March 2024: Warming Wisconsin Weather

Rising Temperature IconI’ve been pretty amazed by the recent temperatures in Wisconsin.  Here in Madison, we hit 70°F in late February.  There have been some colder days this winter, but in general temperatures have been warmer than I remember, particularly compared to what I recall from when I moved to Madison in the mid-1970s.  The warm winter of 2023-2024 followed an extraordinarily warm and dry summer in 2023.  In the context our warmer weather trends, I’ve been trying to think of how plant diseases might be impacted.  Three diseases come to mind that likely could be more problematic in the coming years if warmer conditions continue. 

Southern blight  

This disease is by far the most destructive fungal disease that I see in home landscapes.  The fungus that causes Southern blight (Athelia rolfsii) has a wide host range, and virtually any herbaceous plant in the path of this fungus is likely to be killed.  The fungus can also kill small trees under the right conditions.  The first time I encountered Southern blight was about 20 years ago when it popped up in a flowerbed at the Allen Centennial Garden.  By the time I visited the garden to make a diagnosis, the fungus had killed off every plant in about a 40 square foot area.  As I examined the dead plants, I noted characteristic sclerotia (i.e., resting structures) of the fungus (they look like Osmocote® pellets) all over the bases of the dead plants and in a layer across the mulch in the bed. 

Early in my diagnostic career (i.e., the late 1990s), I occasionally saw Southern blight in Wisconsin.  However, because the pathogen is adapted to tropical and sub-tropical climates, it did not survive the harsh Wisconsin winters at that time.  Extended periods where high temperatures were in the negative digits were common, and these temperatures killed the fungus.  Nowadays, with our relatively mild winters, A. rolfsii seems to be better able to overwinter in the state.  That’s a huge problem because the fungus is easily moved around (via those pesky sclerotia), and if it is not killed by cold weather, then there is increased risk of spread and subsequent wholesale plant destruction. 

Oak wilt

Oak wilt has been present in Wisconsin for the entirety of my diagnostic career, and I have watched the Wisconsin Department of Natural Resources map its spread in the state over the years.  Historically in Wisconsin, sap beetles have moved the oak wilt fungus (Bretziella fagacearum) from location to location.  These insects are attracted to wounded trees and drop off B. fagacearum as they feed on sap in the wounds.  The fungus infects, leading to blockages of a tree’s water-conducting tissue, subsequent branch dieback, and eventual tree death.  Once an oak tree is infected, B. fagacearum can move to nearby oak trees through root grafts. 

I am concerned that our warming weather pattern could contribute to increased risk for oak wilt in a couple of ways.  Warmer summer temperatures are often associated with an increase the frequency and severity of summer storms.  More (and more violent) storms increase the risk of damaged oak trees, which increases the risk of sap beetles visiting these trees and dropping off the oak wilt fungus.  In addition, sap beetles are not the only insect that can move the oak wilt fungus around.  Certain oak bark beetles have been documented to move the fungus as well.  These bark beetles have traditionally had a more southern native range and have not been found extensively in Wisconsin.  However, PJ Liesch, the UW-Madison insect diagnostician, recently commented to me that Pseudopityophthorus minutissimus (one of the oak bark beetles involved in transmission of the oak wilt fungus) has been very abundant in Wisconsin over the last few years.  He’s seen lots of this insect in oak samples that have come into his lab.  How important oak bark beetles are in transmitting Bretziella fagacearum in Wisconsin at this time is not known.  However, they will likely become increasingly important as they become more established and abundant in the state. 

Thousand cankers disease

This fungal disease of black walnut trees has not be reported in Wisconsin to date; the closest state with confirmed reports is Indiana.  Efforts to prevent the introduction of the pathogen (Geosmithia morbida) into Wisconsin currently involve limiting importation of raw walnut wood into the state.  The hope is to prevent introduction of G. morbida, as well as the walnut twig beetle (Pityophthorus juglandis).  This insect moves the thousand cankers disease fungus from tree to tree.  P. juglandis is native to Arizona, California, and New Mexico, but in the last decade or so has been introduced into states east of the Mississippi River (e.g., Indiana, Ohio, Tennessee).  While movement of walnut wood currently poses the greatest threat for introduction of G. morbida and P. juglandis into Wisconsin, milder long-term weather patterns open up the possibility of eventual natural spread of P. juglandis (carrying G. morbida) into the state.  I think it’s only a matter of time before thousand cankers disease pops up in Wisconsin. 

As our long-term weather pattern changes, I’ll be on the lookout for new diseases and changes in established diseases in Wisconsin.  I suggest that you watch your own gardens and landscapes for new diseases that you have not encountered in the past.  You’ll likely see changes.  If you’d like help in identifying your new finds, feel free to contact the PDDC for help.  Check out the clinic website at for details on sample submission, or call (608) 262-2863 or email for advice and guidance.  To keep up to date on clinic activities and resouces, follow the PDDC on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe). 

Happy spring and happy disease hunting, everyone!