The occurrence of favourable environmental conditions can be used to predict infection, but these can overestimate the real risk of disease establishment in crops. Detection tests for ringspot and dark leaf spot in air samples around brassicas have been developed and are useful for assessing risk. HDC project FV 305 investigated if similar tests would be possible for white blister and to give a better understanding of the transmission of the disease under field conditions. White blister fungus produces both water-borne zoospores, which are the main infective agent, and airborne spores (zoosporangia).
The purpose of the project was to demonstrate if these zoosporangia could transmit the disease over wide areas under field conditions. The study confirmed that these relatively large spores are present in detectable quantities in air samples. Numbers of zoosporangia present in spore trap catches varied over the season; however, peaks of more than 1,000 spores could be detected in the air during the early growing period. The numbers were related to when pustules emerged on the leaves of the crop. Numbers declined during September and October as temperature conditions became unfavourable for further white blister infection.
The present study has shown that white blister spores (zoosporangia) are commonly found in the air around infected vegetable brassica crops. White blister was transmitted over a distance of 500m. White blister spores survive in the crop under prevailing temperature conditions for at least 28 days.