Brown Rot of Peach and Nectarine
Brown rot lesions usually show up on fruit near harvest. New infections create a soft brown spot on which tan to gray colored spores develop.
The pathogen survives the winter on previously infected "fruit mummies" that often remain attached to the tree.
This summer, two fruit samples from different locations in Utah County were submitted to the Utah Plant Pest Diagnostic lab. One sample, a nectarine, was diagnosed with brown rot caused by Monilinia laxa. The second sample, peach, was diagnosed with Monilinia fructicola.
Brown rot has, until this year, not been reported from Utah. The outbreak this year may have been associated with heavy rainfall and high temperatures in mid August, two to three weeks before harvest. Both species have probably been in Utah for some time and have gone unnoticed because they did not cause any problems during our usually dry summers.
Both Monilinia species mostly infect peaches, nectarines, sweet and tart cherries, plums and apricots. Rarely, the pathogen can infect apples and pears. During bloom, blossoms can be infected resulting in a blossom and twig blight. Blossom and twig blight is most common in tart cherries but can also occur on other fruit trees. Small cankers with gumming, very similar to cankers caused by shothole disease, develop on twigs.
Infections on fruit are not visible until two to three weeks before harvest. Green fruit is less susceptible unless it is wounded, due to the harder skin and lower sugar content. The initial symptom on fruit is a soft brown spot on which tan colored spores develop. One lesion on an infected fruit can produce thousands of spores, each capable of causing many new infections. With rain, wind, and warm temperatures, the disease can literally spread through an orchard overnight and destroy all fruit within a week.
The fungi survive the winter in fruit that is mummified and left hanging on trees or lying on the ground. The entire mummified fruit can be covered with spores providing inoculum for the following spring that will infect blossoms.
Management of the disease is through good sanitation. Mummified fruit needs to be removed and destroyed to reduce the amount of inoculum. There are fungicides registered for brown rot, including Adament 50 WG, Elevate 50 WGD, Pristine, and Captan 80 WDG for pre-harvest applications, and Scholar for post-harvest application.
It is very important to follow all manufacturer’s labels to prevent resistance (already reported for some fungicides in other parts of the country). It is most important to control initial blossom blight to reduce infection of fruit later in the season.
-Claudia Nischwitz, Extension Plant Pathologist