Recent Changes in Our Understanding of Bitter Rot

Bitter rot is an increasingly important disease of apples and pears in many production regions around the world. This post provides a link to a review of the scientific literature on bitter rot that I recently completed and that formed the basis for a presentation at the Cumberland-Shenandoah Fruit Workers Conference on 1-2 Dec 2016. The powerpoint I used at that conference includes photos and additional comments on research that is needed before we can formulate more effect control strategies for this disease.

The review explains how our understanding of bitter rot has changed in recent years as scientists have used molecular techniques to determine that bitter rot, which in 2006 was believed to be caused by three different fungi, is now known to be caused by at least 18 different Colletotrichum species. Different species predominate in different geographic regions and vary in their susceptibility to fungicides. A table on page 10 of the review provides a useful list of the species causing bitter rot and where and by whom they have been reported.

This literature review for bitter rot will probably be of more interest to scientists, extension specialists, and private consultants than to apple growers who are searching for information on control strategies. A shorter summary covering some of the same information, along with suggested control strategies for bitter rot in northeastern United States, was published 29 August 2016 in Scaffolds Fruit Journal

In addition to the control strategies listed at the end of the Scaffolds article, fruit growers should recognize that the fungi that cause bitter rot can sporulate abundantly in dead twigs that are left in or beneath trees after either winter or summer pruning. Thus, removal of all fallen fruit and pruning debris from herbicide strips beneath trees can help to reduce inoculum and is especially important in blocks with a history of bitter rot problems. Moving prunings to the row middles where they can be shredded with a flail mower is probably sufficient, but more research is needed to determine exactly where and how the fungi causing bitter rot overwinter in our orchards.

Posted 20 Jan 2017; Updated 7 Feb 2017

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