While 2023 was excessively hot and dry, Wisconsin’s spring 2024 weather has been particularly wet. That wet weather has come in the form of regular rounds of severe weather. The third week in May saw a particularly violent set of storms move through the state with heavy rains, high winds (including tornadoes), and hail. With these severe storms, there came extensive physical damage to many trees across the state. Some trees lost large branches, and others completely succumbed to the winds and toppling over. These were the immediate effects of the storms, but other delayed effects are yet to come. Here are three diseases that I suspect we will see an increase in over the summer in the aftermath of May’s severe weather.
Oak wilt
Oak wilt is a lethal fungal disease that affects oaks of all kinds. The disease most rapidly develops in oaks in the black oak group (those oaks with pointed-lobed leaves). White oak group oaks (those with rounded-lobed leaves) are also killed by the disease, although typically more slowly than red oaks. The fungus that causes oak wilt (Bretziella fagacearum) colonizes the xylem (i.e., the watering-conducting tissue) of infected trees, leading to blockages that prevent movement of water to branches. Branches wilt, and trees eventually die from the disease. Introduction of the oak wilt fungus often first occurs via sap beetles, which are attracted to wounded oak trees. Wind damage to oaks during our recent storms, high sap beetle activity, and the prevalence of the oak wilt fungus in oak trees across the state will likely lead to spread of the fungus and an increase of oak wilt this year. Watch for flagging (i.e., dying) branches on oaks, and if you see this symptom, consider submitting a sample to the PDDC for oak wilt testing.
Diplodia shoot blight and canker
Back in 2017, an EF3 tornado cut an 83-mile path through four northern Wisconsin counties (Polk, Barron, Rusk, and Price). The following year, I started receiving red pine samples from areas near where the tornado had traveled. The trees, which had survived the storm, started to exhibit extreme needle browning and eventual tree death. When I examined needles and branches from these trees, I noted extensive numbers of fruiting bodies (i.e., reproductive structures) of Diplodia, the fungus that causes Diplodia shoot blight and canker. Although these trees had survived the 2017 storms, they apparently were damaged by the high winds, and these wounds provided entry points for Diplodia. The fungus subsequently colonized and killed the trees. Given wind damage from our recent storms, I am concerned that we will see a repeat of this scenario in 2024. So, watch your red, Austrian, mugo and jack pines for needle browning and branch dieback. If you see these symptoms, Diplodia may be the cause.
Fire blight
The bacterium (Erwinia amylovora) that causes this disease is most commonly introduced into susceptible trees (e.g., apples, crabapples and pears) via honeybees. These insects pick up the bacterium when they feed on a combination of sap and bacterial cells that oozes from infected tree branches. Honeybees drop off the bacterium in the flowers of healthy trees as they pollinate. Alternatively, Erwinia amylovora can gain entry into trees via wounds created during stormy weather. In particular, I have seen an increase in fire blight after hail. During our recent stormy weather, certain areas of the state experienced quite large hail that did significant damage. These wounds could provide easy access for the fire blight bacterium. If you see rapid dieback of apple, crabapple, or pear branches over the next few weeks, be sure to contact me about testing for the disease. Early detection, followed by pruning to remove infected branches is critical to get this disease under control.
Need Help?
Hopefully, you will not see any of the diseases that I have described above. However, if you see evidence of these or any other plant disease problems, feel free to contact me at (608) 262-2863 or pddc@wisc.edu for advice. For more 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 or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe).