Apple Disease Update: Week of June 12, 2022

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Marssonina Leaf SPot on Apple

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:

What does it look like (i.e. what are the symptoms)?
Early symptoms of Marssonina Leaf Blotch on 'Rome Beauty'

Early symptoms of Marssonina Leaf Blotch on ‘Rome Beauty’

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.

Close-up photo of an MLB lesion. Blue arrow is pointing to acervulus.

Close-up photo of an MLB lesion. Blue arrow is pointing to acervulus.

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.

Advanced symptoms of Marssonina Blotch on Rome Beauty

Advanced symptoms of Marssonina Blotch on Rome Beauty

What cultivars are susceptible?
According to the textbooks all of them. However in NC ‘Rome Beauty’ has historically been the hardest hit over the last two years. So far this season I’ve observed the disease in Fuji, Evercrisp, Rome Beauty, and perhaps Jonagold (not yet confirmed in lab).

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?

Previous studies have suggested that primary infection may actually occur as early as bloom continue approx. 4 weeks. Secondary infections do occur and may continue throughout the season depending on weather conditions.
What should I be spraying to control MLB?
In 2019 and 2021 we evaluated early season fungicide programs (pink-2nd cover) for their potential in controlling MLB in ‘Rome Beauty’. These would be considered the more critical apple scab fungicide timings. In 2019 (graph below) Inspire (Super) and Indar 2F of the DMI fungicides as well as Sercadis (1/2 of Merivon component), and Merivon were the most efficacious fungicides, but did not differ significantly from the captozeb program. Simply put, this suggests that regular Captan covers should provide solid control for MLB, but if we get an extended rainy period it, it would be wise to include one of the aforementioned products in your tank with Captan.
Incidence of Marssonina Blotch on 'Rome Beauty', 2019

Incidence of Marssonina Blotch on ‘Rome Beauty’, 2019

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:

Marssonina table field trial 2021

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….