Effect of Temperature, Dew Period, and Age of Leaves, Spores, and Source Pustules on Germination of Bean Rust Urediospores

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Abstract
Urediospores of Uromyces phaseoli were incubated on wet snap bean leaf disks resting on water agar in seed germination chambers. Minimal germination occurred at 10 and 25.degree. C; none occurred at 4 and 27.5.degree. C. Maximum germination, 93%, occurred within 17.5-22.5.degree. C. At 15-22.5.degree. C, 90% of all germinations occurred within the first 6-8 h of wetness. Spores stored 14-21 days at 21.degree. C in growth chambers showed little or no reduction in germinability. Spores from old leaves and old pustules germinated only 2/3 as well as those from young leaves and young pustules. Germinability after interrupted wetting periods was reduced for well-spaced spores and increased for clumped spores. Spores wetted for at least 2 h continued germination if relative humidity remained > 85%. Spores produced at 24-27.degree. C germinated about half as well as those produced at 16 or 21.degree. C. Equations describing temperature-dew period interactions were developed. Effects of all other treatments could be expressed as fractional responses of the maximum germination at a given temperature and dew period.