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Cucurbit Genetics Cooperative Report 20:54-56 (article 24) 1997

Screening for Silverleaf Resistance in Cucurbita Accessions

Linda Wessel-Beaver

Dept. of Agronomy and Soils, University of Puerto Rico, Mayaguez, PR 00681 U.S.A.

During the past decade the silverleaf whitefly, Bemisia argentifolii Bellows and Perring (= Bemisia tabaci, 'B' strain), has become a serious pest of squash and pumpkins in the Caribbean Basin. Populations of this whitefly have increased dramatically in Florida and Puerto Rico since 1987. Whitefly-induced silvering on Cucurbita species was first observed in Florida in 1987 (5) and in Puerto Rico in 1989 (4). Since then, the silverleaf whitefly has become a problem in other areas as well.

Cultivars of C. pepo show varying levels of resistance to silvering (1,2,3). The purpose of this research was to screen a large number of plant introductions (PIs) from the U.S. National Germplasm System to identify silverleaf resistant is important since silvered plants are less vigorous and silvered fruit are commercially unacceptable. In addition, this germplasm may also be a source of silverleaf whitefly resistance.

Evaluations were done in 1992, 1995 and 1996 in Isabela, Puerto Rico in fields naturally infested by whiteflies. Plots were planted on 60 inch centers and consisted of six plants spaced 30 inches apart. Only the 1992 test was replicated (2 reps). Silverleaf was rated on a plot basis at 6 and 8 weeks post-plant using a 0 (symptomless) to 5 (completely silvered) scale. The tropical pumpkin (C. moschata) cultivar 'Soler' was used as a check.

C. moschata: In 1992, 341 PIs were evaluated. Fifty seven PIs had a rating of 0.5 at 8 weeks. In 1996, 50 of those PIs were retested along with 79 previously untested accessions. In that evaluation 14 PIs remained almost free of silverleaf (Table 1). Six PIs (162889, 211996, 211999, 427214, 483345 and 512153) were highly resistant to silverleaf in both test years. PI 162889 is from Paraguay, PIs 211993, 211996 and 211999 are from Iran, PI 438579 is from Guatemala, PI 483345 is a F1 hybrid from Korea, and the remaining accessions originated from the United States. PI 512153 is 'Tennessee Sweet Potato'. PIs 550689 to 550694 are a related group of crooknecks and butternuts including 'Canada Crookneck', 'New Hampshire Butternut' and 'Ponca Butternut'. 'Tennessee Sweet Potato' and 'Canada Crookneck' are U.S. varieties introduced in the 1800's (6), but not thought to be closely related.

C. pepo: In 1995, 350 PIs were evaluated but heavy rains kept the number of whiteflies low. As a result 251 (72%) accessions showed no leaf silvering at 8 weeks after planting. Nearly the same group (346 PIs) were again planted in February 1996. Although whitefly levels were very high at 6 weeks after planting, a large group of PIs had silverleaf ratings 1.0 at that date (Table 1). Silverleaf readings were not taken at 8 weeks since many plants had developed severe virus symptoms by the date. Resistant PIs originated mainly from Turkey, Iran, the former Yugoslavia and Mexico (although most of the C. pepo collection originated from those same countries). Both PI 234614 and PI 442791 are from South Africa and identified in the GRIN as 'Little Gem'. Paris et al. (2) observed that cocozelle and vegetable marrow summer squash groups, developed primarily in the Old World, were the least susceptible to silvering.

C. maxima: In 1995, 405 PIs were planted ut whitefly numbers were not sufficiently high to cause extensive silvering until January 1996. By then the plants had grown too large to distinguish between plots. In February, 1996 a portion (100 PIs) of the C. maxima were replanted. Only seven accessions were found to be resistant to silverleaf (Table 1). NSL 214307 is identified in the GRIN as originating in Florida while the other PIs are from Turkey, Saudi Arabia and Nigeria.

Adult whiteflies were observed on all accessions. Although no counts were take, some accessions with low silverleaf ratings had much fewer whiteflies compared to highly silvered plants which consistently had large numbers of whiteflies. An initial screening for silverleaf resistance allows the breeder to eliminate a large amount of germplasm and concentrate resources on a much more select group in the search for whitefly resistance. Resistant PIs have been selfed for continued silverleaf and whitefly evaluations.

Table 1. Cucurbita accessions showing a high level of silverleaf resistance at Isabela, Puerto Rico in 1996.

C. moschata (420 PIs tested)
162889y 211999y 483345y 550689 550691 550693
211993 427214y 512153y 550690 550692 550694
211996y 438579        
C. pepo (350 PIs tested)
165047 212214 311103 368603 420328 451853
169430 222786 344358 368605 442294 451854
169450 223354 357928 368609 442297 458730
174188 227298 357936 368615 442299 458731
176961 234614 357937 368616 442304 482593
204693 256063 357952 379309 442307 507888
206957 256064 357963 379310 442311 511982
212000 261610 357967 379311 442318 512188
212001 265560 368592 379314 442321 512189
212013 267664 368594 379316 442791 512192
212014 267756 368595 406679 449350 525181
212060 302418 368602  
C. maxima (405 PIs tested)
169470 182195 183259 249013 244704 NSL214307

z Mean silverleaf rating of 0 to 1 at six (C. pepo, C. maxima) or eight (C. moschata) weeks after planting. Silverleaf rated on a scale of 0 (symptomless) to 5 (completely silvered). Check 'Soler' (C. moschata) had a mean silverleaf rating of 2.8 at six and 4. at eight weeks after planting.
y Tested in 1991-92 and 1995-96.

Acknowledgment: This research was supported in part by USDA/CSREES Grant No. 95-34135-1667 administered by the Caribbean Bastin Administrative Group (CBAG).

Literature Cited

  1. McAuslane, H.J., S.E. Webb and G.W. Elmstrom. 1996. Resistance in germplasm of Cucurbita pepo to silverleaf, a disorder associated with Bemisia argentifolii (homoptera: Aleyrodidae). Florida Entomologist 79(2):206-221.
  2. Paris, H.S., P.J. Stoffella and C.A. Powell. 1993. Susceptibility to leaf silvering in the cultivar groups of summer squash. Euphytica 69: 69:72.
  3. Paris, H.S., P.J. Stoffella and C.A. Powell. 1993. Differential susceptibility to leaf silvering in cucurbita pepo. HortSci 28:657-8.
  4. Segarra-Carmona, A., J. Bird, J. Escudero, G. Fornaris-Rullan and R. Franqui. 1990. Silvering of Cucurbita moschata Duchesne) Poir associated with Bemisia tabaci Genn (Homoptera: Aleyrodidae) in Puerto Rico. J. Agri. Univ. P.R.74:477-8.
  5. Simons, J.N., P.J. Stoffella, K.D. Shuler and R.N. Raid. 1988. Silver-leaf of squash in South Florida. Proc. Fla. State. Hort. Soc. 101:397-9.
  6. Tapley, W.T., W.D. Enzie and G.P. Van Esltine. 1937. The Vegetables of New York: Vol.1-PartIV, The Cucurbits. J.B. Lyon Company, Albany, New York.
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