Answer: Brown rot, caused by the fungus Monilinia fructicola, is a common disease of peaches and plums; it also attacks nectarines, and cherries. The disease may affect blossoms, twigs, and fruit. Generally, there are two major infection periods of the brown rot fungus. They occur during blossom and beginning several weeks before harvest.
The blossom blight phase occurs in early spring during bloom. Young petals first develop brown spots, but the blossoms quickly turn brown or black and die. Small tufts of dusty brown to gray fungal growth can be seen growing on the dead blossoms. The loss of some blossoms in the spring is not serious in itself; however, inoculum produced on the rotting blossoms serves to infect developing fruit later in the season. The fungus also may move from blighted blossoms into the twigs, causing small, elliptical cankers. In some cases, these cankers will girdle and kill twigs. Sap bleeding or gum production often is associated with twig cankers.
The second phase of the disease occurs as fruit begins to mature. Inoculum produced on blighted blossoms, twig cankers, or from nearby wild plums infects maturing fruit. Affected fruit develops light brown spots that enlarge rapidly. The fruit may be completely rotted within a day or two. The fungus sporulates profusely on the rotting fruit, giving the peaches a dusty brown appearance. The rotting fruit shrivels to form a structure called a mummy, which is completely colonized by the brown rot fungus. These mummies may remain attached to the tree or drop to the ground.
Here are some ways to control brown rot:
Clean up fallen leaves and fruit at the end of the season, and remove and destroy any mummified fruit that remains on the tree. Plant trees in full sun; space trees generously and prune annually to maximize air circulation. Thin fruit so they aren't touching. Inspect branches and prune off affected wood. Spray sulfur during bloom and again before harvest if weather is wet and humid. Although no varieties are completely resistant, two varieties, 'Newhaven' and 'Harrow Diamond,' have been shown to have some resistance to the brown rot fungus.
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