Thursday, October 22, 2020

Marssonina blotch

Recently I visited a block of EverCrisp apple trees in an orchard in the hilltowns of western Massachusetts west of the Connecticut River. I went to look at what is probably the largest planting of EverCrisp apples in Massachusetts. A few acres, with more in the works to be planted. The trees are 3-4 years old and on Geneva 41 rootstock. A couple of observations and even more questions.

First, crop load management is essential as EverCrisp can go somewhat biennial if over-cropped. And apple quality is not what it should be on over-cropped trees. What is the best crop load (number of apples) and chemical thining recommendation for EverCrisp?

Second, EverCrisp appears to be quite susceptible to the fungal disease Marssonina coronaria causing the symptom Marssonina blotch. Now, the big question is how important is it to keep this disease under control until the fall harvest? These EverCrisp had not been treated with a fungicide in well over a month, and groups of trees showed signifiacnt Marssonina blotch. Even some partial defoliation. The grower acknowledged that Marssonina blotch has been observed on these EverCrisp trees in the past. A standard fungicide program for apple scab -- that includes Captan and mancozeb fungicides, because it appears these fungicides have good activity against Massonina -- should keep it at bay for the majority of the growing season. Slacking off on fungicide applications towards harvest, however, can result in Marssonina blotch becoming rather "ugly." So my questions include:

  1. Will letting the disease build up -- it overwinters in leaf litter on the orchard floor -- make it more difficult to control in future years? (Remember, sanitation is a basic tenet of plant disease control.) 
  2. What are the ramifications of late-season "leaf blotch" and partial defoliation on tree health and productivity?
  3. What fungicides have best action against Marssonina, and how late into the summer or early fall should fungicide application continue?
  4. What weather conditions are most favorable for Marssonina infection? Surely moisture is an essential ingredient, and in fact, RIMpro has a Marssonina coronario infection risk model. Migh be worth heeding.

I know some of these questions are currently trying to be answered by University researchers in the Northeast, but Marssonina is a relatively new apple disease here and EverCrisp appears particularly susceptible. Both it's parents, Honeycrisp and Fuji, are known to be susceptible to Marssonina. So keep an eye out on those EverCrisp blocks!

Marssonina leaf symptoms on Evercrisp apple, 20-October, 2020

Under magnification, dark spots are diagnostic for Marssonina

Typical Marssonina "hot spot" in EverCrisp trees on 20-October, 2020; trees in background are less afflicted; defoliation of heavily diseased trees is occurring

RIMpro Marssonina model for the UMass Orchard, Belchertown, MA

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