Earlier this year, one of my colleagues called me “giddy” as I was happily expounding about all of the diseases that I was seeing this year due to our wet weather.  The pinnacle of my euphoria came this month when I received a sample of one of my all-time favorite diseases, zonate leaf spot.  This disease is caused by several species of the fungus Cristulariella/Grovesinia, and occurs on a wide range of plants.

My first encounter with zonate leaf spot was back in 2018.  Interestingly, I received two samples of the disease on the same day that year, from two different Wisconsin counties (Marathon and Buffalo), and on different hosts (maple and grape).  I subsequently saw the disease in 2019 on industrial hemp from Langlade County, and the sample I received this year was another maple leaf sample originating in Dunn County. 

In my initial encounter with zonate leaf spot in 2018, a UW-Extension county educator sent me photos of maple leaves with symptoms (roughly round spots with a pattern of concentric rings) that were typical of what I had seen in online photos of the disease.  I REALLY wanted to see a physical sample (and offered to do the diagnosis for free), as zonate leaf spot had been on my plant disease “bucket list” ever since I first saw drawings of the reproductive structures of Cristulariella/Grovesnia (called conidiomata) in one of my reference books.  The Extension educator was quite accommodating. 

Once the physical sample arrived, I again noted typical zonate leaf spot lesions.  I scanned the lesions under my dissecting microscope and relatively quickly found conidiomata of the pathogen.  These structures are HUGE, are multicellular, and are tannish and pyramidal in shape.  Quite frankly, they look like tiny, tan Christmas trees popping up from the leaf surface.  If you saw these (and they are readily visible using a 20X hand lens), you might think they are insect droppings of some kind.  I ended up incubating the maple leaves in a moist chamber for several days, hoping to stimulate additional sporulation.  Interestingly, the fungus eventually formed black sclerotia (resting structures) that were readily visible to the naked eye.  In the recent maple sample that I received, I immediately found both conidiomata and sclerotia as soon as I pulled the sample from the bag. 

When I encountered zonate leaf spot in 2019 on hemp, I decided to go a step further in the identification of the fungus, as I was not able to find any reports of zonate leaf spot occurring on this host plant.  I plucked one of the conidiomata from the leaf surface and placed it on the center of a petri-plate filled with a potato-based growth medium.  The grew quite quickly on this medium.  Sue Lueloff, the molecular diagnostician in my clinic at the time, was able to able obtain a DNA sequence for the fungus, which matched sequences from known isolates of Grovesinia moricola. 

While I tend to get very excited when I see zonate leaf spot, as a home gardener, there’s not much to worry about.  This disease is similar to most other, more common leaf diseases in terms of management.  You begin by making sure to collect up and dispose of any leaves from infected trees after they drop in the fall.  The leaves can be burned (where allowed), buried, or hot composted.  Maintenance pruning of susceptible woody species can help with control of the disease as well.  This pruning opens up the plant canopy and improves airflow, which helps speed drying when leaves get wet.  This drier environment is less favorable for Cristulariella/Grovesnia to infect.  To be honest, zonate leaf spot does not appear to be a serious disease in most instances, so don’t panic if you think you see the disease.  Just let me know if you do, as you will make my day!  

If you have questions about zonate leaf spot (or any other plant disease) feel free to contact me at (608) 262-2863 or pddc@wisc.edu.  As always, 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). 

Go forth and look for supercool, bizarre diseases.  They are out there waiting for you to discover!