In 2024 we had an abundance of spring and early summer rains which led to a plethora of high-incidence plant diseases across Iowa. Because of the prior two years of drought in 2022 and 2023, we became a bit forgetful about what typical plant diseases in the garden and landscape we should keep an eye out for and how to manage them. Of course, managing plant disease begins with an accurate identification as not all management methods work against all diseases and some diseases cannot be managed once they appear. Let's talk about what the plentiful rains this spring could mean for some of the plants in your landscape and garden.
Rust diseases
We're already seeing foliar rusts show up on landscape and fruit trees this year. We've received one sample, numerous other queries, and seeing images on Facebook groups showing crown rust on fine leaf buckthorn this spring. This is not an uncommon disease, but I do suspect that spring weather has increased the incidence this year. Another rust that has already shown up is cedar-apple rust (note that there are other Gymnosporangium rusts that affect other Rosaceous hosts including but not limited to hawthorn, quince, and pear).
Rust-causing pathogens are quite complex in their disease cycle. Of note is that on the Rosaceous host, the fungus does not have a repeating cycle of infection. The spores that cause the symptoms you see came from the fungus growing on the cedar host. So, the symptoms on the Rosaceous host should not increase in incidence. Same for the rust on fine-leaf buckthorn. As such, there is no effective management tactic once the rust symptoms are observed in the landscape.
Management should focus on disrupting the disease cycle. If there is a cedar/juniper host nearby with galls, prune out the galls to help limit the amount of inoculum present. Fungicides can be useful, though are generally not warranted in landscapes, being more useful in fruit production systems. There are cultivars that offer resistance as well. For more on rusts, see Cedar-Apple Rust and Crown Rust Buckthorn.
Fire Blight (Apples, pears, and other Rosaceous species)
Fire blight is a bacterial disease that gets its name because the infected parts of the tree look like they've been literally scorched by fire. It is most known for infecting apple and pear trees, but can also infect crabapples, hawthorns, mountain ash, cotoneasters, quince, and other closely related trees. This time of year, we typically see black, wilted blossoms and flagging shoots. The reason this disease is so problematic, besides looking ugly, is the bacteria will keep moving inside the tree eventually leading to the tree death.
For management, remove any infected tissue 8-12 inches below visible symptoms as the bacteria has already moved further than you can see it. Sterilize any tools used with 10% bleach or 70% alcohol to kill the bacteria; it is easily spread through pruning. Let the pruned material dry out before moving it so the bacteria in the ooze doesn't accidentally spread further. While it is best to burn the infected wood, composting will take care of the bacteria as well. Unfortunately, most home garden tree sprays do not cure active fire blight infections, they are preventative when used at flowering. Check out our encyclopedia article on Fire Blight.
Other foliar fungal diseases in the landscape and garden
This season has been conducive to other types of fungi that cause many other types of leaf spots. While most leaf spots are not problematic, a few can be:
Black spot of roses
While many newer rose cultivars are bred to be disease resistant, heirloom cultivars can be very susceptible. This disease occurs after leaves stay wet for a prolonged period of time; which was common this past spring. Remove infected leaves promptly and determine if a chemical application is needed. Please visit our encyclopedia article on black spot of rose for more information:
Gray mold on strawberry
Caused by the fungus Botrytis cinerea, a fungus that can cause disease on many plants, this disease loves ripe strawberries and their leaves. On leaves, it appears as dead, brown patches and can spread rapidly to fruit causing mushy "moldy" strawberries. Please visit our article on strawberry pests in the home garden to learn more about gray mold and other common strawberry issues.
Below are links to some other foliar fungal diseases that could be bad in the landscape and garden this year. Each of these encyclopedia articles include information on symptoms and how to manage these diseases. Remember that effective disease management begins with accurate disease identification. If you are ever uncertain or want to double-check with someone, be sure to use resources at your local Extension office. Here are the articles:
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 9, 2025. The information contained within may not be the most current and accurate depending on when it is accessed.