Cucurbit Genetics Cooperative Report 21:29-30 (article10) 1998
A Source of a High Level of Tolerance to Squash Mosaic Virus in a Melon from China
Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva, NY 14456
Squash mosaic virus (SqMV) is one of the major viral agents affecting melons (Cucumis melo L.). It is seed transmitted, hence it can be found in every area of the world where this cucurbit is cultivated. In nature, it is spread by striped and spotted cucumber beetles (Acalymna and Diabrotica spp.) (1). Strains of this virus have been classified into two pathotypes. Type I includes melon strains, which cause prominent symptoms on melons and mild symptoms on squashes. Type II includes squash strains, which incite prominent symptoms on squashes and mild symptoms on melons (3).
In the last 35 years, our efforts to locate a melon source of resistance to both pathotypes of SqMV in cultivars, plant introductions, and landraces have been mostly unsuccessful. Some landraces from India, Afghanistan and Pakistan appeared to possess a low level of tolerance (4). Some tolerance was found also in a chinese melon and a few landraces from the Indian subcontinent (7), but none were considered to be suitable for breeding programs. Four years ago we obtained seeds of a melon found in the Fujian Province of China in 1951 from a California gardener. For years, he grew it in his garden and sold some of the fruits in local Chinese market. He also claimed that it was very productive and resistant to diseases. After our seed increase, plants of this melon, designated 'China 51' (C. melo var. makuwa) were tested with each of the following cucurbit viruses: cucumber mosaic (CMV), papaya ringspot (PRSV), squash mosaic (SwMV), watermelon mosaic (WMV) and zucchini yellow mosaic (ZYMV). 'China 51" was found to be susceptible to PRSV, WMV and ZYMV, but it appeared to be uniformly resistant to CMV. A few plants failed to develop systemic symptoms with SqMV. Enzyme-linked immunosorbent assays (ELISA) and recovery tests confirmed the resistance to CMV. Plants that had remained symptomless with SqMV were found to be infected by the virus. The progenies of these few plants were highly tolerant to strains of SqMV belonging to both pathotypes. Considering that in melon, sources of resistance to CMV have been previously reported (2,3,6) our priority was directed to determine whether SqMV was seedborne in 'China 51', and if the high level of tolerance could be retained through breeding.
Individual 'China 51' plants were inoculated with one of four SqMV strains of which two belong to the melon pathotype and the others to the squash pathotype. Seeds from these plants were individually planted in sterilized clay pots containing pasteurized soil. The resulting plants (200 for each strain) remained symptomless, but were periodically assayed to detect any viral infection. All the ELISA tests which were conducted using tissue from the 2nd, 10th, and 20th leaves of each plant, were negative. Hence, it appeared that none of the four strains of SqMV was able to infect any of the embryos of 800 seeds.
For genetic studies, 'China 51' was crossed with 'Honeydew'. Our group of 20 plants after the resulting F1 ('China 51' x 'Honeydew') after two distinct inoculations with a melon pathotype (SqMV-NY93), developed persistent and prominent systemic foliar mosaic and plant stunting. Conversely, a second group of 20 F1 plants of the same cross, reacted with a mild systemic mottling when infected with a squash pathotype (SwMV-NY88). One hundred twenty-eight plants of the F2 ('China 51' x 'Honeydew') tested with the melon pathotype segregated in a ratio of 1 highly tolerant (30) to 3 susceptible (98). One hundred eighteen plants of the same F2 cross inoculated with the squash pathotype segregated in a ratio of 1 highly tolerant (28), 2 partially tolerant (58), and 1 susceptible (32). Thus, the high level of tolerance displayed by 'China 51' with the melon pathotype appeared to be inherited recessively, whereas for the squash pathotype, it appeared to be partially dominant. However, considering that the partially tolerant plants developed some fruit symptoms, the high level of tolerance possessed by 'China 51' should be classified as recessive. More genetic studies are contemplated, including backcross populations. Preliminary studies on the inheritance of resistance to CMV involving crosses of 'China 51' x 'Honeydew' indicate that the resistance to this virus is also recessive.
Of particular interest were the fruits of the F1 ('China 51' x 'Honeydew'), which externally, closely resembled those of the Honeydew, although the flesh was salmon, similar to 'China 51'.
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