Botrytis
Botrytis Blossom and Twig Blight (Botrytis cinerea)
Diseases
Symptoms
Botrytis is a serious fungal disease of blueberries in Northwestern Washington, and can cause damage on stems, blossoms and fruit, especially during wet springs. Infected blossoms show symptoms similar to frost damage, turning brown after a period of high humidity. Under ideal conditions, gray mycelium and spores may be produced on infected blossoms. Botrytis can grow from the infected flower cluster into the stem to cause dieback of the stem. Stem infection can also occur through wounds or areas of frost damage. Branch tips are silver and brittle. Botrytis can also infect green stems causing girdling and eventual die-back. Botrytis causes fruit rot, although the symptoms may not be visible until after harvest. Fruit infected during development will have a decreased shelf live; fresh market fruit packed in closed containers may be unmarketable.
Life History
Botrytis overwinters in infected stems of bushes and in plant debris at the soil line. Copious amounts of spores can be produced during wet springs. Spores are dispersed through wind movement and water splashes. Dead tissues on bushes offer opportunities for colonization. Blossoms present during spore dispersal are vulnerable to Botrytis infection, especially when humid conditions are prominent.
Monitoring
During dormancy, visit 20 bushes per site and inspect 10 shoots per bush for dieback at five sites. Many conditions can cause dieback in previous year’s growth. If 30 or 40% of the bushes exhibit shoots with dieback, consider pruning out dead shoots to reduce potential innoculum. During bloom stage, look for blighted blossoms and infected stems and record infection level. Look for infected fruit during ripening stages.
Management
Branch tips killed by low winter temperatures are easily infected. Remove infected plant material and reduce humidity in the canopy by a regular thinning. If the disease is present in every field it will be hard to eliminate but can be reduced. Apply fungicides during bloom and fruit ripening. Avoid excessive use of nitrogen fertilizer in the spring because the Botrytis fungus will readily infect succulent green growth. Older parts of the plants are rarely attacked. Cool berries rapidly after harvest.
Web Resources
Oregon State University Extension, An Online Guide to Plant Disease Control, Blueberry-Botrytis Blight
http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=177
Michigan State University, Michigan Blueberry Facts, Botrytis Blight
http://www.blueberries.msu.edu/botrytis.htm
Michigan State University Extension, Blueberry Fruit Rot Identification Guide
http://web2.msue.msu.edu/bulletins/Bulletin/PDF/E2847.pdf
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