Utah Pests News Summer 2008

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Watch for Coryneum Blight this Summer 

  The coryneum blight fungus overwinters on infected buds or cankers (top), and infects leaves in spring during moist conditions, leaving the typical “shothole” appearance (middle). Later in the season, developing fruits become infected (in Utah, infections on peach and apricot are most common), leaving sunken pits sometimes associated with gummosis (bottom).
With the arrival of this past springs’ moderate temperatures and rainfall came the onset of conditions for coryneum blight, also known as shothole blight.  This fungal disease causes damage on peach, nectarine, apricot, almond (ornamental as well as nut bearing), and cherry to a lesser degree (ornamental and fruit bearing).  Coryneum blight is caused by the fungus Wilsomyces carpophilus.  Taxonomic changes explain this fungal pathogen’s name as it was once known as Stigmina carpophila or Coryneum beijerinckii.

The pathogen can infect buds, twigs, branches, blossoms, leaves, and fruit.  It overwinters in infected buds and in cankers on infected twigs and branches.  Spores produced from these infected tissues in the early spring are dispersed during rain events to infect new buds.  Later in the season, other susceptible tissues can become infected when there is suitable moisture on leaves and fruit.  The spores of this fungus, called conidia, are extremely durable and can survive in dormancy for months, exposed on the surface of a bud, waiting for just the right temperature and moisture conditions to germinate and infect its host.  Coryneum often surprises growers as it is active early in spring at very cold temperatures.  For example, the fungus can infect a suitable host if moisture is continuous for 24 hours or longer at only 36°F.  This means that infections can occur when host plants are still dormant.  At higher temperatures only 6 hours of continuous moisture at 77°F is necessary for infection.  Coryneum blight can develop very rapidly under warmer temperatures with suitable moisture conditions, and will sporulate from infected tissues throughout the season during wet conditions.

Infections on leaves will develop small round purple to tan lesions or spots that are seldom 1/4-inch in diameter.  Infected tissues can become raised and scurfy and will often drop out as the diseased tissue cannot expand with the growing leaf.  Lesions can be circular to slightly ellipsoid.  These diseased leaf tissues will tear along the lesion margins and may hang on at one attached point, but eventually drop out, giving the shothole appearance.  Infected buds will often develop a canker that can expand to girdle and kill the twig. Infected buds typically will show signs of gumming.  They are easily recognized as they are darker than healthy, non-infected buds. Infection on fruit appears first as small purple spots that later become white to gray lesions, often accompanied with gumming, rendering fruit unmarketable.

Cultural control practices involve pruning infected twigs and branches and destroying the debris.  Thorough pruning during the dormant season is very effective and recommended for the homeowner as a major component for managing this disease.  During irrigation, avoid wetting of branches, twigs, leaves, and fruit.  Chemical controls applied at 50% leaf drop include copper compounds such as Bordeaux mixture, copper based products like Kocide, and fungicides such as Ziram.  Chemical applications such as these in the fall will help to protect buds during the dormant season.  In the spring at shuck fall, fungicides such as Abound, Pristine, Gem, Echo 720, Bravo Weather Stik, and Ziram are effective.  For commercial growers, protective fungicides should also be applied during frequent wet weather.  With regard to any chemicals mentioned, read the label and follow the labeled instructions for their use.  Mention or exclusion of trade names does not represent or imply endorsement nor criticism of any chemical product.

-Kent Evans, Extension Plant Pathologist (No longer at USU)