Thursday, June 25, 2015

Powdery Mildew on Apricot

Apricot fruit with powdery mildew
(Photo: NMSU-PDC)
Featured Diagnosis - Powdery Mildew on apricot, caused by the fungus, Sphaerotheca pannosa. Key symptom: red ringspots and circular lesions on fruit.

On fruit the disease usually appears as round whitish spots a few weeks after shuck fall. The spots will enlarge to cover much of the fruit. Later, the white mycelium sloughs off to reveal rusty colored round lesions. In some cases the center tissue is unaffected resulting in a ringspot pattern. Diseased leaves develop dry brownish patches covered with a white powdery growth. Infected new leaf tissue is dwarfed and distorted. 
Spores of powdery mildew on apricot
(Photo: NMSU- PDC)

Sphaerotheca pannosa is one of two species of powdery mildew that infect apricot fruit and leaves in the spring. This fungus does not overwinter on apricot and primary inoculum comes from infected roses in spring. Powdery mildew spores are moved by wind, water and plant-to-plant contact. Once the spores reach susceptible host tissue they will germinate when there is adequate free moisture (high humidity, rain or there splashing water) and begin to colonize the plant tissue. 


Sphaerotheca pannosa spreads to apricots from infected roses. Ensuring that rose plants near susceptible hosts are treated appropriately for powdery mildew will reduce initial inoculum and disease incidence. Other cultural practices which help to reduce the occurrence of the disease include: selectively pruning branches to increase air flow and reducing humidity in the canopy, removing and destroying all fallen plant material on susceptible hosts, maintaining appropriate plant nutrient levels and selectively pruning other trees and shrubs to reduce shading. Contact and systemic fungicides are available when cultural management practices aren’t successful in reducing disease incidence to an acceptable level. Proper timing and through coverage of all above ground plant parts is critical for control.

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