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  • Author or Editor: Charles C. Block x
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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). 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 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 from resistant accessions. Both the intermediate (moderately resistant) and highly resistant accessions provided excellent protection against powdery mildew in the field.

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Bacterial fruit blotch (BFB) caused by the bacterium Acidovorax avenae subsp. citrulli (Aac) is a seed-borne disease that threatens most cucurbit crops. Although limited resistance has been found in a small number of Plant introductions (PIs) in watermelon (Citrullus spp.), there are no reports of high levels of resistance in germplasm lines of Cucumis spp. In this study, 332 Cucumis spp. PIs were screened for resistance to Aac using a newly developed seed vacuum–infusion assay. Significant differences in the reaction of the PI to BFB were observed. The majority of lines were found to be extremely susceptible to the disease. However, several PIs with lower levels of resistance were also identified. Variability in the reaction of plants within each PI was also observed. Of the 332 PI tested, 16 were selected for additional evaluation using a standard spray inoculation tests. PI 353814, PI 381171, PI 536573, and PI 614401, all belonging to C. melo, and PI 504558 (C. ficifolius) were found to have significantly greater levels of resistance than susceptible control cultivars or other PIs in two independent spray inoculation tests. Germplasm lines developed from these PIs may be useful as sources of resistance to BFB in Cucumis breeding programs.

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