This post is in continuation to my previous post on “Apple Diseases: Apple Scab Disease Cycle”. Here we are going to discuss about the climatic conditions that help the pathogen to over winter and produce primary inoculum for the primary infection thus help us in predicting the disease in advance.
Scab weather monitoring and forecasting:
The scab fungus remained alive through winter in the dead and fallen leaves of apple on the orchard floor, and produced pseudothecia mainly in leaves which fell in September, October and November as the leaves of earlier dates did not survive in the hot autumn days and got decomposed quickly. Cool and moist climatic conditions due to rain or plenty of showers, available at higher elevation and in shady portion of the orchards were congenial for the development of pseudothecia in overwintering leaves within 30-35 days after leaf fall. When moisture was not a limiting factor (12-13 hr) and the tissues of fallen leaves had remained pliable, fungus started producing initials of pseudothecia at low temperature of 4 and 8oC but such pseudothecia which developed slowly throughout the winter season attained full size and became mature at an optimum temperature of 15oC. On the other hand, prolonged dry weather conditions in the winter and spring did not allow normal development of pseudothecia and a month or so delayed even the ascospore maturity.
After the maturation of ascospores on the orchard floor, a light to moderate shower of rain is required to release the inoculum over to susceptible plant stages. This infection came from the late maturing ascospores at high elevation areas where overall development is always slow. Thus, it explains that low temperature due to snowfall is lethal to the matured ascospores and affects the infection process.
In coming post we shall be discussing about the management of disease.