The effects of temperature and wetness period on the development of spinach white rust

Experiments were conducted in controlled environments to determine the influence of temperature and duration of wetness on development of white rust of spinach. Plants of the susceptible cv. Kent were exposed to temperatures of 6 to 28degreesC and interrupted wetness periods that totaled 3 to 84 It following inoculation. Disease severity was assessed following further incubation in a greenhouse at 20 to 30degreesC. Disease was observed at all temperatures and increased with wetness duration. The optimum temperature for disease development ranged from 12 to 18degreesC. Only 3 It of wetness were required for disease development at 12 to 22degreesC. A minimum wetness period of 6 to 12 h was required for disease development at suboptimal temperatures. A multiple regression model describing the response surface of arcsine square root transformed disease severity was developed that had significant quadratic wetness effects, cubic temperature effects, and interaction between temperature and wetness. The resulting polynomial model provided a good fit to the observed data, accounting for 89% of the variation in transformed disease severity.