|Cadle Davidson, Molly|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2011
Publication Date: 12/21/2011
Citation: Ramming, D.W., Gabler, F., Smilanick, J.L., Cadle Davidson, M., Paola, B., Siraprapa, M., Omer, F., Milgroom, M.G., Cadle Davidson, L.E. 2011. Identification of race-specific resistance in North American Vitis species limiting Erysiphe necator hyphal growth. Phytopathology. 102(1):83-93. Interpretive Summary: Powdery mildew is the most important fungal disease of grapes worldwide. In 2008, 10.2 million pounds of sulfur were applied to table and raisin grapes in California. Powdery mildew has developed resistance to a number of other chemicals used for control of powdery mildew. The development of table and raisin grapes resistant to powdery mildew would be a great advantage to growers. The use of mildew resistant cultivars would reduce production costs and the amount of pesticides released in the atmosphere. Resistance to powdery mildew infection is found in a number of grapes species. Race specific resistance against powdery mildew was documented for the first time for two sources of powdery mildew resistance that had been introgressed into table and raisin grape germplasm. The resistance was characterized by programmed cell death of host epidermal cells and arrested or slowed fungal growth. This suggests that powdery mildew resistance mechanisms should be characterized and that multiple resistance genes with different mechanisms be combined when feasible to improve durability of resistance.
Technical Abstract: While race-specific resistance against powdery mildews is well documented in small grains, race specificity against grapevine powdery mildew (Erysiphe necator) is undocumented. In the current study, two sources of powdery mildew resistance introgressed into Vitis vinifera were evaluated in the greenhouse and laboratory: 1) complex hybrid ‘Bloodworth 81-107-11’ of at least V. rotundifolia, V. vinifera, V. berlandieri, V. rupestris, V. labrusca, and V. aestivalis background; and 2) Tamiami cultivar of V. aestivalis origin. Powdery mildew resistance was assessed across progeny of both populations using several pathogen sources in California and New York over multiple years. The site-specific resistance of some progeny suggested the segregation of multiple race-specific resistance genes, which were overcome individually, but the presence of multiple resistance genes appeared effective. Examination of inoculated grape leaves by light microscopy demonstrated that ‘Bloodworth 81-107-11’ and ‘Tamiami’ resistances introgressed into V. vinifera were characterized by slightly reduced penetration success and a programmed cell death of host epidermal cells under appressoria, which arrested or slowed hyphal growth. Low temperature scanning electron microscopy showed that on the leaves of a susceptible grapevine, secondary hyphae grew extensively, forming multiple appressoria at regular intervals along the hyphae, and fungal tissue remained turgid. In contrast, on leaves from both resistance sources, fungal development was often halted at or soon after appressorium formation and was accompanied by collapse of conidia, germ tubes, appressoria, and often secondary hyphae. Controlled inoculation of resistant and susceptible progeny with a diverse set of E. necator isolates demonstrated the presence of fungal races differentially interacting with race-specific resistance genes. This is the first study in grapevine showing the segregation of race-specific resistance to powdery mildew and suggests that grape breeders should characterize the mechanisms of resistance and pyramid multiple resistance genes with different mechanisms when feasible for improved durability.