August: Free Plant Disease Testing Through the PDDC

While the Plant Disease Diagnostic Clinic (PDDC) typically charges a small fee for processing plant disease samples, there are certain diseases where diagnoses are performed for free.  These diseases typically fall into four categories:

Diseases that have not been documented in Wisconsin, but should they be introduced could cause serious damage, and thus early detection is critical for proper management

The disease that currently falls into this category is thousand cankers disease of walnut.  This disease has been, for all practical purposes, lethal on black walnut wherever it has been found.  The pathogen involved is a fungus (Geosmithia morbida) that is transported by and introduced into walnut trees by the walnut twig beetle.  Neither the fungus nor the insect has been found to date in Wisconsin, but I am watching carefully for both.  If you see declining walnut trees with yellow leaves and a thin canopy, particularly with small, pin-sized holes on larger diameter branches, get a sample to the PDDC.  Invoke the words “thousand cankers disease” and the diagnosis is free.

Newly introduced diseases of regulatory importance where documenting how widespread these diseases are can be important for eradication and limit of spread

Boxwood blight falls into this category.  This disease is incredibly destructive to boxwoods (lethal in many cases) and was first documented in southeast Wisconsin in late July of this year.  Typical symptoms include black spots on leaves and stems, progressing to defoliation, dieback and oftentimes shrub death.  Unfortunately, boxwoods tend to be prone to a variety of dieback issues (winter burn being the most common), so I have been telling everyone for several years, “If your boxwood has branch dieback, send a sample to the PDDC, invoke the words ‘boxwood blight’ and get your free diagnosis.”  Submissions of these samples are even more critical now that the disease is in the state to figure out how widely distributed this disease is.

New diseases to that state that aren’t necessarily of serious concern (at least at this point), but should be documented to keep track of their distribution and/or provide samples for researchers

Corn tar spot falls into this category.  Many of you are likely familiar with tar spot of maple.  This tree disease (caused by the fungi Rhytisma americanum and Rhytisma acerinum) has been common in Wisconsin for years.  However, in 2016, a visually similar disease (caused by the fungus Phyllachora maydis) was found on corn.  At this point, corn tar spot has not been a particularly serious disease, but Diane Plewa (my diagnostic counterpart at the University of Illinois) is studying this disease as part of her PhD research and is interested in obtaining samples of the disease from as many locations as possible.  So, if you see corn tar spot in Wisconsin, fill out a corn tar spot survey form, send the form and your sample to the PDDC for a free initial ID (including your complete mailing address so I can get a report to you) and I will forward the sample to Diane.

Diseases of extreme economic importance that recur every year. 

Late blight of tomato and potato falls into this category.  Wisconsin is the third largest grower of potatoes in the US and also has a thriving fresh market tomato industry.  Late blight can be devastating to both (it did cause the Irish potato famine of the 1840’s and 1850’s).  Knowing when the pathogen (Phytophthora infestans) arrives and perhaps even more critically which type(s)/variant(s) of the pathogen is(are) in the state (there are many) are important for choosing appropriate fungicides for control.  Every year I offer free late blight testing, and I forward positive samples to Amanda Gevens, the UW-Madison/Extension vegetable pathologist for typing.  Most of the time late blight is not the problem (typically Septoria leaf spot or early blight), but better to be safe than sorry.  Get those tomato and potato samples in for your free diagnosis.  The first report of late blight for 2018 just came through this week.

Be sure to take advantage of the PDDC’s free services while they last, but also remember that the PDDC needs paying samples as well to help fund the clinic’s operation.  So submit early and often!  As always, if you have questions about plant diseases and their management, or PDDC activities and services, feel free to contact me at (608) 262-2863 or