver-leaf nightshade. Stem and leaf samples of crop and weed species from areas with high N. phyllobia populations were stained with acid fuchsin-lactophenol (3) and examined for nematodes.

Distribution in the soig: Twenty-one soil samples were examined for N. phyllobia.

Received for publication 22 May 1978. 1Technician and Zoologist, Science and Education Admin-

istration, U. S. Department of Agriculture, and Research Scientist, q'exas Agricultural Experiment Station, Route 3, Lubbock, Texas 79401, respectively.

362

Samples came from four silver-leaf night- shade popnlations near Lubbock. Vertical distributions of N. phyIlobia to a depth of 62.5 cm were determined in seven samples by subsampling vertically in 2.5-cm incre- ments. Nematodes were extracted from soil by sieving and sugar flotation (2). Silver- leaf nightshade roots excavated with each sample were stained with acid fuchsin- lactophenol and examined for nematodes.

T o verify that nematode larvae escape abscised galled leaves and enter the soil, two 1,000-ml graduated cylinders (6 cm diam) were filled with steam-sterilized Olton loam soil (55% sand, 21% silt, and 24% clay), with percent moisture adjusted to field capacity. A dry galled leaf was placed on the soil surface in each cylinder and thoroughly wetted. Cylinders were covered with clear plastic to retain moisture. After 7 days, soil was removed in 2.5-cm incre- ments and examined for nematodes.

Inoculation with crushed foliar galls: A 136-sq-m plot was staked in a cultivated field with a high populat ion of silver-leaf nightshade in September 1975. Nematode inoculum was prepared from foliar galls dried to less than 10% moisture, crushed, and sifted through a 2.35-mm sieve. N. phyllobia was introduced by hand- broadcasting 404 g of nematode inoculum evenly over the plot. This achieved a mean nematode density of 8-16 viable larvae/era 2 soil surface. Tile plot was then disked and chiseled and thereafter farmed with the rest of the field.

Inoculation with nematodes in a [oliar spray: A 75-sq-m plot was staked in a popu- lation of silver-leaf nightshade in June 1974. T h e plants initially showed no signs of N. phyllobia infection. Nematodes were intro-

Weed Control with Nothanguina phyllobia: Robinson et al. 363

duced by spraying all leaves of 4 randomly ~a.,,-- / selected silver-leaf nightshade plants to the [ , ~ ~ ~ _ ~ _ ~ ~ . . . . . point of runoff with infective larvae in a \ ,,~.( water suspension (ca. 2,000 larvae per .... plant). These plants were moni tored for , I ~ ,l~'" foliar galls during the next 10 weeks. Leaf ..... samples were removed at biweekly intervals 1~ ) and stained with acid fuchsin-lactophenol ~! .... ~ " to evaluate tissue penetration. Large an- nual plant species were carefully removed to reduce competit ion; otherwise, the plot was left undisturbed. ~ i ¢%

l $LATON Observations were continued in 975 and 1976. All phmts in the plot were staked individually and data were collected on 12 ~[ dates. Plant height, crown area, leaf mor- • ..... phology, and numbers of flowers and berries .... were recorded. Severity of infection was evaluated by assigning a numerical gall- 1 ...... size rat ing and muhip ly ing that by the percentage of leaves galled on each plant.

RESULTS

Geographic distribution: Silver-leaf nightshade plants were present at 95% of the locations observed. T h e populat ions of silver-leaf nightshade were heaviest and in- fections with the nematode were severest in the southwest sector of the surveyed area (Fig. 1). Nematode galls were found at 42% of the locations in 1975 and 64% in 1976. T h e increase in number of loca- tions infected and ratings on the severity of infection at each location indicated twice as much N. phyllobia leaf galling in 1976 as in 1975.

The most common weed species were silver-leaf nightshade, pigweed (Ama- ranthus spp.), Johnsongrass (Sorghum halapense (L.) Pers.), Texas blueweed (Helianthus ciliaris DC.) and kochia (Kochia scoparia (L.) Schrad.). Crop species included cotton (Gos.~ypium hirsutum L.), sorghum (Sorghum bicoIor (L.) Moench.), corn (Zea mays L.), soybean (Glycine max (L.) Merr.), and sunflower (Helianthus annuus L.). Weed and crop plants never showed infection even though they were often tangled with heavily galled silver-leaf nightshade leaves. Laboratory examinations occasionally revealed small numbers of N. phyllobia on leaves and stems of these plants but no evidence of tissue penetrat ion or nematode reproduct ion was found.

Distribution in the soil: Fifteen of the

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FIG. I. Occurrence of S. elaeagni[olium infected by N. phyllobia in the Lubbock area.

21 soil samples contained infective-stage larvae of N. phyllobia. No other juvenile stages or adults were found. Six of the vertically graduated soil-sample series con- tained 100--9,000 individuals each. Larvae were concentrated at the soil surface (maxi- mum density, 8.0 larvae/cc) and at the 15-cm depth (maximum density, 2.3 larvae/cc). In one sample, 1.4 larvae/cc were found 33 cm below the soil surface. N. phyllobia were never found in root tissue. Final vertical distributions in lab- oratory tests were similar to those in the field (Fig. 2).

Inoculation with crushed [oIiar galls: In 1975, late-fall weather prevented re- growth of silver-leaf nightshade. T h e spring anti early summer months of the following year, 1976, were dry (13 cm total precipi- tation), followed by heavy rainfall in July (20 cm). Prior to July, little infection was observed. On July 30, silver-leaf nightshade plants throughout the plot showed extreme infection with N. phyllobia. Infected plants were generally smaller than uninfected plants, and were frequently prostrate from the weight of galled leaves. Nematode in- fectious on silver-leaf nightshade continued for 18 m east of the plot. This area was

864 Journal of Nematology, Volume 10, No. 4, October 1978

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seven plants infected in 1975 wlfich re- sprouted in 1976 were 90% smaller than uninfected plants; fruit ing was similarly reduced. Approximately equal part i t ioning of all plants in 1976 into four infection categories (none, light, moderate, or severe, basetl on numerical ratings) showed that reductions in plant size and fruiting were correlated with the degree of infection (Fig. 3). Moist conditions in 1976 increased nematode symptoms. Th e most active leaf galling of the year occurred after substantial precipitation and continuously high rela- tive humidi ty (Fig. 4). In April and early May (3 cm precipitation), foliar galls were

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slightly downhill and in the direction of the prevailing winds.

Inoculation with a foliar spray: Inocnla- tion in 1974 was followed by :6 weeks of dry weather, during which N. phyllobia larvae, apparently in a quiescent state, were found only on leaf surfaces. After the 6 dry weeks, 2 weeks of rainy weather fol- lowed, and two inoculated plants developed foliar galls containing N. phfllobia larvae. T h e number of galled silver-leaf nightshade plants i n the plot increased from 2 in 1974 to 17 in 1975, and to 157 in 1976. Fifty-nine percent of the plants infected in 1975 died; only 6% of the uninfected plants died. T h e

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~¥eed Control with Nothanguina phyllobia: Robinson et al. 865

observed on 22 plants. All of these plants showed galling within 10 days of emergence. During late May and June (3 cm precipita- tion), many galled leaves abscised, and no additional plants developed galls. Intense new gall formation occurred in July and early August (22 cm precipitation), and the number of galled plants increased to 135 (Figs. 5, 6, 7).

DISCUSSION

Our observations contr ibute the follow- ing information on the life history of N. phyUobia. Adults and preinfective juveniles live and develop only within the moist microhabitat of foliar galls; when galls abscise, adults and preinfective juveniles (lie. Infective-stage larvae within drying galls enter a quiescent state, in which they may remain viable for years and from which they revive to an active state within several hours when rewetted. During sustained moist conditions, these larvae exit abscised galled leaves clinging to the host plant and travel in moisture films on stem surfaces to new infection sites, which are generally actively growing young tissues. If the (lead galled leaf from which larvae escape is on the soil surface, larvae enter the soil and infect preemergent shoots of silver-leaf nightshade. Infective larvae overwinter both in the soil and in galled leaves; the latter are especially subject to dispersal by wind and water. A high biotic potential (often hundreds of thousands of individuals in a single gall) contributes to rapid dispersal by these largely random means.

Silver-leaf nightshade is indigenous to the North American southwest. Its native

range extends from Missouri southward to Louisiana and westward to Arizona and adjacent Mexico (1). N. phylIobia was first reported from central Arizona, at the west end of this range, and from the lower Rio Grande Valley of Texas 1,600 km to the east (6). Our report of the nematode on the Texas high plains, midway between these points, strongly suggests that the geographic distributions of N. phyllobia and silver-leaf nightshade coincide.

N. phylIobia is host-specific for silver- leaf nightshade. In close examinations of numerous plant species from areas with high N. phyllobia populations, no evidence of tissue penetrat ion or nematode repro- duct ion was found. Direct inoculat ion of several plant species in the field (4) failed to produce disease symptoms, while nema- tode galling developed on silver-leaf nightshade control plants. Pronounced host specificity and coincident geographic dis- tr ibutions suggest that N. phyllobia and S. elaeagnifolium are highly coevolved, minimizing the possibility that N. phyUobia would ever switch hosts if populat ions were augmented in a biological control program.

We have demonstrated that large num- bers of infective larvae of N. phyllobia can be readily introduced into silver-leaf night- shade populations. Once introduced, the nematodes spread rapidly, significantly reducing top growth of the host and re- producing prolifically as long as the host plant is present. We speculate that the development of nematode symptoms would be accelerated under aerial irrigation. T h e added moisture would facilitate activation of infective larvae within dead galls or in the soil and allow nematodes to ascend

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366 Journal of Nematology, Volume 10, No. 4, October 1978

p l a n t s t ems . S i l v e r - l e a f n i g h t s h a d e p o p u l a - t i o n s c a n b e r e d u c e d b y d i s p e r s i n g t h e n e m a t o d e a r t i f i c i a l l y , m o d i f y i n g i r r i g a t i o n r e g i m e s a n d s i m i l a r m a n i p u l a t i o n s .

L I T E R A T U R E C I T E D

1. CORRELL, D. S., and M. C. JOHNSTON. 1970. Manual of the vascular plants of Texas. Texas Research Foundation. Renner, Texas. 1881 pp.

2. JENKINS, W. R. 1964. A rapid centrifugation- flolation technique for separating nematodes

from soil. Plant Dis. Rep. 43:692. 3. McBETH, C. W., A. L. TAYLOR, and A. L.

SMITH. 1941. Notes on staining nematodes in root tissue. Proc. Helm. Soc. Wash. 8:26.

4. ORR, C. C., J. R. ABERNATHY, and E. B. HUDSPETH. 1975. Nothanguina phyllobia, a nematode parasite of silver-leaf nightshade. Plant Dis. Rep. 59 (5):461-418.

5. SM1TH, 1). T. ]971. Unpublished data. Texas Agricultural Experiment Station, Rt 3, Lub- bock, Texas.

6. THORNE, G. 1961. Principles of hematology. McGraw-Hill Book Company, New York. 553 pp.