The 2024 growing season was an amazing time for plant diseases. Spring and early summer weather was particularly wet, which was favorable for development of an array of fungal plant diseases. In addition, 2024 was a great year for insect activity, and certain insects that were prevalent last year (in particular aphids and leafhoppers) are notorious for moving pathogens from plant to plant. Of the diseases that I saw at the UW Plant Disease Diagnostics Clinic last year, the following three are diseases that are likely to recur and have a significant impact in 2025.
Last year was the best year for apple scab that I can think of in my 26+ years at the PDDC. The wet weather that occurred as apple and crabapple trees began to leaf out in the spring was perfect for Venturia inaequalis (the apple scab fungus) to infect. These early infections (characterized by dark, feathery-edged leaf spots) set trees up for an apple scab epidemic as wet weather continued later into the growing season. Many trees defoliated due to apple scab by mid-summer. The leaf debris from these trees is where the apple scab fungus has been hanging out for the winter. As soon as rains start this spring and trees begin to leaf out, spores from this leaf litter will lead to new infections, and we’ll be off to the apple scab races once again.
To limit apple scab development, remove and dispose of (by burning, burying, or hot composting) any leaf litter that is still hanging around your apple and crabapple trees. If you have not recently thinned your apple or crabapple trees, consider doing this soon (as long as the buds on the trees have not yet begun to swell). Thinning improves airflow that promotes more rapid drying of leaves when they get wet. Drier leaves are a less favorable environment for the apple scab fungus to infect. For details on pruning trees, check out “Pruning Deciduous Trees” available on the UW-Madison Division of Extension Horticulture website. There are also fungicide treatments that can be used for apple scab control (see the UW Plant Disease Facts fact sheet on “Apple Scab” for details). However, treating for apple scab requires multiple, carefully-timed fungicide applications, which can be difficult to achieve for many home gardeners. If you are planning on planting a new apple or crabapple tree in 2024, make sure you select an scab-resistant variety to avoid apple scab problems in the future.
Septoria leaf spot of lilac
After a hiatus in 2023 (most likely due to the hot, dry 2023 growing season), Septoira leaf spot of lilac was back with a vengeance in 2024. Lilacs all over the state had leaves that first exhibited spotting, then turned partially or fully brown starting at the bottom of shrubs and working up the plant. The culprit was a species of Septoria, a fungus related to (but distinct from) the organism that causes Septoria leaf spot of tomato. The browning caused by Septoria on lilac tends to be quite dramatic, but luckily, the disease is not lethal.
As with apple scab, good clean up of leaves that have fallen from affected lilac shrubs is the place to start in managing Septoria leaf spot. Routine pruning/thinning (see “Pruning Deciduous Shrubs” for details) to open up lilac canopies and promote rapid drying of leaves will also help keep this disease at bay. Use of fungicides (in particular chlorothalonil-containing products labeled for use on woody ornamentals) to manage the disease may also be useful. However, early applications (as shrubs begin to leaf out) and additional regular follow-up applications (if there is wet weather) are critical for control of this disease. Applying fungicides is a lot of work, and if you are unable to commit to the regular applications that are needed, then I recommend foregoing any applications at all.
Aster yellows is a phytoplasma disease that affects 300+ herbaceous plant species in 40+ plant families. Phytoplasmas are bacteria-like organisms that certain insects (most commonly leafhoppers) move from plant to plant. These insects feed in the phloem (i.e., the food-conducting tissue) of plants, and acquire phytoplasmas (and subsequently drop them off) as they feed. In 2024, high numbers of aster yellows leafhoppers (the specific leafhopper that transmits the aster yellows phytoplasma) led to an increase in aster yellows. Once infected, perennial plants remain infected, with the phytoplasma concentrating in plants’ crowns and roots in the fall, as plants go dormant for the year. The phytoplasmas survive the winter in these tissues and recolonize new growth as it emerges in the spring. Given the high incidence of aster yellows in 2024, I expect to see a lot of this disease in 2025. Typical symptoms associated with aster yellows are plant stunting; yellow, orange, or purple leaves; distorted leaves; leafy, green flowers; and brooming (i.e., production for clusters of plant parts).
Unfortunately, the only management for aster yellows is to remove infected plants. You don’t have to be particularly careful about disposing of these plants. Once plants have been dug up and have wilted, they are no longer attractive to leafhoppers (and other insects) that could potential move the pathogen to other plants.
Need help?
Hopefully, my predictions for diseases that will be prevalent in 2024 will be wrong. That said, you will likely encounter plant diseases of some kind this coming growing season. If you need help in identifying whatever diseases you do encounter or need advice on plant disease control, please feel free to contact me at (608) 262-2863 or pddc@wisc.edu. For more information on plant diseases in general and their management, be sure to check out the UW-Madison PDDC website (https://pddc.wisc.edu/). To keep up to date on clinic activities and resources, follow the PDDC on Facebook, Twitter (X), or Bluesky (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe).
Happy gardening, everyone!!
