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Jesica Conrad/Master Gardener: What is that blob on my plum branch?

Plums are the most common stone fruit grown in our area. Stone fruits are characterized by a hard "pit" at the center and include peaches, almonds, cherries and apricots. It’s pretty tough to get stone fruits to grow here in northern Minnesota but there are several varieties that soldier on. This article is about two of the most common stone fruit diseases.

Black knot (sounds like a heavy metal band) is caused by Apiosporina morbosa, on cherries, chokecherries and plums. This happens during wet periods in spring on plants that have been previously affected by spores transmitted by wind or rain splashes. First appear In the spring, the bark ruptures and a light yellowish growth fills the cracks of the swollen area. Late spring brings an olive green layer of spores. By late summer or early fall of the next year, the knots are hard and black and continue to grow girdling the branch.

Small, white blisters on immature fruit appear first. These enlarge as the fruit develops, surrounding the entire fruit. Interestingly, the entire fruit becomes abnormally large (three to four times its normal size), misshapen, with a thick, spongy flesh. A hollow pocket forms in the center of the fruit as the plum seed does not develop. Fruit develops a grey powdery fungal growth and eventually fruit falls off the tree.

Control of this disease is best accomplished by pruning in late winter before the new growth begins. Remove all infected branches 3 to 4 inches beyond the affected areas and discard away from the trees, bagging or burning the lesions. Continue to prune in spring as new swellings are seen in dry weather. The University of Minnesota Extension office recommends a single spray of lime sulfur or Bordeaux mix before the buds swell in the spring. This is a common problem on wild cherries in our area.

Brown rot (Monilinia fructicola) is more of a problem in southern climes, where the disease development is rapid in warm, wet, and humid weather, but with our climate changing here, one should be on the lookout. The North Dakota State University Plant Diagnostic Lab received reports of brown rot affecting stone fruits from Jim Walla, research tree pathologist at NDSU. He noted in 20011 that shoot blight was around Fargo and Bemidji.

This fungus overwinters in dried affected fruit on the tree and ground. Spring arrives and spores are carried by wind and rain and by insects to blossoms and leaves where they start their damage. Leaves and twigs become brown as they are covered by spore masses. One can’t miss the brown rot that occurs on the fruit. If still green, small round light brown spots will be noted. Fuzzy masses of spores cover ripening fruit. They can rot in hours during wet weather and rot also can develop in storage.

Late fall and summer is when management should occur. Carefully remove all remaining fruit and affected twigs/branches away from your trees. If you suspect brown rot, contact the University of Minnesota Extension office for instructions about chemical control products.

To find reliable information about gardening and other horticultural topics, go to the University of Minnesota Extension website

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