Influence of fluctuating temperatures and interrupted periods of plant surface wetness on infection of bean leaves by ascospores of Sclerotinia sclerotiorum

Summary In artificial inoculations of bean leaves (Phaseolus vulgaris), the minimum period of plant surface wetness (PSW) required for the start of infection was temperature dependent. Infection was most rapid at 20–25oC and no infection occurred at 5oC or 27oC. In experiments conducted in a glasshouse or controlled environment chamber, the influence of fluctuating temperatures was taken into account by calculating degree hours (oh) of PSW. A continuous period of PSW was not necessary for successful infection. Successive short periods of PSW were not strictly additive. A dry period following a wet period delayed the start of infection for longer than the duration of the dry period. The length of the delay depended on the preceding oh PSW and on the duration of the dry period. From this information, a model was constructed to predict when first infections could be expected. This model was tested in small plot field experiments in two seasons. The date on which infection was first noticed coincided closely with the date on which oh PSW first exceeded 1200. The use of this model in disease control is discussed.