Over the past 7-10 days it has become clear that a bacterial spot epidemic is underway. It is eerily reminiscent of the epidemic of 1997, although so far it does not appear as widespread or affecting all cultivars as that year. Many blocks are dropping leaves this week, and fruit lesions are beginning to appear. Remember that it takes about a week for leaf infections to appear, and about 2 weeks for those infected leaves to start dropping. It takes about 3 weeks for those same infections to show up on the fruit. Looking back at weather records over at CoCoRaHs (choose your dates and local observations) it appears that the severe winds of May 23, and the rains over the weeks that followed set up a conducive environment for the bacteria in southern counties. We are seeing leaf drop in many locations regardless of treatment. The coming week will tell the story of how much fruit infection there is.
Fortunately this epidemic is coming near pit hardening. We are not quite there yet but should be in the next week or so. Northern counties may be farther off. Fruit is developing some tolerance to bacteria although it still can become infected. Fruit infections before pit hardening tend to be the deep pitted infections that render fruit unsaleable. Fruit infections after pit hardening, while still bad, will be shallow with small lesions that may still make grade. In some years when we see significant leaf drop later in the season we do not see significant fruit infection. Let’s hope this is one of those years.
So what can we do? Take note of this weeks weather. The end of this week may be an important infection if the forecast holds up for the next couple of days since there are many infected leaves still on the tree, and fruit infections are starting to appear. All of those lesions are shedding bacteria. Copper formulations differ as to label restrictions. Dr. Lalancette gave a comprehensive summary in a recent blogpost Copper Bactericides for Peach Bacterial Spot Management. Oxytetracycline products (Mycoshield and Fireline) are labeled for cover sprays. Unfortunately neither copper nor oxytetracycline have much residual protection. Don’t count on oxytetracycline for more than 24-48 hours of protection with light wetting. You can probably count on an inch of rain for both chemistries, but not much after that. The bottom line is frequent applications during inclement weather are necessary and successful in most years, and in bad years may not be enough.