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.
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