ARS | Publication request: POWDERY MILDEW RESISTANCE IN THE U.S. NATIONAL PLANT GERMPLASM SYSTEM CUCUMBER COLLECTION

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 16, 2004
Publication Date: April 1, 2005
Citation: Block, C.C., Reitsma, K.R. 2005. Powdery mildew resistance in the U.S. National Plant Germplasm System cucumber collection. Hortscience. 40:416-420.

Interpretive Summary: Powdery mildew is one of the world's most widespread and damaging cucumber diseases and is caused by a fungus named Podosphaera xanthii. The mildew fungus sends root-like penetrations through the leaf surface, drawing its nutrition from the leaf interior. Susceptible plants allow extensive mildew growth and the leaves eventually turn yellow and die. Resistant plants are able to prevent or greatly limit growth of the fungus. We tested a large collection of cucumbers, 977 accessions in all, from around the world, to find types that were naturally resistant to the disease. The collection is maintained at the North Central Regional Plant Introduction Station at Ames, IA as part of the U.S. National Plant Germplasm System (NPGS). Individual plants from each accession were classified into one of three susceptibility groups based on the amount of fungal growth on the leaves and stems - susceptible (S), intermediate (I) or resistant (R). Ninety-four of the 977 accessions (9.6%) had at least one I or R-type plant. Seventeen of the 20 most-resistant accessions came from Asian countries, including China (PIs 418962, 418964, 432860, and 432870), India (PIs 197085, 197086, and 605930), Japan (PIs 279465, 288238, 390258, and 390266), Pakistan (PI 330628), the Philippines (PIs 426169 and 426170), and Taiwan (PIs 321006, 321009, and 321011). Both the intermediate (moderately resistant) and highly resistant accessions provided very good protection against powdery mildew in the field. This report will be of interest to cucumber breeders and researchers throughout the world, both for finding potentially useful sources of resistance in the U.S. NPGS collection, and for defining methods to quantify powdery mildew resistance.

Technical Abstract: Nine hundred and seventy-seven (977) cucumber (Cucumis sativus L.) accessions from the U.S. National Plant Germplasm System (NPGS) collection were tested for resistance 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 field 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). Ninety-four of the 977 accessions (9.6%) 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, 197086, and 605930), Japan (PIs 279465, 288238, 390258, and 390266), Pakistan (PI 330628), the Philippines (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 field 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 and resistant accessions. Both the intermediate (moderately resistant) and highly resistant accessions provided excellent protection against powdery mildew in the field.