What is causing the leaves of my fruit trees to turn brown and fall off by September each year?
For three years something has been causing the leaves on my ape trees to turn brown and fall off. This begins about the first of August. By late September some of the trees are leafless. It starts on the lower branches and works its way up. It also affects plum and pear trees. Would it be a fungus or an insect? The apples look normal but are much smaller.
There are a number of pests, pathogens, and environmental conditions that might be causing the defoliation you’re seeing, and without seeing photos or samples (obviously not possible this time of year), it’s tough to tell which of them might be affecting your trees. Apple scab is common in New England and is a likely candidate for the apple trees, but it typically doesn’t affect pears or plums (pears can actually suffer scab, but it isn’t the same fungal pathogen). So we may be looking at something else, or multiple causes being driven by common conditions.
Because the symptoms you’re seeing are limited to the leaves and not bark or fruit, I do lean toward a fungal issue of some sort. Last summer in particular was very wet and most trees in the state suffered a higher rate of fungal pathogens, including multiple rounds of infection because of environmental conditions conducive to recurrence. There was a good bit on this issue in this issue of the Maine DCAF newsletter last September–scroll down to “General Fungal Leaf Diseases” and “Unexplained defoliation of Maples and other trees.”
I am going to recommend you reach back out to use next season when you notice the leaf browning/drop–with photos and possibly samples, we should be able to diagnose what’s going on and recommend treatment. Until then, your best bet is to pursue best practices for growing the trees–sanitation (proper pruning for maintenance, pruning out dead portions, promptly removing any debris on the ground, removing leaves in the fall) in particular is important with fungal diseases, as they tend to overwinter in detritus near the trees. You might also consider getting your soil tested through the UMaine Soil Analytical Lab in the spring–changes in pH or nutrition can cause defoliation, as well.