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Black Rot and Frogeye Leafspot

Botryosphaeria obtusa

Black rot and frogeye leafspot, caused by Botryosphaeria obtusa, are important diseases in orchards throughout the southeastern United States, causing extensive fruit loss and defoliation if not controlled. Black rot cankers are important in the northern areas of the eastern United States but are not problems in most orchards in the Southeast.

Symptoms and Disease Cycle

The black rot fungus survives in dead wood and mummified apples left on trees. Apples mummified by thinners the previous season are a good source of inoculum (Fig. 1).

In the warmer growing regions of the Southeast, spores are produced from December to March and washed into buds by rain. When buds start to swell (the silver tip stage), the spores germinate, infect the flower parts, and grow into the core area of developing fruit. The fungus remains dormant until the fruit starts to ripen. It then grows into the flesh and causes a firm brown rot, often at the calyx end (Fig. 2).

In the cooler growing regions of the mountains, spores are not produced through the winter, and infections first occur on the sepals. As the fruit begins to mature, the fungus grows from the sepals into the fruit, often resulting in a firm brown rot on the calyx end (Fig. 2).

After petal fall, additional fruit infections can occur anywhere on the fruit. Infections that occur soon after petal fall appear as raised, black, pimple-like lesions (Fig. 3), while those occurring later in the season are often black, irregularly shaped, and bordered by a red halo (Figs. 1 and 4). Infections are most common in late June, July or August. These infections often do not expand until the fruit begin to ripen (Fig 5).

The optimum temperatures for fruit infection range from 60 to 70 degrees F. Nine hours of wetting are required for infection at these temperatures.

Fig 1 - Infected fruit Fig 2 - Black rot on calyx Fig 3 - Petal fall lesions
Fig. 1. Infected fruit Fig. 2. Black rot on calyx Fig. 3. Petal fall lesions
Fig 4 - Later lesions Fig 5 - Expanding lesion on ripening fruit
Fig. 4. Later lesions Fig. 5. Expanding lesion on ripening fruit

Leaf infection can occur anytime during the spring or summer when the foliage is wet, but is most common during and just after bloom. Leaf infections are brown, circular, 1/8 to ¼ inch in diameter, and often bordered by a purple halo (Fig 7). Symptoms are very similar to those of Alternaria blotch (Fig 8) and captan injury (Fig 9). Sometimes leaf spots are invaded by secondary fungi that enlarge lesions, giving them an irregular appearance. Heavily infected leaves often turn yellow and abscise (Fig 10).

Fig 6 - Frogeye leaf spot Fig 7 - Alternaria blotch Fig 8 - Captan injury Fig 9 - Infected, yellowing leaves
Fig. 6. Frogeye leafspot Fig. 7. Alternaria blotch Fig. 8. Captan injury Fig. 9. Infected, yellowing leaves

Botryosphaeria obtusa can colonize twigs killed by fire blight in the spring and by late summer produces spores in fruiting structures on this dead wood. Infected fire blighted tissues can be a significant source of inoculum as the fruit begin to mature.

Control

To effectively control black rot, remove all dead wood and mummified apples from the tree. If prunings are not removed from the orchard, they should be chopped with a flail mower. Do not pile prunings along the perimeter of the orchard. Current-season fire blight infections should be removed once affected shoots have become necrotic and dried. Since infections can occur as soon as the bud breaks it is important to apply fungicides from silver tip to harvest to control the disease where it is a problem.

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Format updated March 29, 2011