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Bot Rot

Botryosphaeria dothidea

Bot rot, caused by Botryosphaeria dothidea, is also known as white rot or botryosphaeria rot. Bot rot has caused losses of up 50% of the apple crop in some orchards in the Southeast. The fungus also causes a canker on apple trees. The canker phase is most severe in the Piedmont area of Georgia and the Carolinas but can occur throughout the Southeast during hot, dry summers.

Symptoms

Lesions on fruit begin as small, circular brown or tan spots, some­times with a red halo on green or yellow fruit (Fig 1) or a purple to black halo on red fruit (Fig 2). As the lesions enlarge, the rot progresses to the core in the shape of a cylinder (Fig 3). This symptom can be used to distinguish it from bitter rot which forms a V-shaped rot extending to the core. Under warm conditions, the rot progresses rapidly (Fig 4), becoming watery, soft, and a tan to brown color (Fig 5a). Fruit rot completely within a few days . Under cooler conditions, the rot is firmer and a darker brown color (Fig 5b) and is difficult to distinguish from black rot. (In general, it can be difficult to separate bot rot and black rot based on fruit symptoms.) Most rotten fruit fall from the tree, but some mummify and remain attached. Limb and twig cankers can start in lenticels or pruning wounds, particularly stub cuts, and can also start in frost cracks and areas scalded by the sun. When infections occur in lenticels, infected bark becomes depressed, and watery blisters form (Fig 6). Cankers enlarge rapidly on drought-stressed trees; the fungus moves up and down limbs and trunks more rapidly than around them, leading to elongated cankers. As cankers enlarge, they often coalesce and can girdle limbs and trunks (Fig 7). The canker phase of the disease is most severe on Golden Delicious and Rome Beauty but can affect all cultivars.

Fig 1 - Red halo on green fruit Fig 2 - Black halos on red fruit Fig 3 - Rot progression to core Fig 4 - Rot in warm weather
Fig 1. Red halo on green fruit Fig 2. Black halos on red fruit Fig 3. Rot progression to core Fig 4. Rot in warm weather
Fig 5a - Warm weather soft rot Fig 5b - Cool weather firm rot Fig 6 - Bark blister Fig 7 - Girdled bark
Fig 5a. Warm weather soft rot Fig 5b. Cool weather firm rot Fig 6. Bark blister Fig 7. Girdled bark

Disease Cycle

Botryosphaeria dothidea overwinters on dead wood in the tree and on the ground. Spores are produced and released during the summer under rainy conditions. This fungus, like the black rot and bitter rot fungi, can invade fire blighted tissues and produce spores by the end of June. Spores are dispersed primarily by rain and can germinate in 90 minutes at 80° F. Fruit can become infected in as few as 2 hours at this temperature if moisture is present. All cultivars are susceptible, although Golden Delicious appears to be most susceptible. Fruit infections can occur as early as late May but remain latent until the soluble solids reach 10%. This often causes growers to believe that their late-season program is not working, while in actuality most of the infections occurred earlier in the season.

Control

Control of bot rot starts with eliminating dead wood and mummified apples in which the fungus overwinters and produces inoculum. All dead wood, including the current year's fire blight strikes, needs to be pruned out of the tree. Prunings should be removed from the orchard and burned or chopped with a flail mower. Where the disease is a problem, preventive fungicidal sprays should begin at second cover during wet years and continue on a 10- to 14-day schedule. Control of the canker phase also starts with a good sanitation program. Equally important are good pruning practices. Avoid stub cuts; proper pruning saves the branch collar, which is essential for rapid wound closure. During hot, dry weather, trees should be irrigated to lessen stress and reduce the likelihood of infection and canker expansion.

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Format updated March 29, 2011