Apple Disease Update: Week of June 12, 2022
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First off, thanks to all of our speakers, sponsors, and attendees who attended the 2022 S.E. Apple Conference last week. The field day was particularly exciting and informative and from a pathologist’s perspective, slightly surprising in regards to some of the occasional leaf spots I saw! In retrospect, my post last week was quite timely! Marssonina leaf blotch is appearing relatively early this season which helps me to better understand the results of our early season fungicide programs in previous years. A few years ago a posted FAQs on this emerging disease. Below is a “Cliff Notes” version with a few updates:
If you have a hand lens present (or a microscope), you may be able to see small, black fungal structures on the lesions called acervuli. The acervuli contain the conidia (asexual spores) of the MLB pathogen.
As the disease progresses, leaf tissue surrounding the lesion will becoming chlorotic (yellow) and premature defoliation occurs as early as late June.
Mycelium or thread-like structures of the fungus may also appear on older lesions.
Can MLB cause damage to fruit?
Yes, but so far I haven’t seen any symptoms on fruit in NC, even those with severe leaf infections.
When should we be protecting against infections?
In 2021, we also evaluated early season programs at the same application timings. I didn’t have time today to put the results into a graph but click on this link for fungicide trial results:
Basically, early season applications of Cevya, Captozeb, and Miravis reduced MLB by 65-70%. Again, in both trials these trees were not sprayed after 2nd cover with any fungicide. Imagine how a full fungicide program would perform?
For this weeks fungicide applications, I would suggest continuing with your captan alone, or captan + ziram, or captan+ phos. acid program. IF you have not made two applications yet of any of the following: Merivon, Luna Sensation, Pristine, or Flint, it may be a good idea to include one of these materials with a half rate of captan for your weekly spray. Looking at the forecast, it looks like the heat and wet will continue for a few days….