Inheritance of the Resistance to Squash Leaf Curl Virus
in Cucurbita pepo L.

Clara E. Montes-García and Sergio Garza-Ortega

Department of Agriculture and Animal Science, University of Sonora, P.O. Box 305,
Hermosillo, Sonora 83076 México

Judith K. Brown

Department of Plant Science, University of Arizona, Tucson, AZ 85721

Additional index words. whitefly, Bemisia tabaci

Abstract. Squash leaf curl virus (SLCV) is a threat to cucurbit growing regions of Southwest United States and Northwest Mexico. SLCV-resistant lines 214 and 264 were crossed with susceptible Gray Zucchini (GZ) to study the nature of the resistance. When line 264 was used as the resistant parent, a 3:1 and a 1:1 resistant to susceptible ratio was obtained for the F2 and for the backcross respectively. Using line 214 as the resistant parent did also fit the expected values except for perhaps a mis-labled F2 family from the cross GZ x 214. PCR tests for geminiviruses were positive for all materials during the fall but negative during the spring. It is concluded that SLCV resistance in Cucurbita pepo (L.) is controlled by a single, dominant gene.

Viral diseases cause devastating losses in cucurbits. Squash leaf curl virus (SLCV) in particular is destructive in all cucurbits produced in the southwestern United States (Brown, 1994; Brown and Nelson, 1986; Brown and Nelson, 1989; Flock and Mayhew, 1981; Nameth et al., 1986). We found that this virus infects cucurbits during the fall in Sonora, Mexico (Montes et al., 1998). SLCV-resistant lines of summer squash were developed at the University of Sonora (Ramírez, 1997; Tacho and Garza, 1986). The resistance, which apparently behaves as a dominant trait, was introduced to Cucurbita pepo (L.) from a local landrace of Cucurbita moschata (Duch.) Duch. ex Poir. However, most commercial hybrids of summer squash are susceptible to SLCV (Bracamonte, 1998; Tacho, 1995). Farmers are concerned about growing SLCV-resistant hybrids during the fall, which could be accomplished by using parents with resistance. The objective of this study is to determine the inheritance of the resistance to SLCV in C. pepo resistant progeny.

Materials and methods

Summer­fall 1996. Gray Zucchini-type, SLCV-resistant lines 214 and 264 and susceptible Gray Zucchini (GZ) were established in early Septem

ber. Reciprocal crosses to obtain the F1 generation were accomplished by hand-pollination (Whitaker and Robinson, 1986). Symptoms of SLCV developed on GZ, whereas, lines 264 and 214 were free of the characteristic upward curling of leaves, thickening of veins, reduction in leaf size, yellowing, and stunting. PCR assays for whitefly-transmitted geminiviruses (Brown et al., 1995; Wyatt and Brown, 1996) were positive in both resistant lines and susceptible GZ.

Winter­spring 1997. Parents and F1 hybrids obtained during the fall were planted on 12 Mar. Self-pollinations and backcrosses were performed but due to a severe papaya ringspot virus infection there was no fruit set from the backcross with the resistant parent. PCR assays (Wyatt and Brown, 1996) indicated that SLCV was not present during this season (Montes et al., 1998).

Summer­fall 1997. Materials shown on Table 1 were direct seeded on 10 Sept. Previously to this date (20 August), SLCV-susceptible GZ was planted in beds adjacent to each of the treatments. By the time the experimental materials were planted, all nearby GZ plants exhibited silvering of leaves due to infestation by the silverleaf whitefly (Costa and Brown, 1991). Symptoms of SLCV were clear on the materials reaching 99% infection in GZ by early November, at which time adjacent

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Table 1. Segregation of crosses between resistant (R) summer squash lines 214 and 264 and susceptible (S) 'Gray Zucchini' (GZ) to squash leaf curl virus infection, summer­fall season 1997.

Observed Expected

Entry R S R S c2 P

GZ 1 133 0 134

214 130 3 133 0

264 157 2 159 0

F1 214 x GZ 110 38 148 0

F1 GZ x 214 142 13 155 0

F2 214 x GZ 89 38 95 32 1.6404 0.200

F2 GZ x 214 52 68 90 30 21.5111 <0.001

60 60 2.1333 0.154

BC(214 x GZ) GZ 77 69 73 73 0.4384 0.508

F1 264 x GZ 147 4 151 0

F1 GZ x 264 143 14 157 0

F2 264 x GZ 104 39 107 36 0.394 0.540

F2 GZ x 264 109 34 107 36 0.114 0.740

BC (264 x GZ) GZ 75 58 67 67 2.173 0.150

 

GZ plants were removed and whiteflies were killed by application of insecticides. Plants were rated as resistant if free of symptoms and as susceptible if they had either slight or severe symptoms. Fruit were removed from plants two times during the season, and the remaining crop was allowed to develop uniformly to maturity for all treatments. Again, as with summer-fall of 1996, all plants examined by the PCR gemminivirus assay were positive for SLCV.

Results

As shown in Table 1, when line 264 was used as the resistant parent, the F1 generation was resistant. The F2 segregated in a 3:1 resistant to susceptible ratio. A 1:1 ratio was observed for the backcross to the susceptible parent. This result clearly indicates the presence of a single dominant gene responsible for the resistance.

However, when line 214 was used as the resistant parent, the F2 generation of the cross GZ x 214 did not fit the expected ratio, while the reciprocal clearly fit the 3:1 resistant to susceptible ratio. This F2 family may have been mislabeled. It fits well to a 1R:1S ratio expected in a BC family.

In materials examined in this study, resistance to SLCV behaved as a dominant trait. The resistance is conferred by a single dominant gene, as results with line 264 would suggest. Perhaps a larger F2 population would provide a more accurate information if line 214 is used as the resistant parent.

Literature cited

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