Severity of rust infection in soybean genotypes with partial resistance as a function of temperature and leaf wetness duration
Soybean cultivars with resistance against Asian soybean rust (ASR) are necessary to maintain plant yield when control methods become ineffective. We examined the influence of temperature (18, 21, 24, 27 and 30°C) and leaf wetness duration (0, 6, 12, 24 and 48 hours) on the penetration and temporal progress of ASR in soybean genotypes with different levels of partial resistance from the Federal University of Uberlandia breeding program. The genotypes were selected in the field from 100 progenies during 15 years under ASR epidemic conditions. Information on inheritance and molecular markers for disease resistance was available for our study. There were significant differences (P=0.001) in the area under the disease progress curve (AUDPC) between genotypes under the interaction of the binomial (temperature x leaf wetness duration - LWD) and for incubation period at different temperatures. No signs or symptoms of rust appeared at temperatures below18°C or above 30°C. The relationship between temperature and LWD affect the AUDPC and temperature affects incubation period. The highest AUDPC values occurred at 24°C and leaf wetting period of 24 hours, and the lowest values were achieved at temperatures above 27ºC. (Cultivar Desafio RR 8473 RSF) showed susceptibility to ASR.ASR lesions started to appear on average at least 15 days after infection in partially resistant genotypesF8 BRSGO Luziânia X Potenza, F8 BRSGO Caiapônia X Potenza, and F8 BRSGO Caiapônia X IAC100, which also had lower AUDPC and severity values, during the monocycle experiment. We conclude that ASR is shifting to a longer incubation period and fast sporulation induction after uredinia formation. These genotypes can be used in the field, with less need for fungicides helping avoid the emergence of new ASR pathotypes and races with low sensitivity to currently used fungicides.