Thursday, May 22, 2014

Diseases of Apple: Disease Cycle of Premature Leaf fall

This is in continuation to my previous post on Diseases of apple: Premature Leaf Fall. In today’s post we are going to discuss about the Disease cycle and predisposing factors of Premature Leaf Fall.

Disease cycle

The fungus overwinters in the form of apothecia on fallen leaves. Primary infection is caused by ascospores produced on the overwintered leaves. The ascospores discharge begins during spring season but causes infection at a particular leaf age stage. The disease first attacks plants growing in shady and high moisture conditions

Air temperature, light intensity and soil moisture are the predisposing factors to trigger the disease outbreak, however, severity index increases with the maturity of the leaves.  The disease is severe when a cool, rainy period of 4 to 5 days proceeds several hot sunny days in the orchard. The inoculum of the disease get released at the pink bud stage but it causes the infection only at a particular leaf maturity stage when plant physiological processes start producing ethylene, the ripening / ageing hormone. The pathogen therewith the hormonal changes induce yellowing and abscission of the leaves and defoliation occurs.

The scientists have developed disease cycle key on the rate of infection conditions for this new disease on apple plant to monitor it. It therefore concludes that the blotch started its symptoms on the upper side of the leaf first in the 2nd week of June irrespective of the weather conditions. However, the disease symptom on leaves followed by temperature above 20oC and relative humidity above 70 per cent continuously for 5-6 days, induce yellowing and defoliation process. Prolonged hot and dry weather affect the primary infection and disease conditions adversely. Under favourable weather conditions the leaves start defoliation within 3-5 days whereas under interrupted/un-favourable conditions the infected leaves could retain up to 3 months.

In coming post we shall be discussing about the disease monitoring system for farmers.