Effect of Temperature and Duration of Leaf Wetness on Two Disease Components of Alfalfa Rust

D.H. Webb^l, F.W. Nutter, Jr.^l, and D.R. Buxton^2
1Department of Plant Pathology, ²USDA-ARS and Department o f Agronomy
Iowa State University, Ames, IA 50011

The Standard Tests used to Characterize Alfalfa (Medicago sativa L.) Cultivars published by the North American Alfalfa Improvement Conference recommends a constant temperature of 25 C for all phases of the alfalfa rust (Uromyces striatus Shroet.) monocycle when testing alfalfa genotypes for resistance to this pathogen. Following inoculation, a 24-hour leaf wetness period under darkness at 25 C has been recommended to facilitate the infection process (germination and penetration). During the latent and infectious periods of a monocycle, it is recommended that plants be maintained at 25 C with a 16 hour photoperiod. While the methods recommended in the Standard Tests have proven useful in differentiating the resistance and susceptibility of alfalfa genotypes based on the size and number of pustules, further information is warranted on the environmental conditions necessary to optimize individual disease components such as infection efficiency and latent period. Therefore, the objectives of this study were to:

(i) quantify the effects of leaf wetness duration and temperature on infection efficiency and
(ii) quantify the effect of temperature during the latent period of infection efficiency, latent period and the rate of pustule appearance.

Alfalfa plants were inoculated with urediospores of Uromyces striatus and then subjected to leaf wetness durations of 4, 8, 12, 16, 24 or 32 hours at 20 C. There was a significant positive, linear relationship between hours of leaf wetness and infection efficiency (pustules/leaf) from 4 to 24 hours (P<0.001, r² = 0.96). To determine the effect of temperature during a 24-hour period of leaf wetness following inoculation, inoculated plants were subjected to temperatures of 17, 19, 22, 25 or 28 C. There was a significant negative and linear relationship between temperatures during the leaf wetness period and infection efficiency (P<0.001, r² = 0.98). Infection efficiency was more than twenty times higher at 17 C than at 28 C. Constant temperatures of 15, 18, 21, 24, 27, or 30 C after the initial 24-hour leaf wetness period did not effect infection efficiency (pustules/cm²), but did effect latent period (the time from inoculation to the time when 50% of the pustules were visible). There was a significant negative and linear relationship (P < 0.001, r² = 0.94) between temperature and latent period (ln T₅₀). Thus, temperature during the initial 24-hour leaf wetness period is more critical and has a greater impact on infection efficiency than post-infection temperatures. The rate of pustule appearance increased as temperature increased and the rate of pustule appearance (as affected by temperature) was best described by the Gompertz population growth model.

1995 Central Alfalfa Improvement Conference Proceedings