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.
Carrot (Daucus carota L.) seed germination may be erratic or reduced under high temperatures (above 35 °C). Even in tropical genotypes (tolerant to high temperatures during crop development), the negative effects of high temperatures on carrot stand establishment have been observed, especially during summer. The objectives of this study were to characterize commercial carrot cultivars and accession lines for their ability to germinate at high temperature and determine the ethylene production during imbibition at high temperature. Seeds from 34 commercial cultivars and 125 carrot accessions from the North Central Regional Plant Introduction Station were germinated at 25 °C (optimal) and 35 ± 0.5 °C (high) in constant light. Ethylene production during seed imbibition at high temperature was evaluated in some genotypes. Many of the commercial cultivars had reduced germination at 35 °C. ‘XPC-3617’, ‘Alvorada’, ‘Brasilia’, and ‘Esplanada’ had the greatest germination at 35 °C. A greater number of accessions germinated at 35 °C than did the commercial genotypes. The accession PI 319858 germinated 95% at both temperatures and was considered thermotolerant. Six accessions (Ames 7665, Ames 7698, Ames 25031, PI 167082, PI 294637, and PI 319858) germinated above 80% at 35 °C and were also identified as potential sources of thermotolerance. Fifteen other accessions (Ames 7694, Ames 25031, Ames 25036, Ames 25049, Ames 25705, PI 167082, PI 179687, PI 180834, PI 261782, PI 269486, PI 273658, PI 277710, PI 288242, PI 294637, and PI 319858) had thermotolerance ratios of T35/T25 0.85 or greater (where T35 = germination at 35 °C and T25 = germination at 25 °C) and were identified for further testing. The identified thermotolerant genotypes might be useful for carrot seed germination mechanism studies as well as for breeding programs. Ethylene production during seed germination at high temperature was greater in thermotolerant genotypes than in thermosensitive genotypes. High correlations were observed between first germination count at 35 °C and ethylene production, total germination at 35 °C and ethylene production, and thermotolerance ratio and ethylene production.