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 are C. fioriniae, C. chrysophilum, C. siamense, C. noveboracense, and C. fructicola. These species vary somewhat in their optimal growth temperatures, but an orchard is managed the same regardless of which species are present. 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, but it has not been found in Pennsylvania.

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 green tip, with the highest numbers of spores produced during warm weather. Spores are washed by rains to developing fruit. The optimal conditions for the disease to develop are surface wetness and a temperature of 70 to 90°F. Frequent rain events, which lead to extensive wetness hours combined with warm temperatures, have resulted in significant bitter rot outbreaks. Apples are susceptible to infection throughout the growing season, but they seem to be slightly more susceptible during the latter half of the season (mid-June through September), at least in part because that is when temperatures are the highest.

Disease Management

Cultivars vary in their susceptibility, with the most susceptible being Rome, Honeycrisp, Jonagold, Empire, Nittany, McIntosh, Liberty, Northern Spy, and Summer Rambo. 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, will need to be monitored closely starting at bloom.

Protectant fungicides should be applied before warm and wet periods. A strong apple scab management program from bloom through first cover will be enough to keep early infections in check. Since high disease pressure is favored by warm temperatures and prolonged periods of moisture, fruit is most susceptible from mid-June through harvest when these conditions are most likely to occur. During this period, growers are encouraged to tank mix stronger chemicals with their broad-spectrum fungicides during cover sprays, keeping in mind the products sprayed during the early season to follow fungicide resistance management guidelines.

Fungicide sensitivity assays and field trials performed at Penn State have shown few products control the fungal species causing bitter rot in Pennsylvania. The most effective products for managing bitter rot are the broad-spectrum fungicides sulfur, mancozeb, ziram, and captan, and the single site fungicides pyraclostrobin (FRAC Group 11; found in Merivon and Pristine), trifloxystrobin (FRAC Group 11; found in Flint Extra and Luna Sensation), benzovindiflupyr (FRAC Group 7; Aprovia), and if applied at the highest rate, fluazinam (FRAC Group 29; Omega). Penthiopyrad (FRAC Group 7; Fontelis) used at the 20 fl oz/A rate is effective when tank mixed with captan; however, it is fair when used alone.  

To prevent bitter rot infections, growers should time their fungicide applications before rain events during mid-June through harvest. Tank mix captan (3 pounds per acre) or ziram (4 pounds per acre) with Merivon, Luna Sensation, Flint Extra, Aprovia, or Omega. Growers are encouraged to be mindful of the PHI for these products; products with 0 to 14 day PHI should be used near harvest to prevent postharvest disease issues that may occur during storage. During storage, Scholar (fludioxonil; FRAC group 12) is the most effective postharvest fungicide to prevent bitter rot. For organic options, sulfur is the most effective.