Cucurbit Genetics Cooperative Report 10:56-57 (article 30) 1987

The Search for Sources of Resistance to Squash Mosaic Virus In Melon: A Preliminary Report

R. Provvidenti

Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456

M. A. Humphreys

Vegetable Research Department, Harris Moran Seed Company, Rochester, NY 14624

In nature, squash mosaic virus (SqMV) is confined to Cucurbitaceae. Squash (Cucurbita spp.) and melons (Cucumis melo) are particularly affected and most of the outbreaks of this virus can be traced to their infected seeds. Seed transmission may range from 1 to 10%, but a much higher percentage has been also recorded (1,5,7). From plant to plant, SqMV is efficiently spread by the striped and spotted cucumber beetles (Acalymma spp. and Diabrotica spp.), or less frequently by contact (2).

Through the years, seed producers have succeeded in limiting the presence of SqMV from their seed stocks, but lately, some well known melon cultivars have been found to be carriers, with alarming consequences. Thus, the development of resistant cultivars would be of great benefit to the seed industry.

Since all domestic and foreign cultivars are susceptible to SqMV, a major effort is under way to locate sources of resistance in foreign introductions, primitive cultivars or land races of C. melo and its botanical varieties. Resistance is being sought against the two known pathotypes, the melon strain (SqMV-I) and the squash strain (SqMV-II) (3). Both occur in melons, and can be differentiated by host reaction and serology (3). Generally, SqMV-I causes more severe symptoms in melons than the SqMV-II However, this distinction is not absolute, since some cultivars appear to react similarly to both pathotypes.

Sixty accessions of C. melo from several areas of the world have already been screened with both pathotypes, using from 15 to 20 plants of each line for each strain. None has been found resistant or tolerant to the melon strain, but a few lines have yielded plants tolerant to the squash strain. This tolerance is expressed as a mild foliar mottle with limited plant stunting. Particularly promising appears PI 157080 (China), which was previously reported to be tolerant to an isolate of SqMV (9).

The results with the melon strain have been disappointing, but the search for resistance to both pathotypes is continuing. The plant introduction collection of C. melo is very rich in numbers and in genetic diversity, hence, it may eventually reveal valuable SqMV-resistant germplasm. In breeding for resistance or tolerance, any genetic factor(s) that will preclude seed transmission and prevent or minimize systemic infection of the virus can be considered very useful.

Several years ago, Provvidenti and Robinson (6) reported resistance to SqMV in Cucumis metuliferus, commonly known as 'jelly melon' or 'horned cucumber'. This feral species reacts to both pathotypes of the virus with chlorotic lesions on inoculated leaves, but is systemically resistant (6). Both C. metuliferus and C. melo possess the same number of chromosomes (n=12) (5), but it is very difficult to obtain interspecific crosses. The only successful attempt was reported by Norton, but progenies of that cross (4) have not been made available. Thus, it is not known whether they still retain the resistance to SqMV and watermelon mosaic virus 1 possessed by their wild parent (6).

Literature Cited

1. Alvarez, M., and R. N. Campbell. 1978. Transmission and distribution of squash mosaic virus in seeds of cantaloupe. Phytopathology 68:257-263.
2. Campbell, R. N. 1971. Squash mosaic virus, in Descriptions of Plant Viruses No. 43. C.M.I./A.A.B. Kew, Surrey, England.
3. Nelson, M. R., and H. K. Knuhtsen. 1973. Squash mosaic virus variability: Review and serological comparisons of six biotypes. Phytopathology 63:920- 926.
4. Norton, J. D., and D. M. Granberry. 1980. Characteristics of progeny from an interspecific cross of Cucumis melo with C. metuliferus. J. Amer. Soc. Hort. Sci. 105:174-180.
5. Powell, C. C., and D. E. Schlegel. 1970. Factors influencing seed transmission of squash mosaic virus in cantaloupe. Phytopathology 60:1466- 1469.
6. Provvidenti, R., and R. W. Robinson. 1974. Resistance to squash mosaic virus and water-melon mosaic virus 1 in Cucumis metuliferus. Plant Dis. Rep. 58:735-738.
7. Rader, W. E., F. H. Fitzpatrick, and E. M. Hildebrand. 1947. A seedborne virus of muskmelon. Phytopathology 37:809-816.
8. Shimotsuma, M. 1965. Chromosome studies of some Cucumis species. Seiken Ziho. 17:11-16.
9. Webb, R. E., and G. W. Bohn. 1962. Resistance to cucurbit viruses in Cucumis melo. Phytopathology 52:1221.