Powdery Mildew Resistance in the U.S. National Plant Germplasm System Cucumber Collection

Nine hundred and seventy-seven (977) cucumber (Cucumis sativus L.) accessions from the U.S. National Plant Germplasm System (NPGS) collection were tested for resis- tance to powdery mildew, caused by the fungus Podosphaera xanthii (Castagne) Braun and Shishkoff, formerly known as Sphaerotheca fuliginea (Schlecht.) Poll. Plants from each accession were evaluated in the greenhouse following inoculation with fi eld isolates of P. xanthii. Each plant was placed into one of three susceptibility classes based on the amount of fungal growth and sporulation on the hypocotyl, stem, petioles and leaves - susceptible (S), intermediate (I) or resistant (R). Of the 977 accessions (9.6%), 94 contained at least one I or R-type plant. Seventeen of the 20 most-resistant accessions came from Asian sources, including China (PIs 418962, 418964, 432860, and 432870), India (PIs 197085, 197088, and 605930), Japan (PIs 279465, 288238, 390258, and 390266), Pakistan (PI 330628), the Philip- pines (PIs 426169 and 426170), and Taiwan (PIs 321006, 321009, and 321011). A quantitative study was conducted to compare mildew reproduction on S, I, and R-type cucumbers in the greenhouse and under fi eld conditions in Ames, Iowa. Leaf disks were removed weekly and microscopic counts made of spore populations. The leaf disk method was superior to visual rating for ranking and differentiating intermediate from resistant accessions. Both the intermediate (moderately resistant) and highly resistant accessions provided excellent protection against powdery mildew in the fi eld. Powdery mildew is one of the world's most widespread and damaging diseases of green- house and fi eld-grown cucumbers (Cucumis sativus L.). The most frequently cited causal agents are Golovinomyces cichoracearum Gelyuta (syn. Erysiphe cichoracearum DC) and Podosphaera xanthii (Castagne) Braun and Shishkoff (syn. Sphaerotheca fuliginea (Schlecht.) Poll. ). In early reports (Barnes and Epps, 1956; Whitaker and Davis, 1962), G. cichoracearum was identifi ed as the primary causal agent in the United States, but more recent studies indicate that P. xanthii is the more common pathogen in the U.S. (Ballantyne, 1975; McCreight et al., 1987). Both species were observed on Cucumis melo L. in France, with P. xanthii isolated alone in 79% of the cases, G. cichoracearum isolated alone in 18% of the cases, and the remainder a mixture of both fungi (Pitrat et al., 1998). The primary causal agent in the U.S. may have shifted toward P. xanthii over time, but there were probable misidentifi cations in some of the older literature. The two species can be diffi cult to differentiate in the absence of the perfect stage, but useful identifi cation methods are now available. Podosphaera xanthii can be distinguished from G. cichoracearum by the presence of well-defi ned fi brosin bodies in the conidia and the occasional production of forked germination tubes (Boesewinkel, 1980; Kable and Ballantyne, 1963). Fibrosin bodies are visible with a light microscope, appearing as conical, disc and rod-shaped structures when the conidia are mounted in 3% aqueous KOH (Kable and Ballantyne, 1963). No races of P. xanthii have been reported from cucumber, although multiple races have been reported from muskmelon (C. melo) (Hosoya et al., 2000; Mc- Creight et al., 1987; Pitrat et al., 1998). Disease resistance from introduced germ- plasm has long been a valuable tool for control- ling powdery mildew in cucumber. Much of the early breeding work in the U.S. and sources of resistance was summarized by Peterson (1975) and was recently updated by Jahn et al. (2002). Two of the early examples of cucumber powdery mildew resistance from introduced germplasm