A conducive day concept to explain the effect of low temperature on the development of Scleroderris shoot blight.

The apparent latent period in the disease cycle may result from the need for winter conditions before symptom development can occur. The occurrence of symptoms primarily on lower branches, and the restriction of the disease to latitudes that receive sustained snow cover in the Lake states are consistent with this observation. Comparison of literature descriptions of outbreaks of the disease to weather records revealed a strong association between conducive periods, usually single conducive winters, and the occurrence of symptoms. The natural range of the disease may be restricted by the need for recurrence of conducive periods within 3 yr to avoid breaking the disease cycle