Apple Disease Update: May 14, 2020

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Frogeye leafspot on evercrisp

Apologies for the delayed post this week. Between the cold temps and paucity of good thinning this year, it seems like my hort. sci. colleagues were the ones in the hot seat this week. Nonetheless, there are a few disease updates/observations that I think are important to point out this week.

First-as far as normal maintenance applications go…The forecast next week looks like rain and daytime temperatures in Western NC  of 65-80 F are predicted. If there are ascospores of the GLS/bitter rot pathogen that are hanging out on tree litter on  the ground and have not yet been released, the rain will likely release them. Make sure your trees are protected. Since it also looks like we will have 4 to 5 days of rain in a row, you may want to consider a Merivon application no later than this Sunday (5/17) if you did not apply it yet this season or if you did not apply it last spray. Make sure to combine with either mancozeb (77 day PHI) or half-rate of captan for resistance management. If you did apply it last week, consider either captan + mancozeb OR mancozeb + Aprovia or Fontelis. If you go the captan + mancozeb route, just keep in mind you may have to go out and reapply once the rain ends.

Developing apple scab lesions on immature fruit

I’ve recently heard a bit of chatter that apple scab is not a big concern across the southeast this year because of the cool weather. As your apple pathologist, I beg you to please refrain from letting your guard down. At this time last year, we had 6 scab infection events, compared to this year in which 12 infection events have been forecasted. Certainly, both active ingredients in most SDHI/QoI premix products (e.g. Merivon, Luna Sensation) provide excellent scab control. Aprovia and Miravis (SDHI fungicides) have provided an outstanding level of scab control. If you are scouting and notice any infections, considering an application of one these products with either captan or mancozeb.

black rot on apple

Lastly, I want to bring your attention to Frogeye leaf spot (photograph at top of this post). For those of you who are not as familiar with Frogeye, it is caused by the fungus Botryosphaeria obtusa- the same pathogen causing black rot on your fruit (see photo directly above) and the pathogen we most commonly associate with black rot of the woody tissue in apple trees. So far, Cameo, Honeycrisp, and Evercrisp have been particularly hard hit.

The first symptoms of frogeye leaf spot appear as small purple spots. The lesions will expand as the disease progresses and remain fairly circular. More mature lesions become more of a tan color with a purple margin. Frequently, a dark dot is evident in the center of the lesion, which gives the lesion a frogeye appearance. Since this is the second season in a row I’ve seen higher than normal levels of diseases caused by B. obtusa, I want to alert you to the issue and make sure you are prepared to manage the disease. Captan, strobilurin (FRAC 11) fungicides, and thiophanate methyl are currently the best options. Sanitation measures such as mummy removal, pruning of dead wood, and limiting tree stress should also mitigate disease. If you have problems in some of your blocks this year, a fungicide program aimed at frogeye control should be addressed next year as green tissue begins to emerge.