Over the last decade or more, we have seen shifting weather patterns in Wisconsin. Winters, in general, have been milder. Precipitation of all forms (winter and summer) have been quite variable, with some years wetter than expected, others drier, and this variability dependent on geographical location within the sate. Also, the frequency of severe storms seems to have increased. What does this variable weather mean in terms of plant diseases? It is hard say exactly, but the following diseases are ones I tend to wonder about as our weather patterns evolve.
As the name implies, this disease has been traditionally considered more of a problem farther south in the US, where warmer temperatures are more favorable for the causal fungus (variously called Agroathelia rolfsii, Athelia rolfsii, or Sclerotium rolfsii) to survive and thrive. During my tenure at the Plant Disease Diagnostics Clinic (PDDC), I have occasionally encountered this disease, and the cases that I have observed have been devastating. The pathogen is extremely aggressive and has a wide host range (particularly among herbaceous ornamentals), so damage has tended to be widespread and severe.
My first encounter with Southern blight was back in the 1990s at the Allen Centennial Garden (ACG) on the UW-Madison campus. The fungus was likely introduced via transplants produced in warmer climates and then brought into Wisconsin for sale. By the time I was called to make a diagnosis, the Southern blight fungus had wiped out every plant in a roughly 40 square foot area in a prominent annual bed. The disease was relatively easy to diagnose, as the fungus produces copious numbers of small, spherical, tan resting structures (called sclerotia) that coat the stems of dead plants and the surrounding mulch. The staff at the ACG were deligent in removing the dead/dying plants, as well as soil in the affected area, to manage the disease. I think that our more bitterly cold winters at that time also helped in suppressing the pathogen, but even so, ACG continued to have issues with the disease for several years thereafter, likely due to spread of sclerotia to other areas in the garden.
With the warmer winter temperatures that we have experienced in recent years, I expect that the Southern blight pathogen will more readily survive in Wisconsin from year to year, which likely will lead to longer-term infestations and more severe outbreaks of the disease that are more difficult to manage.
Oak wilt is definitely not new in Wisconsin. I have been diagnosing this disease since I first started at the PDDC back in 1998. The fungus involved in oak wilt (called Bretziella fagacearum) colonizes the water-conducting tissue (i.e., xylem) of oaks and leads to blockage of water flow in the trees. This results in branch wilting and eventual tree death. Historically, in Wisconsin, the oak wilt fungus has been moved around by sap beetles. These insects visit infected trees where they feed and mate. After acquiring the oak wilt fungus, the insects are then attracted to healthy oak trees that have been wounded in some way (e.g., by storm damage, by pruning), and they drop off the fungus as they feed in the wounds. Further spread of the fungus can occur in oak groves via root grafts that form between the root systems of adjacent oak trees.
In more southern areas of the US, the oak wilt fungus has also been moved from tree to tree by certain types of oak bark beetles. These insects were not historically found in Wisconsin, because Wisconsin’s cooler conditions (both in the summer and winter) were not favorable for their survival and reproduction. More recently, however, certain oak bark beetles (in particular Pseudopityophthorus minutissimus) have been more consistently reported in the state. It is unclear exactly what impact oak bark beetles will have on the number of cases of oak wilt in Wisconsin, but their presence opens up the possibility of an increased number of infections and thus a greater impact of the disease in the state.
Canker diseases
With an increase in severe weather (e.g., hail, high winds), I am also concerned about eventually seeing an increase in a range of canker diseases on trees and shrubs. Canker diseases are caused by fungi and bacteria that locally infect branches and eventually girdle these branches causing branch dieback. Often these pathogens take advantage of compromised or wounded tissue to gain entry into a plant.
As an example, fire blight is a common bacterial canker disease of apples, crabapples, and pears. The causal bacterium (Erwinia amylovora) is most typically introduced into trees by honeybees. These insects acquire the bacterium from infected trees and then drop the bacterium off in the flowers of healthy trees as they pollinate. Alternatively, Erwinia amylovora is known to gain entry into susceptible trees via branches that have been damaged by hail or other storm-related damage.
Amongst the fungal canker diseases, Diplodia shoot blight and canker is one where storm-related outbreaks have been documented. In May 2017, a tornado traveled 83 miles through Polk, Barron, Rusk, and Price Counties causing severe damage in many areas. In the years that followed, red pine stands that were in the path of the storm began to develop outbreaks of Diplodia shoot blight and canker. Physical damage to the trees due to the storm likely provided entry points for the fungus, with the accompanying rain providing the wet conditions optimal for spores of the fungus to germinate and infect.
As our current weather pattern continues to generate more, and more violent, storms, I suspect that outbreaks of canker diseases will likely become more prevalent and cause increased damage in our forests, landscapes, and orchards.
At this point, my conjectures on potential plant disease changes are speculative, but I’ll be watching to see if any of my predictions are supported through by the diagnostic samples that are submitted through the PDDC. However, as I have joked in the past, if I truly had accurate predictive abilities, I would be on a beach somewhere, with drink in hand, having made millions on the stock market. All of this said, if you would like to keep abreast of my findings, and other PDDC-related resources and activities, consider following the PDDC on Facebook, Twitter (X) or Bluesky (search for “UWPDDC”), or subscribe to my clinic listserv, UWPDDCLearn (just email me at pddc@wisc.edu to get on the list). I provide routine updates on all of these platforms. Finally, you can always contact me directly for information by email or by phone at (608) 262-2863.
Have a great 2026 growing season!
