The Q&A Archives: Plums develop rusty skin, fruit rot

Question: My plums develop a rust on the skin and as the fruits expand, the skin cracks and they rot. What's the cause of this rust? John Mapp Nassawadox, VA

Answer: It's most likely brown rot, says Richard Marini, Extension tree fruit specialist at Virginia Polytechnic Institute in Blacksburg. Brown rot is very common on plums and other stone fruits in the East and it spreads easily. The fungus overwinters on mummified fruit and plant debris and reinfects developing fruit three to four weeks before harvest. Spores released during warm, humid weather penetrate the skins of the fruits, says Marini. Once the plum is infected, russet patches appear and rot soon follows. In dry years, very little brown rot is seen. This year, however, has been wet, so we'll probably see a lot of the disease, says Marini. To reduce the spread of brown rot, remove and destroy all mummified fruit in fall, keep the tree pruned to an open center to encourage air movement and thin the fruits when they are one inch in diameter, says Marini. For a sever infection, spray every two to three weeks after bloom with Captan or sulfur, he adds.

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