Effects of different dew periods and temperatures on infection of Senna obtusifolia by a Brazilian isolate of Alternaria cassiae

Senna obtusifolia (Syn. Cassia obtusifolia; Fabaceae), commonly known as sicklepod, is a problematic weed in the USA and tropical regions of Africa, Asia, and South America. It is very common in Brazilian agricultural lands and causes major problems in west-central soybean production areas. The fungus Alternaria cassiae was discovered in 1992 in a disease survey of various regions of Brazil. The objective of this research was to study the effects of different dew durations and temperatures on infection of S. obtusifolia by a Brazilian isolate of A. cassiae. Sicklepod plants were inoculated with conidia of A. cassiae and incubated at 25 °C and different dew durations (4, 6, 8, 12, 24, or 48 h). The disease incidence (number of dead leaves) was greater on plants submitted to 24- and 48-h dew periods. Plants that were not submitted to a dew period did not show any disease symptoms. Dry-matter accumulation in fungus-treated sicklepod plants decreased as the dew duration increased. The combined effects of dew duration and temperature were evaluated by submitting A. cassiae-inoculated sicklepod plants to five different temperatures (17, 20, 23, 26, or 30 °C) and three dew durations (8, 12, or 16 h). Under the lower temperatures (17, 20, and 23 °C), disease incidence was greater at 8 and 12 h of dew compared to the higher temperatures (26 and 30 °C). When plants were incubated at 16 h of dew, the different incubation temperatures did not influence disease incidence, with the exception of 30 °C. Inoculated sicklepod plants accumulated greater dry matter under higher temperatures. A. cassiae requires lower temperatures and dew durations greater than 12 h for maximum expression of its biological control effects on sicklepod.