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(Bill Turechekwwt3@nysaes.cornell.edu & Cathy Heidenreich mcm4@nysaes.cornell.edu, Plant Pathology, Geneva)

Warm, wet weather after shuck split may result in disease problems for stone fruit growers caught off guard. Be sure to keep a weather eye open to avoid potential problems with bacterial spot of peach, and possibly peach scab.

Bacterial spot is a disease that affects virtually all stone fruits, but is particularly damaging to peaches, nectarines, and apricots. The disease is caused by the bacterium Xanthomonas arboricola (previously known as Xanthomonas campestris pv. pruni).

Temperatures above 65 F and wet conditions favor disease development. Bacterial spot may particularly be a problem in orchards that have a history of disease. Some of our western New York orchards were hit hard last year with bacterial spot, increasing the potential for repeat infections this season. Primary fruit and leaf infections occur as a result of frequent wetting events from full bloom to 4 weeks after shuck split. Wind-driven rain or debris can damage leaves and developing fruit, creating small wounds that the bacteria can enter, and significantly affecting the occurrence and severity of fruit and leaf infection. Disease does not develop under hot and dry conditions.

Bacterial spot affects the fruit, foliage, and young woody growth (twigs). Severe foliar infections result in leaf drop, which may significantly reduce tree vigor and winter hardiness. Bacteria from leaf infections may move into the current year's twig growth, leading to canker formation and providing an overwintering site for the bacteria. On fruit, the bacteria cause unsightly, dark brown lesions and/or blemishes. Lesions often become sunken and the skin of the fruit cracks, causing deep pits that leave the fruit unmarketable.

Peach scab, caused by the fungus Cladosporium carpophilum, is more of a problem downstate, and on later peach varieties. It is capable of infecting all cultivars of peach, and is known to affect apricots, plums and nectarines. Once established, this disease can be extremely damaging to trees. The fungus overwinters in small, slightly sunken, grayish twig lesions on the previous year's growth. Spores (conidia) produced from these stem lesions in early spring are splashed by rain onto small fruits and twigs. Infection and fungal growth are most rapid during periods of rainfall with temperatures between 65-75 F. Symptoms develop after a very long incubation period of 40-70 days. Because of the long incubation period, it is most often only the infections occuring between shuck split and pit hardening that develop fruit symptoms before harvest. Secondary infections may occur on twigs, and on late-season cultivar fruit.

PEACH SCAB

Although leaves and twigs may become infected, the fruit exhibit the most obvious evidence of the disease, developing small, greenish circular spots that gradually get bigger and darken as spore production begins. These spots appear when fruit are half grown, and are most common on the stem end of the fruit, but can occur over the whole surface. Enlarging lesions, if numerous, may coalesce, causing the fruit to be misshapen or cracked as it expands. This cracking opens an infection site for other fungi, such as brown rot. Scabby fruit do not ship or store well, and may drop prematurely.

Disease management

The most effective way of managing bacterial spot is to plant varieties resistant to the disease, yet this not always practical in respect to market demand. Maintaining proper fertility is essential, as excessive growth or poor nutrition increases a tree's susceptibility. Our limited choice of cultural control methods makes chemical control necessary when spot is a problem. If you had disease problems last year and/or weather predictions look favorable for infection, applications of oxytetracycline will be necessary. If warm and wet weather conditions persist, oxytetracycline can be applied on a 7-10-day schedule from now until 3 weeks before harvest. Oxytetracycline is intended to be used in a preventive mode; it has very limited to no kickback activity. If conditions prohibit you from making an application 24 hours or longer after a known infection event, save your money, as an application here will probably be ineffective against these infections.

To prevent peach scab, pruning is helpful because it facilitates air movement through the canopy to reduce the length of wetting periods, and improves spray penetration into trees. When control measures are needed, apply fungicide sprays at 10-14-day intervals starting 10 days after shuck split and continuing until 6 weeks before harvest. These intervals may be lengthened during extended periods of dry weather. Several products are labeled for use in N.Y. for peach scab control, including Captan 50WP (2 lb/100 gal) or Captan 4L (1.5-2 pt/100 gal), Indar 75WS (0.8 oz/100 gal), Sulfur 95WP (5 lb/100 gal), or Topsin M 70WP/Captan 50 WP combination (6 oz/100 gal and 1 lb/100 gal, respectively). All of these fungicides are labeled for control of brown rot through either petal fall or shuck split. So, if you have been using these fungicides in your schedule for control of brown rot, you have been managing scab as well. It is important to maintain protection beyond shuck split through pit hardening for control of peach scab, especially under favorable weather conditions. These fungicides will also offer protection against brown rot during this period.

 

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