During July and August, Crabapples, ornamental pears, and ash trees experience leaf loss (refer to the image below) due to infections caused by apple scab, apple rust, pear scab, and anthracnose. These fungal diseases target young leaves during the early spring. Regrettably, there are currently no effective treatments available to halt the ongoing leaf loss at this stage of the season.
The abundant rainfall during the wet spring and persistent precipitation throughout the summer created optimal conditions for the proliferation of these fungi, enabling them to disperse spores and infect the emerging, healthy leaves from the buds (refer to image 2). Once the leaves are infected, there is no cure available for the fungi. As the growing season progresses, the fungi continue to propagate within the leaves, consuming the living cell tissue. The spreading hyphae of the fungi result in the formation of lesions, causing dead tissue on the infected leaves and fruits (refer to image 3).
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Ultimately, when the entire leaf reaches the end of its life cycle, it undergoes senescence and detaches from the tree, eventually falling down (refer to next image).
Over the course of several years, this repeated leaf loss will affect the visual appeal of the trees (refer to images 5 & 6) and can compromise their overall health by diminishing the photosynthetic uptake. The fallen leaves also serve as a shelter and protective habitat for the fungi during the winter. Once winter passes, the fungi within the deceased leaves generate spores that disperse through air currents and water splashes, subsequently infecting new tissues. While this is the primary method of spore dissemination, infected leaves that remain attached to the tree and branches carrying infected foliage can also transmit the disease-causing agents during the following spring.
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Through plant breeding initiatives, numerous plant cultivars have been developed to possess “resistance” against foliage diseases. Several crabapple cultivars have been specifically propagated for their ability to resist apple scab. Typically, this resistance remains effective for approximately ten years, until the scab fungi adapt and overcome the resistant characteristics. It is advised to plant crabapples that are resistant to scab, although it is important to acknowledge that even these resistant varieties will eventually succumb to the disease as the scab fungi adapt over successive generations.
In addition to incorporating disease resistance through breeding, the primary method of managing these fungal diseases is by preventing their infection of the foliage. Removing fallen leaves from around the trees can reduce the dispersal of spores in the following spring, but it is insufficient to completely prevent infections in susceptible trees.
The most effective approach to prevention is the application of fungicides during spring. These fungicides create a protective barrier on the leaf surface, eliminating any spores attempting to germinate. In wet seasons, such as the current one, it may be necessary to repeat the treatments as new foliage emerges and rainfall keeps the new tissues moist. Unprotected foliage is susceptible to infection, and once a leaf is infected, there are no available treatments to cure the infection. It is essential to ensure thorough coverage of leaves with fungicides, and the timing of the applications is crucial for successful prevention.
The experienced plant health care professionals at Ostvig Tree Care have extensive expertise in treating these diseases over many years. They possess the necessary equipment and knowledge to protect trees of all sizes. They are also proficient in diagnosing various foliar disease problems that can affect trees in the Twin Cities area.
If you are concerned about leaf drop in your trees, it is recommended to contact an Ostvig Tree Care expert to identify the potential issues affecting your tree. While leaf drop due to spring fungal infections is common during this time of year, other factors can contribute to early leaf drop, some of which may pose serious risks to your tree’s health.
Dr. John E. Lloyd, Ph.D., serves as the President of Plant Health Doctors, LLC in the Twin Cities and can be reached through Ostvig Tree Care at info@OstvigTree.com.
