Cucurbit Genetics Cooperative Report 5:46-47 (article 23) 1982
R. Provvidenti
New York State Agricultural Experiment Station, Cornell University, Geneva, New York 14456
Sources of resistance to cucumber mosaic virus (CMV) and watermelon mosaic virus 2 (WMV-2) , and tolerance to watermelon mosaic virus 1 (WMV-1) were recently found in three foreign accessions of Cucurbita maxima.
Resistance to CMV. A single plant selection of PI 234608 (cv. Queensland Blue from Johannesburg, South Africa) appears to posses an adequate level of resistance to isolates of this virus. Following foliar inoculation, plants reacted with chlorotic spots which turned necrotic, involving the entire leaf area. The virus, however, failed to move systematically and this condition persisted after 3 to 6 subsequent inoculations. Under field conditions, the progeny of 234608-1 remained unaffected by CMV, whereas most of the plants of other cultivars of C. maxima developed mosaic and foliar distortion. However, during late cotyledonary stage tended to develop severe mosaic and stunting. This shift toward susceptibility could be attributed primarily to reduced light intensity and/or quality, since temperature was adequately controlled.
Resistance in ‘Queensland Blue’ to CMV had been reported by Greber (1) from Queensland, Australia, where this cultivar is the most commonly frown ‘pumpkin’. Two lines of it were recently obtained from Greber, but they yielded mostly susceptible plants when inoculated with our isolates of CMV. Those few plants which appeared to be free of systemic infection segregated for susceptible and resistant individuals in the next generation. In Queensland, this cultivar is affected by WMV-1 and WMV-2 (1). Our field and greenhouse tests have confirmed its susceptibility to these two viruses and to others such as squash mosaic virus (SqMV) and tomato ringspot virus (TmRSV), which occurs in New York State.
Resistance of plants of 234608-1 to CMV requires further field evaluation, particularly under the pressure of severe CMV epidemics.
‘Queensland Blue’ is a very productive cultivar with round and ribbed fruits which have gray-blue skin and thick, dark orange flesh.
Tolerance to WMV-1. A single plant selection of the cultivar Zapallito Redondo, from Uruguay, has been determined to have a good level of tolerance to isolates of this virus from New York, Florida, Virginia, and Hawaii. Inoculated plants responded with scattered, small, chlorotic spots involving 2 to 5 leaves; subsequent growth was free of symptoms. Plants remained vigorous and productive. This selection of ‘Zapallito Redondo’ (ZR-1) is susceptible to CMV, WMV-2, SqMV, and TmRSV.
Plants of ZR-1 have bush habit; fruits are small (about 15 cm in diameter) with green skin and yellow flesh.
Resistance to WMV-2. A single plant of PI 419081 (cv. Pai Yu) (White Jade) from China was selected because it was free of symptoms when an epidemic of WMV-2 affected all other plants of domestic and foreign accessions of C. maxima at the Northeast Regional Plant Introduction Station, Geneva, NY, in 1978.
In field trials in subsequent years, the progeny of 410908-1 has remained free of symptoms of WMV-2. Similar results have been obtained in greenhouse tests using isolates of this virus from New York Florida, California, and China. However, recovery tests have revealed a low level of symptomless infection confined to the inoculated leaves. In late autumn and early winter tests, plants of 419081-1 tended to develop scattered, systemic, chlorotic spots, some ring-like, on 1 to 4 leaves, with further growth free of symptoms.
Plants of 41908-1 have a vine habit and produce medium to large fruits with white skin and light orange flesh. No resistance to other viruses was found in this selection or in the parent, PI 4109081.
During the summer of 1981, plants of 234608-1, ZR-1, and 419081-1 were crossed with each other and with those of CVS. Buttercup and Emerald, both accessions of C. maxima. The F1 and relative parents will be evaluated under greenhouse and field conditions in 1982.
Although resistance to viruses had been found in feral Cucurbita spp. (2), our findings provide additional sources of resistance to CMV and WMV-2, and tolerance to WMV-1 in cultivated accessions of C. maxima.
Literature Cited
- Greber, R.S. 1969. Viruses infecting cucurbits in Queensland. Qd. J. Agr. Anim. Sci. 26:145-171
- Provvidenti, R., R.W. Robinson, and H,M, Munger. 1978. Resistance in feral species to six viruses infecting Cucurbita. Plant Dis. Reptr. 62:326-329.