Why are There Holes in My Cheese?

Do you ever wonder what is causing your cherrylaurel to look like Swiss cheese? Clearly, it’s not because Anatole has been hanging around. More than likely the shothole symptoms you see are caused by one of several pathogens and abiotic stresses common to other species of Prunus.

Shothole symptoms in cherrylaurel. Photo: Richard Buckley, Rutgers PDL

The bacterial pathogens Pseudomonas syringae pv. syringae and Xanthomonas arbicola pv. pruni have been implicated as possible causes of shothole “disease.” Several fungi are also known to cause similar symptoms in Prunus spp. including: Cercospora, Blumeriella jaapii, and Wilsonomyces carpophilum.

Bacterial leaf spot? Photo: Richard Buckley, Rutgers PDL

Symptoms of all these diseases are similar. Leaf spots are characterized with circular to irregular margins that are often surrounded by a reddish border or with a yellow halo. Abscission layers quickly develop around the spots, which causes the injured tissues to drop away, leaving holey and tattered leaves.

Early spring leaf spots in cherrylaurel appear to be caused by a bacterial pathogen. Photo: Richard Buckley, Rutgers PDL

In our laboratory, the fungal diseases, particularly the disease caused by Blumeriella jaapi, are more commonly diagnosed on cultivated (ex: Prunus cerasus) and ornamental cherries (Prunus serrrulata) rather than cherrylaurel (Prunus laurocerasus).

Abscission layers form and spots drop out. Photo: Richard Buckley, Rutgers PDL

As for the cherrylaurel samples, we have occasionally diagnosed the bacterial disease caused by Xanthomonas arbicola pv. pruni. Quite frankly, proof of the presence of that pathogen is often scant and an accurate diagnosis is nearly impossible to obtain once the injured tissues drop from the leaves. These “diagnoses” are often qualified as “tentative” and as such, we shy away from recomending pesticide interventions. A major complication here is that cherrylaurel commonly shows shothole symptoms resulting from unidentified cultural or environmental stresses. Watching the plants in my neighborhood the past couple weeks (these photos), I am beginning to wonder if most of the shothole we see in New Jersey cherrylaurel plantings isn’t simply due to winter injury.

“You are so ugly, I can stick your face in some dough and make gorilla cookies.” Cherrylaurel has become another plant I love to hate! Photo: Richard Buckley, Rutgers PDL

Remember that after the tissues drop, it is difficult to impossible to determine specifically what caused the problem. Rapid submission of damaged leaves to a diagnostic lab, before the spots drop out, is our best hope for an accurate diagnosis. Once the diseased tissues disappear, “unknown physiogenic disorder” might be all you get out of your local cheese taster!