Apple and Pear Disease - Bitter Rot

Bitter rot on apple and pear fruit is caused by the pathogenic fungi Colletrotrichum gloeosporioides and C. acutatum.
Apple and Pear Disease - Bitter Rot - Articles
Apple and Pear Disease - Bitter Rot

Bitter rot occurs only on fruit and can penetrate unbroken fruit skin. Photo by K. Peter.

The same causal pathogens are also responsible for anthracnose disease on peach, anthracnose fruit rot on blueberry and strawberry, ripe rot on grape, anthracnose on pepper, and blossom-end rot of green burrs on chestnuts. The sexual stage of C. gloeosporiodes, Glomerella cigulata, can also cause fruit rot and often is associated with a leaf spot disease. The discussion below is limited to the disease as it affects apple and pear trees.

Symptoms

Bitter rot occurs only on fruit. Cankers can form on twigs, but they are rare. The fungus is one of the few fruit rot organisms that can penetrate the unbroken skin of the fruit. Maturity of the fruit, temperature, humidity, and presence of disease are factors that determine when the disease manifests. Bitter rot typically manifests in July and August and fruit susceptibility increases as it begins to mature.

The fungus does not require fruit wounding to establish an infection and can directly penetrate the fruit skin.

Rot spots usually appear on the side of the apple directly exposed to the sun. The disease is noticed first as a small, light brown, circular spot. One or many spots may appear; if temperature and humidity are high, they enlarge quite rapidly and soon change to a dark brown. By the time the spots are 1/8 to ¼ inch in diameter, they are distinctly sunken or saucer shaped. When they reach ½ inch in diameter, small black dots, the fruiting bodies of the fungus, appear in the sunken lesion. These may be arranged in concentric rings. Later, they ooze a gelatinous, salmon-pink mass of spores, washed by rains to other fruit. When cutting through the lesion on the horizontal axis of the apple, the flesh is light brown and watery in a cone-shaped area, with the small end of the cone toward the fruit center. As the fruit ripens, it decays rapidly and finally shrivels into a mummy. Diagnostic symptoms of the leaf spot disease, caused by Glomerella cigulata, include irregular-shaped, dark purple-brown lesions with concentric rings, which may coalesce. Leaves can turn yellow and fall off of the tree. Gala, Golden Delicious, Pink Lady, Goldrush, and Pristine are known to be susceptible.

Disease cycle

Bitter rot spores overwinter in mummified fruit, cracks and crevices in the bark, and cankers produced by either the bitter rot fungus or other diseases. Jagged ends of broken limbs are ideal sites. With the advent of warm weather the fungus produces spores washed by rains to developing fruit. Often the first infections appear as a cone-shaped area on the tree and can be traced to a source of spores at the tip of the cone. The optimal conditions for the disease to develop are rainfall, relative humidity of 80 to 100 percent, and a temperature of 80 to 90°F.

To manage and control bitter rot in your orchard, remove old fire blight cankers and dead wood from your orchard. Alternatively, mulching the brush so it decays over the year also promotes sanitation. Remove apple mummies remaining on the tree from the previous season since the mummies serve as an inoculum source. Low areas of the orchard where drying is slower are susceptible areas. Bitter rot disease management is most effective by applying fungicides on a 10- to 14-day interval schedule through harvest and more frequently under favorable conditions. Captan and ziram provide good protective control and are most optimal when combined with another fungicide. The fungicides Pristine and Merivon (FRAC Groups 7 and 11) are excellent for controlling bitter rot; however, to reduce the risk of resistance, alternating a fungicide with another FRAC Group is encouraged. Summer fungicide applications should not be extended beyond 14-day intervals.

Disease management

To manage and control bitter rot in your orchard, remove old fire blight cankers and dead wood from your orchard. Alternatively, mulching the brush so it decays over the year also promotes sanitation. Remove apple mummies remaining on the tree from the previous season since the mummies serve as an inoculum source. Low areas of the orchard where drying is slower are susceptible areas. Bitter rot disease management is most effective by applying fungicides on a 10- to 14-day interval schedule through harvest and more frequently under favorable conditions. Captan and ziram provide good protective control and are most optimal when combined with another fungicide. The fungicides Pristine and Merivon (FRAC Groups 7 and 11) are excellent for controlling bitter rot; however, to reduce the risk of resistance, alternating a fungicide with another FRAC Group is encouraged. Summer fungicide applications should not be extended beyond 14-day intervals.

Authors

Apple and pear diseases Peach, cherry, other stone fruit diseases Tree fruit disease management

More by Kari A. Peter, Ph.D.