Conditions for Infection of Apple by Phytophthora syringae

The processes leading to Phytophthora fruit rot were divided into two main stages for the purposes of investigating the effects of temperature and duration of wet periods on pathogen development: oospore germination and infection of fruit by zoospores. It was found that the first stage was markedly affected by temperature over the range 10–20°C and required a wet period of 4–7 days. At 18 and 20°C, activation was low regardless of the length of the wet period. Once oospore germination (first stage) had occurred, free water was necessary for only a few hours for fruit infection (second stage) to occur, but the incidence of infection rose rapidly over the first 48 h, regardless of temperature over the range 10–20°C. From the data obtained, mathematical models were produced relating the incidence of Phytophthora fruit rot to the two weather variables. These models can be used to develop a weather‐based risk assessment system for the disease.