Development of Disease-resistant Cucurbit Crops

Claude E. Thomas

U.S. Department of Agriculture, Agricultural Research Service, U.S. Vegetable Laboratory,
Charleston, SC 29414

Additional index words. germplasm, evaluation, enhancement, powdery mildew, downy mildew, anthracnose, fusarium wilt

Abstract. Resistance to diseases has long been the focus of cucurbit breeding programs. Physiological specialization of fungal pathogens will continue to be a problem for powdery mildew, downy mildew, anthracnose, and fusarium wilt.


Literature cited

Armstrong, G.M. and J.K. Armstrong. 1978. Formae speciales and races of Fusarium oxysporum causing wilts of the Cucurbitaceae. Phytopathology 68:19­28.

Bardin, M. 1996. Diversite phenotypique et genetique des oidiums des cucurbitacees, Sphaerotheca fuliginea et Erysiphe cichoracearum. Thesis, L'Universite Claude Bernard-Lyon I.

Goode, M.G. 1958. Physiological specialization in Colletotrichum lagenarium. Phytopathology 48:79­83.

Iwata, Y. 1942. Host plants of Pseudoperonospora cubensis (Berk. et Curt.) Rostow. Ann. Phytopathol. Soc. Jpn. 12:48­50.

Jagger, I.C. and G.W. Scott. 1937. Development of powdery mildew resistant cantaloup. No. 45. USDA Circ. 441.

Jagger, I.C. and G.W. Scott. 1938. A new biologic form of powdery mildew on muskmelons in the Imperial Valley of California. Plant Dis. Rptr. 22:275­276.

Jenkins, S.F., N.N. Winstead, and L.C. McCombs. 1964. Pathogenic comparison of three new and four previously described races of Glomerella angulata var. orbiculare. Plant Dis. Rptr. 48:619­623.

Martyn, R.D. 1996. Fusarium wilts, p. 13­17. In: T.A. Zitter, D.L. Hopkins, and C.E. Thomas (eds.). Compendium of cucurbit diseases. APS Press, St. Paul, Minn.

Palti, J. 1974. The significance of pronounced divergencies in the distribution of Pseudoperonospora cubensis on its crop hosts. Phytoparasitica 2:109­115.

Thomas, C.E. 1978. A new biologic race of powdery mildew of cantaloups. Plant Dis. Rptr. 62:223.

Thomas, C.E., T. Inaba, and Y. Cohen. 1987. Physiological specialization in Pseudoperonospora cubensis. Phytopathology 77:1621­1624.

Wasilwa, L.A., J.C. Correll, T.E. Morelock, and R.E. McNew. 1993. Reexamination of races of the cucurbit anthracnose pathogen Colletotrichum orbiculare. Phytopathology 83:1190­1198.

The development of disease-resistant cucurbit crops has been the focus of many germplasm evaluation and enhancement studies. This presentation especially emphasizes some of these studies that have dealt with the recognition of physiological specialization within fungal pathogen species that attack cucurbit crops. This recognition has been basic to and has driven many efforts to identify sources of resistance and to incorporate this resistance into cucurbit crops.

Jagger and Scott (1937) developed 'PMR 45' cantaloupe with resistance to powdery mildew (Sphaerotheca fuliginea). Race 2 soon appeared and so began the long saga of developing melons with resistance to this pathogen (Jagger and Scott, 1938). Since that early work, an additional race has been described on melon in the U.S. (Thomas, 1978) and three additional races in France (Bardin, 1996). All of the races identified have been based on the use of differential melon genotypes.

Iwata (1942) recognized the host specificities exhibited by different isolates of the downy mildew pathogen (Pseudoperonospora cubensis). Palti (1974) further analyzed the significance of divergence of the pathogen on various cucurbit hosts. Thomas et al. (1987) described five pathotypes of P. cubensis based on host compatibilities between isolates and different cucurbit species and subspecies.

Goode (1958) demonstrated three races of anthracnose (Colletotrichum orbiculare) based on virulence of pathogen isolates against cucumber, watermelon, and butternut squash. Jenkins et al. (1964) noted four additional races based on virulence against additional cucurbit host genotypes. Using vegetative compatibility studies in conjunction with virulence tests on different cucurbit hosts, Wasilwa et al. (1993) distinguished three races of the pathogen from a wide array of tested isolates.

Armstrong and Armstrong (1978) distinguished five forma speciales of the fusarium wilt pathogen (Fusarium oxysporum) on the Cucurbitaceae. Three races of F. oxysporum f. sp. niveum; four races of F. oxysporum f.sp. melonis; and three races of F. oxysporum f. sp. cucumerinum are recognized attacking watermelon, melon, and cucumber, respectively (Martyn, 1996).

Cucurbitaceae '98