Summer Fungal Fruit Rots and Their Management
By: Lianna M. Wodzicki, Graduate Research Assistant, and Dr. Melanie Lewis Ivey, Assistant Professor, Department of Plant Pathology, The Ohio State University, Wooster, OH
As the final cover sprays are applied in apple orchards, you may notice some unsightly spots on your fruit. Fungal fruit rots can cause significant losses in yield and reduce fruit quality. These diseases often appear just prior to harvest, during harvest, or in storage.
If these rots are not prevented through the integration of sanitation and cultural practices, and often fungicides, they can cause severe losses. The expression “one bad apple spoils the whole barrel” is a good description of what fruit rot can do.
Bitter rot, black rot, and white rot are the three disease that cause the most damage to fruit, often rendering them unsalable.
Bitter rot (caused by the Colletotrichum acutatum fungal complex) is characterized by brown, sunken spots. After rain periods, these spots can develop masses of orange spores on the surface. Internally, the discoloration of the flesh tapers down to the core in a V shape (Image 1). The discolored flesh is corky or spongy to the touch. Bitter rot is a major problem in Ohio due to prevalence of warm and humid summers. Diseased fruit, infected twigs and mummified fruit (in the tree and on the ground) are the primary sources of these fungi.
Black rot (caused by the fungus Botryosphaeria obtusa) is a dry rot that almost always starts at the calyx (blossom end) of the fruit. The spots are dark brown and black in color with black specks (fungal fruiting structures) scattered over the lesions. Unlike bitter rot lesions, black rot lesions are not sunken. In addition to fruit symptoms, this fungus can cause a leaf spot (frogeye) and/or sunken cankers on branches. Sporulating cankers (areas of infected and dead bark), diseased leaves and mummified fruit are all sources of the fungus, although cankers are the primary source of new infections in the spring.
White rot is caused by a similar fungus to black rot - Botryosphaeria dothidea. However, unlike black rot, white rot is a soft rot. Infected fruit quickly turn to mush and have a strong sour smell. Light tan lesions form on the fruit and lesions on light skinned varieties can have a reddish halo. White rot lesions on red skinned varieties appear bleached. Internally, the lesions are soft and extend all the way to the core. The disease frequently appears
after periods of hot and wet weather in mid-summer. The fungus also causes a stem canker that girdles the branches. While fruit on the orchard floor can serve as a source of spores, cankers are the primary source of new infections in the spring.
Fruit Rot Management
The fungi that cause bitter rot, black rot and white rot can be stealthy, infecting the fruit without showing symptoms until environmental conditions are just right. Because there are no resistant fruit varieties and infections can occur anytime from petal drop through storage, prevention is key to successful management.
Any practice that helps to maintain trees in a healthy, vigorous condition is critical for controlling the canker phases of fruit rot diseases. Cankers generally develop only on stressed or weakened trees, especially winter-injured trees. Prune trees annually and maintain a balanced fertility program based on soil and foliar nutrient analyses.
Orchard and tree sanitation are critical to preventing new infections in the late spring. Dormant season removal of dead wood and mummified fruit in the tree as well as cleaning up the orchard floor after harvest will reduce inoculum levels in the orchard and ultimately reduce the amount of fruit rot. However, during Ohio growing seasons warm temperatures, high humidity, and rain promote infections and disease development, necessitate fungicide sprays. Fungicides* are recommended from petal fall through harvest. When frequent rains and warm weather persist leading up to harvest, it’s advisable to apply a fungicide that targets all three diseases and that will protect the fruit after harvest and during storage. Fungicides in FRAC group 11 (i.e. Merivon, Pristine or Luna Sensation) are particularly effective at this stage of production. One of these fungicides mixed with Captan is recommended for the last cover spray of the season. Because growers will often use FRAC 11 fungicides for primary scab management early in the season, they may need to use a fungicide with a different FRAC code (i.e. FRAC 7 or 9) to manage primary scab in order to meet fungicide registration requirements for the total number of applications permitted in a season.
In addition to a late season application of a FRAC 11 fungicide, postharvest fruit rots can be managed using best sanitation and handling practices. Harvesting practices that minimize bruising or wounding, clean and sanitized harvest bins, and storage temperatures below 40 F with low humidity will reduce the risk of fruit rot infections and rot development.
*Product information including FRAC codes, Preharvest Interval (PHI), and rates are provided in the Midwest Fruit Pest Management Guide. Before using fungicides consult the product label and follow all usage requirements.
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