Thursday, October 30, 2014

A disease of peach trees: Coryneum Blight

Photo of Reliance Peach by Julie Elliot
Shot hole disease or Coryneum blight will occur on twigs and buds when spring weather is wet.  This disease can occur on peach, nectarine and apricot trees.  This disease can reoccur in these fruit trees, especially if the fruit trees have a history with the disease. 

Watch for small, purplish black spots which first appear on the twigs and can be concentric.  These spots will expand and their centers will be brown with small tiny dark brown spore-forming structures at the center.  These spore-forming structures are called sporodochia.  If you have a hand lens, they can be seen best with a hand lens. 

Besides the twigs, the spots on the leaves can be very similar in nature.  They are small spots starting out as purple and then developing a tan center which will fall out.  Hence the name “shot hole” disease.  Sometimes, the spots may be surrounded by a light green or yellow margin.

It is important to note that this disease can winter over on twigs and buds.  The lesions can continue to spread even at temperatures of 45 F even though the optimal temperatures are ranging from 70 F to 80 F.  If there are periods of prolonged wetness in the fall and continuing to mid-winter, you need to be proactive and use a preventative spray.  According to Colorado State University Extension, specialist Harold Larsen recommends fungicides such as Bravo containing chlorothalonil or copper-containing products such as Bordeaux mixture, Kocide or Fixed Copper can be used as a preventative.  Spraying a preventative, means just that.  What is not already affected will be protected.  A word of warning, copper-containing products should only be used when the leaves are off the trees and during fall is the best time.  Follow the label for usage.  The label is the law.

For those who have sprinkler systems where the water may come in contact with the trunk or lower limbs, this encourages the disease.  The lower limbs of the tree will be where the moisture from a rain dries out last.  This is an area where there can be the most infection from the disease.  It is equally important to rake any fallen infected leaves or twigs so that irrigation or rain does not allow the spores to be spread back on the tree.  The spores can be carried back to the tree by wind during a rain storm. 

For more information on Coryneum blight, you can access CSU Fact Sheet number 2.914 on-line or visit your local Extension Service Office.


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