brown spots on honeycrisp apple trees
I have 2 honeycrisp apple trees that are fairly young (got them 2 years ago as small trees). Last season, all of the leaves developed brown spots and basically looked dead by the end of the summer. I sprayed with neem oil at that time but figured it was too late. I had assumed the trees were going to die, but then they came back this spring and started out looking great, however now those same brown spots are coming back again. I sprayed with Bonide fruit tree spray about a week ago, before the spots were there (they just showed up a few days ago). Can anyone provide me some tips on what this could be and what I can do now, in the growing season to mitigate this issue? I originally thought it was apple scab until I read honeycrisp trees are resistant . Appreciate any feedback!
Montgomery County Pennsylvania
Apple scab is caused by a fungus, Venturia inaequalis. Understanding the fungus's life cycle is key to controlling the disease. The fungus overwinters on dead apple leaves and fruit left on the ground. When conditions (rain and temperature) are suitable spores are forcibly discharged up onto leaf and flower buds in the tree. There the spores germinate in the water film (from rain) if temperatures are suitable. Once the primary infection is established, a second type of infectious spore is produced which damages the leaves and fruit. In the fall infected leaves and fruit fall to the ground and the fungus forms protective capsules, which overwinter. If we have a very dry spring, apple scab is not much of a problem. Unfortunately, dry springs are rare in western Oregon.
Control of apple scab depends on 1. sanitation (removal of fallen leaves and fruit is very important to limit the source of the infection), 2. pruning to open up the tree to air drying, and 3. spraying with fungicides. Fungicides can be either protective or postinfective. A combination of the two can be used. The protective antifungicide is the most important. Spraying should start when flower buds swell. At least 2 sprays are recommended. Postinfective sprays have limited efficacy. Sprays for organic production include: fixed copper, Bordeau mixtures, copper soaps (copper octanoate), sulfur, mineral or neem oils and mycloburanil. All these products have pluses and minuses, so you should research the one which will work best for you. This article gives good information on organic methods of scab control, Apple Scab.
Honeycrisp apple trees are considered resistant (Apple Scab), at least in one references. I'm not sure what the problem with your tree is, but I would look at the roots because the tree doesn't seem to be getting needed moisture from the roots. You can dig down in a spot around the tree and see if the roots look healthy, firm and light colored. Scrape back part of the root to see if it's alive. If the roots are soft and dark you might want to remove the tree and plant another honeycrisp in a different area, since the original area is probably contaminated.
If your trees are planted within the last 2-3 years they probably need supplemental water in the summer. The rule of thumb is 1" water over the area once per week. The main goal is to get a good root system established, so watering helps keep the roots healthy. If your trees look healthy and are putting on good growth, your watering system is working.
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Portland Metro Area www.metromastergardeners.org