Apple Disease Update: June 16, 2020
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Well Southeastern Apple Growers, it was only a matter of time. This past weekend I observed small purple lesions in our research trial (cv ‘Gala’) and by today, several more lesions characteristics of Glomerella leaf spot were found. Although it is cool outside, the pathogen is still able to germinate and infect with temperatures in the 60s and these extended leaf wetting events. If you have not applied a fungicide your trees in the past 7 to 10 days, I would strongly encourage you to get a cover spray applied to cultivars that are highly susceptible to GLS and bitter rot. Tomorrow (Wednesday) as it looks like there will be occasional breaks in the rain. Even during a misting/light rain like we had today (6/16), applying captan (3/4 to full rate) OR captan (1/2 rate) + ziram (1/2) rate should be effective. Plus the mist could even be advantageous in helping to redistribute the fungicides throughout the canopy.
You may be inclined to add a spreader/sticker to the tank if spraying under light rain conditions. If you’ve had prior experience with a certain adjuvant in combination with captan, and you’ve sprayed that combo under slow drying conditions (e.g. rain, mist, fog, cooler temps) without observing phytotoxicity on the fruit and the leaves, then I won’t discourage you from continuing to do that. Just keep in mind that certain cultivars are more susceptible to chemical injury than others. I would strongly urge you to NOT experiment with new tank mixtures during this weather pattern. In 2019, I presented some research on surfactant/captan injury that Kerik Cox and I had completed while I was a student in his program at Cornell. Sprays were made under misting conditions and temps in the mid-upper 50s on cv ‘Gala’. A picture is worth 1000 words:
This is not to say that all adjuvants + captan will cause injury-just be careful and be smart!
If you get the urge to head out to your see if GLS is present in your blocks, early symptoms (purple flecks/spots) can easily be mistaken for other diseases such as frogeye leaf spot, Marssonina leaf blotch, and even Alternaria leaf blotch. Take a look at the photos below for example. The one on the left is early symptoms of Glomerella leaf spot, in the center early symptoms of frogeye leaf spot, and on the right I’m fairly confident it’s early symptoms of Marssonina blotch. As you can tell, the purple spots all look quite similar. What usually helps me at this stage and recognizing the cultivar in which symptoms are occurring and also try to inspect areas of the tree that do not get as much air movement (toward trunk and lower canopy). You may be able to observe more mature symptoms in this region and these are usually more diagnostic of the disease.
Lastly, captan by itself is so-so for control of flyspeck/sooty blotch. If you only applied captan or captan/ziram last cover spray, you may want to consider adding thiophanate-methyl (e.g. Topsin) or a DMI (e.g. Inspire Super, Cevya) or ProPhyt into the tank.