Scaffolds 00 index

(Dave Rosenberger dar22@cornell.edu, Plant Pathology, Highland)

Rusty spot, peach scab, and bacterial spot are peach diseases that occur only sporadically in New York State, but they cause severe losses when they do appear. All three of these diseases are initiated at or shortly after petal fall. Peach scab and bacterial spot are more severe in New Jersey and other southern states than they are in New York. Peach scab and bacterial spot occur in the Hudson Valley in years following unusually mild winters or when spring conditions are especially conducive to disease development. All three diseases can be prevented with fungicides or bactericides.

Rusty spot is believed to be caused by Podosphaera leucotricha, the same fungus that causes powdery mildew on apples. Rusty spot appears as large rust-colored spots on the sides of green peach fruit. It is best prevented by locating peach plantings away from mildew-susceptible apple cultivars that provide the inoculum for infections on peach. Thus, peaches should never be planted adjacent to mildew-susceptible apple cultivars such as Ginger Gold, Cortland, Rome Beauty, or Idared. Warm, dry weather conditions that favor powdery mildew on apples will also favor development of rusty spot on peaches.

Fig. 1. Rusty spot lesion on immature peach.

If rusty spot is a problem in existing plantings, it can be controlled by applying mildewcides starting at shuck split. Sulfur is probably the most cost-effective mildewcide for peaches, although the SI fungicides registered for controlling brown rot will also control rusty spot. Fruit become more resistant to rusty spot after pit hardening, but additional infections may occur after pit hardening if peaches are grown in an area with high levels of inoculum.

Peach scab, caused by Cladosporium carpophilum, overwinters in lesions on peach twigs and as spores on the bark. The fungus causes velvety brown spots to develop on peach fruit. Scab is more severe in southern areas than in the northeast. In the northeast, spraying for peach scab is unnecessary in most years. However, scab occasionally causes considerable damage in orchards left unprotected after shuck split. Peach scab is easily prevented by applying fungicides from shuck split until early July. Sulfur added to several cover sprays is the cheapest and most cost-effective fungicide for controlling peach scab. As an alternative, one spray of Bravo applied at shuck split to control brown rot will probably provide full-season control of scab in the northeast.

Fig. 2. Peach scab lesions appear as small black spots on immature fruit and make fruit unmarketable.

Bacterial spot is caused by the bacterium Xanthomonas campestris pv. pruni. Bacterial leaf spot is rarely a significant problem in New England and New York (except for Long Island), but can cause significant losses in peach orchards in some years. X. pruni infects leaf scars on twigs as leaves drop in the fall, then initiates infections the next year if spring weather conditions favor infection. Infections on green fruit cause the fruit to secrete gum. Multiple infections can cause the fruit to crack as they ripen. Infections on leaves are often most numerous on the edges and lower half of the leaf because the bacteria are carried to the lower half of the leaf during rains. Severely affected leaves turn yellow and drop prematurely. Infected leaves supply inoculum for further infections of fruit.

The best control for bacterial spot is to avoid planting varieties that are highly susceptible to this disease. Where susceptible varieties are already planted, bacterial leaf spot can be reduced by applying a copper spray at leaf fall in autumn to reduce overwintering populations of the bacteria. The primary control for susceptible cultivars is application of Mycoshield (terramycin) at 10-14-day intervals between shuck split and fruit ripening.

> 5.1 Insects