Apple and Pear Disease - Bitter Rot

For the last several years, bitter rot has become an important fruit rot on apple in Pennsylvania. Bitter rot on apple and pear fruit is caused by the pathogenic fungi in the Colletrotrichum genus, specifically those species in the C. acutatum and C. gleosporioides species complexes. Similar 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 predominant species causing bitter rot on apple in Pennsylvania is C. fioriniae. Glomerella cingulata (sexual stage of C. gleosporioides) causes a leaf spot disease and this does not occur in Pennsylvania because we do not have the fungal species in apple orchards. G. cingulata is predominantly found in southern states, such as North Carolina.

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. When the spore penetrates the skin, the infection will then go dormant (quiescent phase) for a period of time. During this time, the spore does not grow and is not susceptible to fungicides. Consequently, fungicides need to be applied prior to the initial infection of the spore.  Maturity of the fruit, temperature, humidity, and presence of disease are factors that determine when the quiescent period ends and the disease symptoms manifest. Bitter rot typically manifests in July and August, and fruit susceptibility increases as it begins to mature. 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 1/4 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.

Disease cycle

Bitter rot spores are suspected to overwinter in buds, mummified fruit, cracks and crevices in the bark, and cankers produced by either the bitter rot fungus or other diseases.. Spores have been detected as early as pink bud. With the advent of warm weather the fungus produces spores washed by rains to developing fruit.. 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. Frequent rain events, which lead to extensive wetness hours, during bloom and during the latter half of the season (July through September) have yielded significant bitter rot outbreaks.

Disease management

Cultivars vary in their susceptibility with the most susceptible being Rome, Honeycrisp, Jonagold, Empire, Nittany, McIntosh, and Liberty. Sanitation is important for any kind of fruit rot management: removal of old fire blight cankers, dead wood, mummified fruit, and encouraging breakdown of brush. Currently, full season management is necessary to prevent bitter rot. Warm temperatures and frequent wetting events, which cause prolonged wetness hours, during bloom will need to be monitored closely. Fungicide sensitivity assays performed at Penn State have shown few products control the predominant fungal species causing bitter rot in Pennsylvania (C. fioriniae). To date, the most effective products managing bitter rot include mancozeb, captan, and pyraclostrobin, which is found in Merivon and Pristine. The pathogen is resistant to trifloxystrobin (Flint Extra and Luna Sensation), kresoxim- methyl (Sovran) and thiophanate methyl (Topsin-M). Tank mixtures of Merivon and mancozeb are encouraged to be applied from bloom through first cover (as least two complete applications). Rain events and prolonged wetness hours will have to monitored closely during the summer, as well. During summer cover sprays, regular captan (3 lb/A) applications are necessary. To control late season infections and postharvest rots, another application of Merivon is recommended near harvest.