Leaf Blotch of Horse-Chestnut

Pest Management Fact Sheet #5094

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Introduction

Leaf blotch of horse chestnut is caused by the fungus Guignardia aesculi. This is a common disease which causes browning of the leaves especially during years with wet springs. It is usually not of concern to the health of the tree although young trees and nursery stock may suffer due to complete defoliation.

Environmental Conditions

During the spring, ascospores are released during wet weather. These spores require several hours of wetness in order to germinate and infect leaf tissue. Wet springs lead to a large number of initial infections. As these infections develop they will produce spores which cause additional infections during wet summer weather.

Symptoms

The initial leaf infections appear as water-soaked areas which discolor and eventually turn brown. These lesions may be small or large and may coalesce involving the entire leaf. Often the border of these lesions is yellow. The leaves may become curled and distorted and dry out and drop from the tree. Occasionally reddish spots may be observed on the petioles and fruit. This disease is readily distinguished from drought injury by the presence of the tiny black spore producing bodies within the infected tissues.

Survival and Dispersal

The fungus survives in infected leaf tissues throughout the winter. In the spring, the development of the sexual spore-producing structures (pseudothecia) is timed according to temperature so that the ascospores are mature when the new leaves are developing. When these young leaves become infected the fungus produces a different type of spore (conidia) which are splashed by the rain spreading from leaf to leaf causing additional infections. Where nursery stock is in close proximity it allows for the easy and rapid spread of the disease during wet weather.

Control

Once the leaves have been infected there is no cure. Control depends on prevention.

Rake and destroy infected leaves in the fall to remove the overwintering fungus from the area.
Because dense foliage retards drying of the leaves, nursery stock should not be planted close together.
Pruning of trees to obtain a more open growth habit will help decrease drying times.
At the time of bud break during wet springs, fungicides may be applied to protect developing foliage. Repeat at 7-10 day intervals depending on the weather. If the foliage has been successfully protected against the spring ascospores, no conidia will be produced to continue the infection process and further sprays may be unnecessary. Some fungicides which are recommended are: chlorothalonil (Daconil), and mancozeb (Fore).

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