Colletotrichum acutatum J.H. Simmonds
Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. in Penz.
Bitter rot appears on young fruit as small, circular brown lesions. Lesions expand rapidly and radially under wet and warm conditions. As they age, they turn darker brown and become sunken (A). When several lesions occur on a fruit they tend to coalesce and no longer appear circular. The spores of the fungus are creamy, white to pink, and tend to form in concentric circles within the lesion (B). The rotted flesh is often watery and appears V-shaped in cross section (C). The fruit eventually dries, mummifies, and may fall to the ground or remain hanging from the tree throughout the duration of the winter.
Common to all fruit-growing regions in eastern North America. It is an important fruit rotting disease in warmer apple growing regions, such as the mid-Atlantic and southern US.
Early symptoms can be confused with early symptoms of black rot or white rot.
The bitter rot fungi overwinter as saprophytes in many wild hosts and in mummified fruit within the orchard. Spores from these sources are produced in abundance during periods of warm (>20°C) and wet or humid weather. Mummified fruit and dead wood should be removed to reduce the source of inoculum. Regular fungicide applications from early summer through harvest are usually necessary to manage disease in problem orchards; the strobilurin fungicides and captan are the most effective fungicides. No apple variety is completely immune to disease; however, some varieties like Fuji, Golden Delicious, and Empire are more susceptible.
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