Relationship of Watermelon Genotype and Ploidy to Incidence of Yellow Vine Disease

S.D. Pair1 and B.D. Bruton2

USDA­ARS, South Central Agricultural Research Laboratory, P.O. Box 159, Lane, OK 74555

Additional index words. plant resistance, cucurbits, triploid, vine decline

Abstract. Producers lack satisfactory control strategies for yellow vine, a serious vine decline disease of watermelon, Citrullus lanatus (Thunb.) Matsum & Nakai, and other cucurbits in Oklahoma and Central Texas. A phloem-limited bacterium has been associated with the disease and evidence suggests that the organism may be vectored by leafhoppers. In 1997, 21 watermelon cultivars representing open-pollinated, hybrid, and polyploid genotypes were screened for resistance to the yellow vine disease. Results indicated that watermelon cultivar did not influence populations of cucumber beetle or squash bug. Percentage of yellow vine was, however, significantly influenced by both cultivar and by ploidy level. Incidence of yellow vine affected plants ranged from 1.7% to 45.0% in 'Sugar Baby' (4n) and 'Klondike', respectively. Diploid open-pollinated and hybrid cultivars had 63% and 41% higher incidence of yellow vine disease, respectively, compared to triploid seedless cultivars. Increased leaf pubescence and other factors are discussed as potential mechanisms for the observed difference in incidence of yellow vine disease.

Mention of a trademark, propriety product, or vendor does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products of vendors that may also be suitable.

1Research entomologist.

2Research plant pathologist.

 

 

From 1988 through 1990, a new vine decline disease of squash and pumpkin (Cucurbita pepo L.), known as yellow vine, was observed in Central Texas and Oklahoma (Bruton et al., 1995a). Subsequently, the sudden appearance of yellow vine disease in watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] and cantaloupe (Cucumis melo var. cantalupensis Naud.) in 1991 proved devastating for watermelon producers in Oklahoma and Central Texas (Bruton et al., 1992). Initially, yellow vine disease appeared to be confined to the Cross Timbers vegetational zone in central Texas and Oklahoma. In 1997, yellow vine caused near 100% losses of watermelon grown in Anderson County, Tex., located in the Post Oak Savannah vegetational area of eastern Texas. This was the first confirmed incidence of yellow vine disease outside of the Cross Timbers area. Since 1991 the disease has appeared sporadically, but is still considered a limiting factor to expansion of the industry in these regions of Texas and Oklahoma.

When yellow vine disease first appeared, symptoms were attributed to numerous abiotic and biotic agents including herbicide damage, nutrient imbalance, fusarium wilt, charcoal rot, Monosporascus vine decline, and unknown seedborne pathogens. In addition, the disease was described as whitefly-borne lettuce infectious yellows (Halliwell and Johnson, 1992) in the aftermath of sweetpotato whitefly outbreaks in South Texas. Observations in yellow vine affected areas never revealed whitefly infestations (Pair et al., 1998). Subsequent research (Bruton et al., 1995b) demonstrated that rates of yellow vine disease in squash were suppressed in insecticide-treated plots, but not in fumigated soil, suggesting that the disease was vectored by insects. Numerous experiments with cucurbit insect pests including cucumber beetles, squash bug, and aphids, failed to achieve transmission of the disease (Pair et al., 1993), thus, neither confirming nor identifying potential vectors.

 

 

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The nature of the suspected causal agent associated with yellow vine has now been described as a phloem-limited bacterium (Bruton et al., 1998). The bacterium was consistently associated with symptomatic yellow vine plants. Symptoms of the disease generally appear 2 weeks before harvest and are characterized by general leaf yellowing, upturned terminal leaves, and eventual death of the plant. The one characteristic symptom associated with yellow vine is the honey-brown phloem discoloration in the lower crown and primary root of affected plants. In some cases, infected vines may collapse in one day without developing the leaf yellowing symptom. Unfortunately, these bacteria cannot be cultured in the usual manner, presenting significant obstacles in terms of conducting transmission studies or in screening germplasm for resistance. In most cases, plant diseases associated with phloem-limited bacteria typically are transmitted by leafhoppers (Hopkins, 1977). Indeed, recent data suggests that a deltacephaline leafhopper is involved in the transmission of yellow vine disease to cucurbits (Pair et al., 1998).

The cucurbit host range of yellow vine currently appears to be limited to squash, pumpkin, watermelon, and cantaloupe. Symptoms have not been observed in cucumber or any of the gourds. Evidence suggests that squash, pumpkin, watermelon, and cantaloupe, in order of decreased

susceptibility, are most affected.

No satisfactory control strategy exists for yellow vine disease. Insecticidal applications offer some degree of suppression, but results are erratic and usually uneconomical. Soil fumigation does not influence the incidence of yellow vine, although disease severity is generally much higher in cucurbit crops planted on plastic mulch. Watermelon and cantaloupe crops planted before 1 June generally exhibit a higher incidence of yellow vine than those planted later. In contrast, a high incidence of yellow vine has been observed in squash planted after 1 July. Identification and development of resistant germplasm for use in an integrated system offers the most reliable and efficient long-term management strategy for many plant pests. Resistance in triploid seedless watermelons, particularly in 'Tri-X 313' has been reported (Bruton et al., 1995a; Pair et al., 1998). Therefore, we initiated field studies to expand our knowledge of potential resistance to yellow vine disease among watermelon cultivars of different genotypes and ploidy levels.

Materials and methods

The experiment was conducted in a 0.5 ha field of Bernow fine sandy loam soil located at the Lane Research Center, Lane, Okla. Land preparation, fertilization, and application of herbicides were performed according to Oklahoma State University Extension guidelines. The experiment consisted of 21 watermelon cultivars (Table 1), which included 8 diploid open-pollinated, 7 diploid hybrid, 5 triploid, and 1 tetraploid. The open-pollinated 'Hairless Joe' is a male-sterile mutant used as a genetic marker and segregates 1:1 for glabrous and pubescent progeny (Watts, 1962). Seeds of each cultivar were planted in seedling trays containing potting mix, watered, and maintained in a greenhouse at 24 to 29 oC. At the third true leaf stage, seedlings were transplanted to the field on 12 May 1997. The experiment was arranged in a RCB design with 6 replications and consisted of ten plants per cultivar 0.9 m apart in a single row 9.1 m in length, 3.6 m between cultivars, and with a 3.6 m alley between plots. Plots containing 'Hairless Joe' contained equal numbers of both glabrous and pubescent plants.

Table 1. Watermelon cultivars evaluated for resistance to yellow vine disease during 1997 at Lane, Okla.

Cultivar Genotypez Seed source

Dumara 2n, hy Nunhems

Jonia 2n, hy Nunhems

Red Comet 2n, hy Nunhems

Angela 2n, hy Intersemillas

Azabache 2n, hy Intersemillas

Sorpresa 2n, hy Intersemillas

Pata Negra 2n, hy Petoseed

Ace of Hearts 3n Petoseed

Deuce of Hearts 3n Petoseed

Tri-X 313 3n American Sunmelon

Crimson Jewel 3n Rogers

Scarlet Trio 3n Rogers

Black Diamond 2n, op Wilhite

Royal Sweet 2n, op Petoseed

Allsweet 2n, op Hollar

Charleston Gray 2n, op Hollar

Klondike 2n, op Hollar

Hairless Joe 2n, op Hollar

Crimson Sweet 2n, op Dewitt

Sugar Baby 2n, op Sunseeds

Sugar Baby 4n Sunseeds

zDenoted as 2n = diploid, 3n = triploid, 4n = tetraploid, op = open-pollinated, and hy = hybrid watermelon cultivars.

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Table 2. Incidence (%) of yellow vine in watermelon cultivars, Lane, Okla., 1997.

Cultivar Yellow vine (%)z

Klondike 45. 0 a

Charleston Gray 33. 3 ab

Hairless Joe 33. 3 ab

Royal Sweet 28. 3 ab

Black Diamond 26. 7 ab

Pata Negra 26. 7 ab

Red Comet 25. 0 ab

Dumara 21. 7 ab

Crimson Sweet 18. 3 ab

Ace of Hearts 13. 3 ab

Scarlet Trio 13. 3 ab

Azabache 13. 3 ab

Allsweet 13. 3 ab

Sugar Baby (2n) 11. 7 b

Jonia 11. 7 b

Angela 8. 3 b

Tri-X 313 8. 3 b

Sorpresa 8. 3 b

Crimson Jewel 6. 7 b

Deuce of Hearts 6. 7 b

Sugar Baby (4n) 1. 7 b

zMeans separation within a column by Ryans Q test at P 0. 05.

(P > 0.05) cucumber beetle, squash bug, aphids, or leafhopper populations. Slight numerical differences for cucumber beetle and squash bug were observed before vining, but these populations were primarily confined to small areas of the field and were attributed to aggregation behavior of these insects. Melon aphid and leafhopper populations were not observed on the test plants. However, a leafhopper swept from bermuda grass in the field borders and tested with PCR showed positive for the bacterium associated with yellow vine disease (S.D. Pair and B.D. Bruton, unpublished data).

Watermelon plants symptomatic for yellow vine disease were first observed on 17 June and last on 20 Aug. when the study was terminated. Incidence of yellow vine was influenced by cultivar (F = 2.91; df = 20, 105; P = 0.0002) and ranged from 1.7% and 45.0 % diseased plants in 'Sugar Baby'(4n) and 'Klondike', respectively (Table 2). In general, open-pollinated cultivars had the highest incidence while some 2n hybrid and most polyploid cultivars showed some resistance to yellow vine disease. For example, 'Sugar Baby' (2n) incurred six times more yellow vine than it's 4n counterpart. Interestingly, glabrous 'Hairless Joe' plants exhibited 3.5¥ more yellow vine disease than the pubescent isoline (data not shown).

Polyploid cultivars were generally more resistant to yellow vine disease than diploid types. When yellow vine disease data were analyzed according to ploidy level, significant differences were noted (F = 8.22; df = 3, 122; P = 0.0001) among diploid open-pollinated and polyploid cultivars (Table 2). There were also substantial numerical differences in percentage yellow vine disease among each of the ploidy levels possibly reflecting

At weekly intervals following transplanting and ending at harvest, the numbers of cucumber beetles, squash bugs, aphids, and leafhoppers were determined from examination of five randomly selected plants in each plot. Twice weekly, all plants in each plot were observed for the appearance of yellow vine disease. The crown and tap root were excised from symptomatic plants and transported to the laboratory for microscopic examination. Plants exhibiting a honey brown discoloration of the phloem, characteristic of yellow vine disease (Bruton et al., 1995), were submitted for polymerase chain reaction (PCR) (Avila et al., 1998) to confirm the presence of the bacterium associated with the disease. Insect population and percentage yellow vine disease data for each watermelon cultivar were analyzed using PROC GLM and means were separated using Ryans Q test (SAS Inst., Cary, N.C.). Percentage yellow vine for each cultivar was calculated by multiplying the number of confirmed yellow vine affected plants by 100 and dividing by the total number of watermelon plants.

Results and discussion

On a seasonal basis, cultivar did not influence

Table 3. Influence (%) of watermelon genotype and ploidy on incidence of yellow vine disease, Lane, Okla., 1997.

Genotypez n Yellow vine (%)y

2n, op 48 26. 2 a

2n, hy 42 16. 4 ab

3n 30 9. 7 bc

4n 6 1. 7 c

zDenoted as 2n = diploid, 3n = triploid, 4n = tetraploid, op = open-pollinated, and hy = hybrid watermelon cultivars.

yMean separation within a column by Ryans Q test at P 0. 05.

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hybrid cultivars appeared to have more dense and longer leaf trichomes especially along midveins. The tetraploid 'Sugar Baby' was substantially more pubescent than the diploid cultivar (S.D. Pair and B.D. Bruton, unpublished data). The effects of ploidy on cucurbit susceptibility to pests , especially insects, have received little attention. Polyploid watermelon lines have been reported as more resistant to anthracnose (Henderson, 1977) and to bacterial fruit blotch (Hopkins and Elmstrom, 1995). Since triploid seedless watermelons exhibit superior resistance to yellow vine in addition to other diseases, perhaps they should be considered by growers in affected production areas.

While watermelon leaf pubescence may play a role in conferring resistance to yellow vine disease, physiological traits imparting tolerance or antibiosis due to genotype cannot be discounted at this point. Research efforts at this laboratory have been initiated to confirm the identity of the insect vector and to investigate more thoroughly the morphological and physiological factors in watermelon germplasm that affect insect feeding behavior and susceptibility to yellow vine disease.

Literature cited

Avila, F.J., B.D. Bruton, J. Fletcher, J.L. Sherwood, S.D. Pair, and U. Melcher. 1998. Polymerase chain reaction detection and phylogenetic characterization of an agent associated with yellow vine disease of cucurbits. Phytopathology 88:428­436.

Bruton, B.D., B. Cartwright, S.D. Pair, J. Fletcher, and M.E. Shaw. 1992. Status of watermelon decline in Oklahoma and Texas, p. 130­132. In: B.D. McCraw (ed.). Proc. 11th Annu. Okla. Hort. Ind. Conf., Okla. State Univ., Stillwater.

Bruton, B.D., S.D. Pair, and E.V. Wann. 1995a. Yellow vine disease of watermelon and cantaloupe in central Texas and Oklahoma, p. 155­159. In: G.E. Lester and J.R. Dunlap (eds.). Proc. Cucurbitaceae 94. Gateway Printing, Edinburg, Tex.

Bruton, B.D., S.D. Pair, T.W. Popham, and B. Cartwright. 1995b. Occurrence of yellow vine, a new disease of squash and pumpkin, in relation to insect pests, mulches, and soil fumigation. Subtrop. Plant Sci. 47:53­58.

Bruton, B.D., J. Fletcher, S.D. Pair, M. Shaw, and H. Sittertz-Bhatkar. 1998. Association of a phloem-limited bacterium with yellow vine disease in cucurbits. Plant Dis. 82:512­520.

 

varying degrees of heterosis. Diploid open-pollinated and hybrid cultivars had 63% and 41% higher incidence of yellow vine disease, respectively, compared to triploid seedless cultivars.

Results from this study suggest that substantial variability exists among and within, watermelon genotypes in their level of resistance to yellow vine disease. The mechanism of the apparent resistance was not investigated here. Assuming that the vector is a leafhopper as evidenced by their association with similar diseases caused by phloem or xylem-limited bacteria, it is likely that morphological characters such as leaf pubescence may account for the differential levels of resistance exhibited by the watermelon cultivars tested in this study. The presence of trichomes apparently limit leafhopper access for feeding in the leaf veins and is the most thoroughly studied factor associated with plant resistance to leafhoppers (Tingey, 1985). Leaf trichome variability among watermelon germplasm sources for potential resistance against insect attack has not been studied, but is often considered an important character in the genetic improvement of other crops. The presence or absence of pubescence in cucumber determines the ovipostion preference of pickleworm and melonworm (Diaphania sp.) (Elsey and Wann, 1982). In cotton, resistance to Empoasca facialis Jacobi was found directly related to length and density of trichome cover on abaxial midveins (Parnell et al., 1949). Conversely, leaf hairiness is associated with increased silverleaf whitefly (Bemisia argentifolii Bellows & Perring) populations in cantaloupe (Riley, 1995). The increased yellow vine disease incidence in pubescent 'Hairless Joe" in our study contradicts the hypothesis that the degree of leaf hairiness may influence disease levels in watermelon. We observed that the pubescent 'Hairless Joe' plants were much more vigorous than the glabrous isoline. Consequently, the glabrous vines tended to be covered rapidly by the pubescent isoline canopy, which may have afforded some degree of protection from leafhopper feeding on the glabrous plants.

Ploidy was a significant factor in resistance to yellow vine disease in our study. While leaf hairiness among the different genotypes was not measured, many of the polyploid, as well as several 2n

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Elsey, K.D. and E.V. Wann. 1982. Differences in infestation of pubescent and glabrous forms of cucumber by pickleworms and melonworms. HortScience 17:253­254.

Halliwell, R.S. and J.D. Johnson. 1992. Lettuce infectious yellows virus infecting watermelon, cantaloupe, honeydew melon, squash, and cushaw in Texas. Plant Dis. 76:43.

Henderson, W.R. 1977. Effect of cultivar, polyploidy, and "reciprocal" hybridization on characters important in breeding triploid seedless watermelon hybrids. J. Amer. Soc. Hort. Sci. 102:293­297.

Hopkins, D.L. 1977. Diseases caused by leafhopper-borne rickettsia-like bacteria. Annu. Rev. Phytopathol. 17:277­294.

Hopkins, D.L. and G.W. Elmstrom. 1995. Comparison of triploid and diploid watermelon cultivars and breeding lines for susceptibility to bacterial fruit blotch, p. 152­154. In: G.E. Lester and J.R. Dunlap (eds.). Proc. Cucurbitaceae 94. Gateway Printing, Edinburg, Tex.

Pair, S.D., B.D. Bruton, B. Cartwright, and J. Duthie. 1993.

Status of search for insect vectors of yellow vine in cucurbits, p. 149­151. In: B.D. McCraw (ed.). Proc. 12th Annu. Okla. Hort. Ind. Conf. , Okla. State Univ., Stillwater.

Pair, S.D., B.D. Bruton, F. Mitchell, and J. Fletcher. 1998. Advances in yellow vine research, p. 287­290. In: B.D. McCraw (ed.). Proc. 17th Annu. Okla. Hort. Ind. Conf. , Okla. State Univ., Stillwater.

Parnell, F.R., H.E. King, and D.F. Ruston. 1949. Jassid resistance and hairiness of the cotton plant. Bul. Entomol. Res. 39:539­575.

Riley, D. 1995. Melon cultivar response to Bemisia. Subtrop. Plant Sci. 47:39­45.

Tingey, W.M. 1985. Plant defensive mechanisms against leafhoppers, p. 217­234. In: L.R. Nault and J.G. Rodriguez (eds.). The leafhoppers and planthoppers. Wiley, New York.

Watts, V.M. 1962. A marked male-sterile mutant in watermelon, Proc. Amer. Soc. Hort. Sci. 81:498­505.

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