Friday, May 16, 2014: Our warm, humid, misty, wet weather is providing excellent infection conditions for bacterial spot of stone fruits. (Our current weather also favors brown rot infection, but you already knew that!) Bacterial spot can destroy peaches, nectarines, apricots, and plums by causing lesions on fruit. The critical period for control is petal fall through shuck split, although control measures may need to be continued into the summer on susceptible cultivars. (See the Rutgers University assessments of peach-nectarine cultivar susceptibility here and susceptibility ratings plus control recommendations from Michigan here.)
The best control is achieved with applications of the antibiotic oxytetracycline (Mycoshield, FireLine), but these products are not labeled for plums and apricots. Low rates of copper can also be used to control the disease on peaches and nectarines, but I really don’t know what impact copper sprays might have on apricots and plums. On peaches and nectarines, even low rates of copper will cause leaf spotting that can be almost as severe as the damage caused by bacterial spot itself, but copper applications can keep bacteria from infecting fruit. Because of the phytotoxicity caused by copper sprays (especially if applied too close together during periods with no rain), I suggest that stone fruit growers use Mycoshield or FireLine in at least the first few applications of the season.
For more details managing bacterial spot, see the excellent article by Dr. Norm Lalancette that just appeared in the Rutger’s Plant and Pest Advisory. Note that his comments on using Mycoshield/FireLine appear in the last paragraph of the article. I am not certain if all of the copper products listed in the article are registered in New York, so check product labels carefully, as always.