As I commented in my March 2025 web article, the 2024 growing season was incredibly wet, and as a consequence, I observed an uptick in foliar diseases such as apple scab and Septoria leaf spot of lilac.  But, it wasn’t only leaves that had problems due to the wet weather.  A lot of last year’s rain ended up in the soil, and the resulting wet soil conditions contributed to an increase in root rots and related diseases. 

Rot roots

Root rots are caused by a wide variety of organisms including (but not limited to) Rhizoctonia, Pythium, and Phytophthora,.  Rhizoctonia is a true fungus, while Pythium and Phytophthora fall into a group of fungi-like organisms called the water molds.   Water  mold root rot pathogens tend to be particularly aggressive and also tend to be long-lived.  They produce thick-walled resting spores (called oospores) that can survive for years, if not decades, in the soil.  Under hotter, drier conditions, these spores germinate to form hyphae (i.e., fungal threads) that can directly infect plant roots.  Under cooler, wetter conditions, oospores germinate in a different manner, eventually producing numerous smaller, swimming spores called zoospores.  These spores swim to plant roots where they infect.  Production of zoospores significantly increases a water mold’s ability to infect, leading to an increase in the number of infections and subsequent disease severity. 

Root rot pathogen infections directly lead to root decay and deterioration.  Roots are typically discolored (e.g., brown, black), as well as soft and mushy.  This decay interferes with a plant’s ability to take up water and nutrients.  Indirect effects due to root rots include leaf symptoms that might be misidentified as being due to poor fertility (e.g., yellowing or reddening), symptoms that might be mistaken for drought stress symptoms (e.g., marginal leaf browning), wilting and branch dieback, and, ultimately, plant death. 

Once symptoms of root rots are observed, often there is little one can do to correct the problem.  Root rots are best managed preemptively at the time that gardens and landscapes are established.  Modifying soils to improve drainage (e.g., by adding organic matter), and choosing plants that are adapted to the soil moisture conditions at a particular site are two ways to prevent root rot issues.  There are potentially fungicides that can be used for management of root rots.  However, for these treatments to be effective, early diagnosis of the problem (i.e., before there is extensive root damage) is critical.  Fungicides, for the most part, do not cure existing infections, but help protect new roots from being infected.  Before applying fungicides, proper identification of the organism(s) involved in a root rot problem is extremely important; the fungicides needed to manage fungal vs. water mold root rot pathogens are quite different.  Often, plants have problems with both fungal and water mold root rotters simultaneously, so use of multiple active ingredients is needed for control.  Note that root rot fungicides typically are not readily available to homeowners, so contracting with a professional pesticide applicator to make treatments  will likely be necessary.  As you can imagine, using fungicides for root rot control can get very expensive, very quickly. 

Related diseases

A variant of root rots that I encountered several times last summer and have already encountered in 2025, is bleeding cankers on trees (e.g., maple, beech, birch).  Typical symptoms of bleeding cankers are a series of wet, oozy spots on tree trunks.  Sometimes these areas are near the soil surface; other times, the oozing occurs farther up on the trunks.  The most typical cause of bleeding cankers on common landscape shade trees is Phytophthora (which I mentioned above in the context of root rots).  When people contact me about bleeding cankers, I request that they send me blobs of the ooze and/or small bits of the ooze-soaked bark (about the equivalent of a 2 inch by 2 inch bark piece).  I test this material with a dipstick serological test for Phytophthora.  This test uses similar technology to that used in home pregnancy tests and home COVID-19 tests. 

The real fun begins if I get a positive reaction from my initial testing (and many of the samples I have received have tested positive).  At that point, I have to forward the sample to the WI DATCP Plant Industry Lab for further testing.  One of the species of Phytophthora that can cause bleeding cankers is Phytophthora ramorum.  This organism causes sudden oak death/Ramorum blight and is highly regulated at both the state and federal level, because of how destructive it can be.  I need to make sure that samples that test positive for Phytophthora are not infected with Phytophthora ramorum.  Phytophthora ramorum has occasionally be detected by WI DATCP in nursery stock brought into Wisconsin from other states.  However, Phytophthora ramorum has never been detected in Wisconsin in established landscapes plants.  Luckily, none of the samples that I have submitted to WI DATCP has tested positive for Phytophthora ramorum.  Typically, the Phytophthora species present in my samples has been Phytophthora cactorum, a common Phytophthora species that can cause root rots on a wide range of plant species.

Need help?

Hopefully, you will never have problems with root rots or bleeding cankers in your gardens and landscapes.  If you do however, I encourage you to submit samples for a proper diagnosis.  If you have questions about any plant disease that you are seeing or questions about sample submission, please feel free to contact me at (608) 262-2863 or pddc@wisc.edu.  For additional information on plant diseases 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). 

Keep on gardening, and have a great spring and summer!