Peach Scab
Cladosporium carpophilum  
Peach scab lesions on twigs.
Peach scab lesions on fruit.

 

I. Introduction: Peach scab is caused by a fungus which can be extremely damaging to trees throughout the mid-Atlantic region because of the typically warm, wet weather during the day through the mid-season period. The disease appears to affect all cultivars of peach and is known to occur on nectarines and apricots as well.

II. Symptoms: The most notable symptoms of peach scab occur on the fruit, where small, greenish, circular spots gradually enlarge and deepen in color to black as spore production begins (photo 2-60). Fruit lesions are most common on the shoulders of the fruit, but can occur anywhere on the surface. Where numerous, they often coalesce and may lead to cracking of the skin as the fruit enlarges, allowing rot organisms to enter. The overwintering twig lesions (photo 2-61) are clearly visible during the early season as small, grayish, more or less circular, slightly sunken lesions on the previous season's shoot growth.

III. Disease Cycle: The pathogen overwinters in small twig lesions on last season's shoots. Conidiospores, produced in these cankers during the early spring, are splashed by rain to young fruits and new shoot growth. Rain is required for infection and a very long incubation of 40 to 70 days is needed for symptom development. Although the fruits remain susceptible through harvest, it is usually only infections that occur during the shuck split to pit hardening stage of development that have an opportunity to show symptoms before harvest. Twig infections that result in the formation of small overwintering lesions can occur throughout the season. Secondary infections may occur on twigs but usually do not appear on fruit, except on late season cultivars.

IV. Monitoring: Monitoring for peach scab begins with an awareness of where there was a problem the previous year. Be observant for overwintering twig lesions (photo 2-61) while pruning, record their presence on sample trees, and mark several locations to monitor for sporulation activity later. The critical time for effective disease control begins at the shuck split stage of fruit development. By the time the disease appears, it is too late to do anything about it during the current growing season.

Beginning in mid to late season, monitor 25 fruit on each sample tree for lesions (photo 2-60) which are most common on the shoulders of the fruit, but can occur anywhere on the surface. Where numerous, they often coalesce and may lead to cracking of the skin as the fruit enlarges, allowing rot organisms to enter. A total of ten to 20 fruit infections and greater than 20 fruit infections represents moderate and high risk, respectively. These damage levels indicate that improvements in disease management are needed.

V. Management:  Proper and regular pruning facilitates air movement, reduces length of wet periods, and improves spray penetration into trees.  Fungicide sprays, applied at 10- to 14-day intervals, should be made beginning at petal fall and continuing until 40 days before harvest. 


Chemical control - commercial growers

Chemical control - home orchardists (pdf file - Acrobat Reader required)

Text prepared by P. W. Steiner and K. S. Yoder

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