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Bitter Rot/Glomerella Leaf Spot

Colletotrichum gloeosporioides and C. acutatum (sexual stage of C. gloeosporioides is Glomerella cingulata)

Bitter rot, caused by Colletotrichum gloeosporioides and C. acutatum, is the most important summer rot disease of apples in the Southeast. The sexual stage of C. gloeosporioides, Glomerella cingulata, can also cause a fruit rot and has recently been associated with a leafspot disease. Under favorable conditions, Colletotrichum spp. can cause extensive losses in just a few weeks. All cultivars of apple grown in the Southeast are susceptible to the disease.

Symptoms

Fruit infections can occur soon after bloom and appear as small gray or brown flecks, which may not enlarge until later in the summer (Fig. 1). During the summer, infections result in small, sunken, brown lesions, sometimes surrounded by a red halo that is especially visible on green or yellow fruit (Fig. 2). When lesions are about 1/2 inch in diameter, the fungus forms small, black fruiting bodies, called acervuli, about the size of a pinhead, around the interior of the diseased area. In wet weather masses of cream to salmon-pink spores are produced on the surface of the lesions by these fruiting structures (Fig. 3). As the lesions enlarge, the rot progresses to the core of the fruit in a V-shaped pattern (Fig. 4, left). This pattern differs from bot rot that forms a cylindrical rot pattern extending to the core (Fig. 4, right).

Leaf spots caused by G. cingulata begin as small purple flecks that enlarge to irregular necrotic areas 1/8 to ½ inch in diameter (Fig. 5). Severely affected leaves turn yellow and abscise. Fruit infections may appear as small scab-like lesions about 1/8 to 1/4 inch in diameter (Fig. 6), but often develop into typical bitter rot lesions.

Fig 1 - Early infection
Fig 2 - Summer lesions
Fig 3 - Lesion with spores
Fig 4 - Bitter rot (left), Bot rot (right)
Fig. 1. Early infection
Fig. 2. Summer lesions
Fig. 3. Lesion with spores
Fig. 4. Bitter rot (L), Bot rot (R)
 
Fig 5 - G. cingulata on leaf
Fig 6 - G. cingulata on fruit
 
 

Fig. 5. G. cingulata onleaf

Fig. 6. G. cingulata on fruit
 

Disease Cycle

The bitter rot fungi survive the winter in dead wood and mummified fruit that remain on the tree. Spores are produced on overwintering sites during the spring and summer and are released by rainfall. The sexual spores are airborne while the more important asexual spores are waterborne. The optimum temperature for spore germination is about 80 degrees F, with infection occurring within 5 hours at this temperature. The amount of infection increases as the wetting period lasts longer (up to 60 hours). Epidemics occur during prolonged periods of wet warm weather. Fruit infection can occur anytime (from soon after petal fall through harvest), but most infection occurs in the latter half of the season. The most severe epidemics occur when there are warm, wet periods early in the season, followed by similar periods in mid to late season.

Control

Bitter rot control is dependent on a good sanitation program. Dead wood, including the current year's fire blight strikes, needs to be removed from the tree and destroyed. All mummies hanging on the tree must be removed. If practical, remove diseased fruit from the tree during the growing season to reduce the spread of the disease. Fungicides applied from first cover until harvest on a 10- to 14-day schedule are effective if a good sanitation program is followed. If periods of warm, wet weather occur, it is imperative to spray more frequently than every 14 days.

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Format updated March 29, 2011