Focus — Plant Diseases & Pests 2020
Four Fungal Diseases for Spring by Heather Prince
With a mild winter and cool spring, fungal plant
diseases are popping up in landscapes far and wide. As you assess client spaces, be on the lookout for these four fungal problems so you can be proactive in treatment. Fungal pathogens benefit from wet spring weather, and we chose two relatively new problems and two that regularly alarm clients every year. We consulted with university plant clinics in Wisconsin, Illinois, Michigan, and Indiana to provide a regional update on four problems that should be on your radar this spring. Although it seems to make the news every year, only a smattering of boxwood blight cases occur throughout the Midwest. With the prevalence of boxwood in our landscapes, however, it’s a good idea to be able to recognize this devastating pathogen as immediate treatment is key to slowing and stopping its spread. Blight has been reported on cut boxwood greens used for Christmas decorations, so there is the potential for inadvertent introduction into the landscape. Boxwood have taken a number of hits the past couple of years, and it may also be a good idea to look at alternatives. The challenge is finding something with as much versatility and formal appeal.
Boxwood blight (Calonectria pseudonaviculata) rep-
resents first as dark leaf spots that may have a slight ring pattern that progress to twig blight and rapid defoliation. Look for dead spots in hedges and individual plants and inspect plant parts. Boxwood blight isn’t the only fungal pathogen that can cause twig dieback or dead patches, and it’s easy to classify winter damage, volutella blight, macrophoma leaf spot, or phytophthora root rot as blight. Clients may also lump all boxwood problems into ‘blight’. The big difference is that boxwood blight causes extensive defoliation and rapid death. The other pathogens listed cause foliage to brown or go tan, hold on the plant, and are far less likely to cause plants to die. They are easily pruned out and controlled. Boxwood blight first presents as dark roundish splotches with yellow halos on leaves that may have white spores on the underside as the disease progresses. Rapid defoliation starts at the bottom and moves upward into the canopy. A key symptom that differentiates boxwood blight from other boxwood diseases is that narrow, black streaky cankers develop on green stems. During periods of high humidity, 10
white, fuzzy masses of numerous spores will develop from these black stem cankers and can be observed with a hand lens. This pathogen does not attack roots, but repeated defoliation and dieback from stem cankers can kill young plants quickly. If you suspect boxwood blight, send samples to your state plant clinic for confirmation. Once the disease is detected, good sanitation is critical. Remove and bag diseased plants and fallen leaves and dispose or bury them. Where permitted, you may burn infected plants. Do not compost infected plants or plant debris as it will not kill the pathogen. Research has found that the fungus can persist in the soil for five years or more, which (continud on page 12)
The Landscape Contractor June 2020
