A: Your plums are showing the early stages of brown rot, which is caused by a fungus called Monilina fructicola. Brown rot can attack blooms, fruit, leaves, and stems. It over-winters on blighted stems and mummified fruit. Brown rot can be a problem during wet weather. I know what you are thinking – we have had very little rain but remember the spores have been there all along and they are just now causing issues with the fruit. In addition, we create an artificial environment by applying irrigation to the trees – sometimes twice a week. It is best to irrigate any tree and shrub at the root keeping the water off the leaves. Infections will appear as masses of brown to brown gray spores on infected tissue if left on the tree long enough. It is important to prune the tree in an open vase-shape to encourage good air flow which reduces the spore problem. At this point, we have no plum cultivars resistant to brown rot, and fungicide sprays may be necessary during wet weather or irrigation landscapes. The best time to apply fungicide is just at the new leaves are forming, just after flower budding and fruit setting. Use a fungal spray specifically developed for fruit tree species. It is also very important to remove any fruit from the tree and not allow it to go from one season to the next. Clean away any leaf, stem and fruit debris from around the base of the tree as these can be the source of the fungal spores, creating the perfect environment for future spore dispersal to the tree.

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