Diseases of Fruit Plantation Medicinal and Aromatic Crops

Phytopthora Crown Rot, Collar Rot, and Root Rot of Apple

Causal organisms: fungi in the genus Phytopthora
(Collar rot affects the scion portion of the tree, crown rot affects the rootstock portion of the tree. Root rot affects the root system away from the crown region.)

Symptoms

Rottening of may be seen at or below the ground line and may extend from the original site of infection into the root system and up the trunk to the bud union and above the bud union if the scion is also susceptible. Infected bark is brown and often slimy when wet. When infected bark is pulled, the cambium and phloem will be seen as an orange, reddish brown color. The rottening of tissues caused by the fungus girdle the tree resulting in poor vegetative growth of the tree with chlorotic foliage that may turn purple in the autumn. A severely infected tree may die. Trees may be killed in one growing season or may survive for a number of growing seasons.

Dissemination

Pathogen survives in soil for several years as spores, especially in old orchard soils. The pathogen also gets disseminated through infected nursery stock.

Disease cycle

The fungus survives in the soil as thick-walled spores (oospores) that are resistant to drought and relatively resistant to chemical treatment. The fungus may enter the orchard through infected nursery stock and also through irrigation water. Mobile spores (zoospores) originating from oospores move to the tree and colonize the apple bark tissue. The fungus may build up high level inoculum in the soil in a short period under favourable conditions - i.e. during wet, cool periods after harvest and in spring.

Management

The best time to manage Phytopthora diseases is during the initial stage of orchard establishment while selecting the site, root stock, site preparation and planting. Cultural Controls: While planting a new orchard, select the site and rootstocks carefully. Be sure the orchard site has adequate drainage system throughout the year. When irrigating the orchard, do not irrigate the soil for long. Use rootstocks which are resistant to the disease. Recommended scion cultivars may only be chosen for resistance.
Chemical: Mancozeb 2g/lit spray has been found effective.