Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Current weather conditions are favorable for the development of bacterial spot in susceptible stone fruits including peaches, nectarines, apricots, plums, pluots, apriums, and plumcots. Bacterial spot is caused by the organism Xanthomonas campestris pv. pruni. It is found on leaves, twigs, and fruits. This time of year, we see the results of spring twig infections with shoot tips that are dead. Leaf symptoms after infection are most readily seen as a “shot hole” appearance, where the small, infected areas dry up and fall out, or as tattered leaf edges. Fruit infections are dark colored small spots on fruit skin in that then lead to fruit cracking later as spots coalesce.
Bacterial spot on nectarine leaf. Note shot hole appearance
The bacteria overwinter in twigs that were infected in the previous fall from diseased leaves. In the spring, during warm, wet conditions, the bacteria ooze out and can be splashed onto leaves and fruit. Fruit and leaf infections start around shuck split and then can continue throughout the season in susceptible varieties. Infections only occur during wet conditions.
In bacterial spot susceptible varieties, sprays of copper fungicides are applied in early spring prior to bloom to reduce surface bacteria numbers. To control the disease during the season, sprays should be applied from petal fall until 2 weeks before harvest. In wet conditions, applications should be close together (5-7 days), in dry conditions, applications can be spread further apart. Use antibiotic products (Mycoshield, Fireline) or use fixed coppers with low phytotoxicity potential. Copper can cause leaf damage so care should be taken with their use.
The best management strategy for bacterial spot is to use resistant varieties. Many eastern bred varieties have good bacterial spot resistance. Western bred varieties, developed in lower humidity areas, are often very bacterial spot susceptible and will be difficult and expensive to produce on Delmarva.