Squash leaf curl (SLC) is a virus disease of squash transmitted by the sweetpotato whitefly [Bernisia tabaci (Germ.)]. 'Cucurbita maxima Duch. ex Lam., C. mixta Pang, and C. pepo L. cultivars and the wild taxon. C. texana Gray exhibited severe symptoms in response to SLC in greenhouse and field tests. Symptoms on C. moschata (Duch.) Duch. ex Poir. cultivars were much more severe in greenhouse tests than in field tests. Three wild species, C. ecuadorensis Cutler and Whitaker, C. lundelliana Bailey, and C. martinezii Bailey, were virtually immune in greenhouse tests, but were infected in field tests. Cucurbita foetidissima HBK expressed moderate symptoms in a field test. Benincasa hispida (Thunb.) Cogn., C. ficifolia Bouche, Lagenaria siceraria (Mol.) Standl., Luffa acutangula (L.) Roxb., Luffs aegyptiaca Mill., and Luffs graveolens Roxb. were resistant to SLC in greenhouse and field tests.
When the melon aphid, Aphis gossypii Glover, was collected from 3 field host species at 3 different localities and then reared on 3 different laboratory host plants, their larviposition was the greatest when the field host, maintenance, and the test plants were the same species. Those collected from Cucurbita sp. and subcultured on either Cucumis melo L. ‘PMR 45’ muskmelon, Citrullus lanatus (Thunb.) Matsum and Nakai ‘Crimson Sweet’ watermelon, or Cucurbita pepo L. ‘Small Sugar’ pumpkin, initially larviposited less when compared to the stock cultured on ‘PMR 45’ for 8 years. After 6 months of acclimation, those reared on either watermelon or muskmelon larviposited equally as well as the stock culture. Those reared on pumpkin, however, did not appear to acclimate. Larviposition of aphids cultured on muskmelon, watermelon, and pumpkin on 5 common test plants suggest watermelon the most acceptable as a transition host. Regardless of aphid source or rearing host, reproduction was lowest on progeny 91731 selected from our aphid-resistance source PI 414723, which was used to develop 3 released aphid-resistant breeding lines of muskmelon.
Lettuce (Lactuca sativa L.) cultivars and breeding lines differed in tolerance to lettuce infectious yellows virus. Correlations of symptom severity in field tests with fresh weight of control plants in a greenhouse test indicate a relationship between symptom severity and the inherent vigor of a cultivar. ‘Climax’ had the mildest symptoms and was the most vigorous cultivar. The level of tolerance exhibited by ‘Climax’ and a few other cultivars will not prevent occurrence of lettuce infectious yellows, but could ameliorate its effect on lettuce production.
The spread of watermelon mosaic virus by the melon aphid (Aphis gossypii Glover) was 31%, 74%, and 71% less to a melon aphid-resistant muskmelon (Cucumis melo L.) breeding line than to the susceptible recurrent parent in a field cage study. Aphid-resistant and susceptible plants served equally well as the virus source. The highest rate of infection (97.9%) was noted when target plants were all melon-aphid susceptible, least (26.7%) when the target plants were all melon-aphid resistant, and intermediate (69.4%) when the target plants were an equal mix of aphid-resistant and susceptible plants. The number of viruliferous aphids per plant required to cause a 50% infection varied from five to 20 on susceptible controls and from 60 to possibly more than 400 on a range of melon aphid-resistant populations. An F family from a cross of the melon aphid-resistant AR Topmark (AR TM) with the susceptible `PMR 45' had significantly less resistance to virus transmission than AR TM. Breeding line AR 5 (an aphid-resistant population with `PMR 5' as the recurrent parent) had significantly greater resistance to transmission than other aphid-resistant populations.
Seven previously undescribed genes in muskmelon (Cucumis melo L.) for resistance to powdery mildew [Sphaerotheca fuliginea (Schlecht. ex Fr.) Poll., races 1 and 2] are reported. Progeny 92417 has a recessive gene for resistance to race 1, which is nonallelic to Pm-1. Breeding line WMR 29 has a gene for resistance to race 1 that is allelic to the recessive gene in 92417, but it is not known whether the two genes are identical alleles. Plant introduction 414723, 92417, and WMR 29 differentiated isolates of S. fuliginea race 2 at Montfavet, France, and Riverside, Calif. Comparative responses of F1, F2, and BC progenies from crosses involving 92417, PI 414723, and WMR 29 revealed six new genes for resistance to race 2. Genetic relationships among these seven genes are not fully known. Allelic and linkage relationships of these seven genes with the five previously known genes for powdery mildew resistance are also unknown.