Late Summer/Fall Leaf Spots
Leaf spot diseases caused by fungal pathogens, and in some cases, by bacteria, are most common in the late summer and fall on landscape trees and shrubs and in forest settings.
As trees cease active growth in late summer or fall, fungal pathogens take advantage of lower resistance to fight infection. At the same time, fungal and bacterial pathogens multiply and spread very quickly from one tree to another due to favorable temperature and humid weather conditions.
Depending on the weather variables (i.e., excessively rainy and humid weather, unseasonably cool temperatures, or high heat) and initial infection timing, diseased leaves can defoliate, or fall off, earlier than normal. The main danger of losing leaves earlier than normal is that tree can produce less food energy through photosynthesis that is stored in the root system for next year’s growth.
If leaf spot disease kills too many leaves on a recurrent basis, the tree's health can gradually decline. If the problem can’t be corrected over many years, the tree may even die, although it is kind of rare.
Leaf spot disease is a concern to many homeowners as it is aesthetically unacceptable and creates uncertainty about the outcome for the tree. Early defoliation and spots (Figure 1) on leaves also can compromise fall color displays of deciduous landscape trees such as maple, oak, horse chestnut, sycamore, etc.
Figure 1. Common leaf spots on maple a) Phyllosticta leaf spot (Photo credit: MM Rahman); b) Tar spot (Photo credit: https://www.wpr.org/wet-weather-increases-tar-spot-disease-maple-treesa)
How to separate leaf spots from other fatal tree diseases?
As mentioned above, leaf spots in most cases are not fatal to the tree. However, a few other diseases, such as Verticillium wilt of maple, oak wilt or leaf scorch can kill a tree within a very short time.
If leaf spots are properly diagnosed at the early stage of infection, they should appear as individual spots with necrotic tissues, usually with gray centers and brown margins. As the spots enlarge, they can merge to kill large areas, yet individual spots should be prevalent in other areas.
In contrast, wilt disease or leaf scorch will generally show leaf areas dried up, starting from the edge or tips and progressing inward (Figure 2). Verticillium wilt doesn’t have any recommended treatment options, but oak wilt can be treated by injecting the tree with fungicides, such as Alamo, if detected early enough.
Figure 2. Oak wilt symptom on red oak (Photo credit: https://www.vtinvasives.org/news-events/news/oak-wilt-an-aggressive-disease-kills-thousands-of-trees)
What can be done for managing leaf spot?
The pathogen that causes leaf spot disease survives on infected fallen leaves from one growing season to another. Lots of spores are produced and released in the air and can cause new infection depending on the host’s susceptibility and conducive environmental conditions. Thus, raking up and discarding fallen twigs, branches and dead leaves in trash bags as soon as they fall or at the end of the growing season is an effective way to ensure removal of infection source and reduce leaf spot diseases in the next season.
Healthy trees are better able to withstand diseases. Fertilizing trees every spring and irrigating as needed, especially during a prolonged period of dry weather, should support healthy growth of trees to minimize stress caused by leaf spots.
In most cases fungicides are not needed or recommended for controlling leaf spots, as it may not be feasible to try to spray on large trees. However, if it is a recurrent problem and trees are too small to tolerate leaf spot pressure, fungicide use may be necessary. There are several fungicides, including thiophanate methyl, chlorothalonil, ferbam and mancozeb, that are registered for use against leaf spot. Users are suggested to decide on feasibility and follow label directions for dosage rates and safety precautions.
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