Boxwood blight has been confirmed in at least half the states in the U.S., and it’s spreading. The Horticultural Research Institute (HRI) and the Boxwood Blight Insight Group (BBIG) hosted an informational webinar on the disease, featuring Dr. James LaMondia, Connecticut Agricultural Experiment Station, who explained the challenges and offered management tips.
LaMondia said for boxwoods in the landscape, management is for minimal disease while aiming for 100% control. In nurseries, management is for zero tolerance.
“When boxwood blight first occurred, I was involved in screening fungicides to control the pathogen,” said LaMondia. “We incorporated fungicides in media and exposed the pathogen in different concentrations and followed with testing on plants.”
Testing isn’t specific for any single stage of the fungus because it grows as fugal hyphae. “When there’s enough growth and it’s ready to spread, it produces masses of sticky cylindrical spores that are held together in a gooey mass,” said LaMondia. “They also have survival structures, or microsclerotia, which are masses of resistant hyphae that are long-term survival structures. Different life stages have different susceptibility to fungicides.”
As new fungicides have become available, the BBIG has developed a list of effective materials in a variety of FRAC groups representing different modes of action. Some treatments are systemic while others are protectants.
In experiments to determine the spray application intervals of fungicides, LaMondia recorded the protectant qualities of systemic products at two-week intervals and achieved close to 100% control at two weeks. With applications at three-week intervals, control treatments were less effective.
Subsequent research involved interactions of fungicide treatments, variances among boxwood cultivars and level of protection over time. “Fungicides gave us longer periods of protection, both in infection and sporulation,” said LaMondia. “If we have a more susceptible variety, we’ll have to spray more often.”
When LaMondia tested treatments, he pulled microsclerotia from infected plants and exposed them to different fungicides. He found that microsclerotia are resistant to environmental conditions as well as fungicides.
“The biggest challenges with fungicides are coverage, area and time,” said LaMondia. “It’s very difficult to get complete coverage throughout all the leaves and interior of the plant, on and into the boxwood tissue. We also have to protect over a long period while the infection period might range from spring through fall.”
He said fungicides should be applied when there’s a risk of spread, when moving plants and at pruning.
LaMondia studied conidia dispersal by placing a drop of water on a leaf with conidia present then blowing on the water to force conidia movement. Knowing how the causative pathogen is dispersed is important because it dictates means of controlling the pathogen.
“Long-distance dispersal is by way of infected plants,” said LaMondia. “Conidia don’t become airborne either singly or in sticky clumps, but they do disperse readily in water.” He added that water splashed from sporulating leaves is why epidemics occur in wet weather.
“There’s been a lot of study of dispersal of pathogens in strawberries and other crops,” said LaMondia. “Typically, [travel] is less than one foot. Splash doesn’t go very far unless there are extreme conditions like hurricanes. Mechanical dispersal can also happen – moving through plants when they’re damp or wet, equipment contact, moving plants or pruning.”
To minimize long-distance dispersal, LaMondia suggested growers destroy infected plants, isolate incoming plants, have plants inspected and perform diagnostic testing.
Cultural controls involve awareness of irrigation type and frequency, temperature, moisture and plant spacing. Pathogen spread is more likely with rain or irrigation, infected debris, mulch and dropped leaves from infected plants. The critical conditions for infection are eight to 12 hours of continuous leaf wetness and temperatures from 64º to 80º F.
Incidence of disease varies by cultivar and region. “In Oregon there are isolates that infect different cultivars at different temperatures,” said LaMondia. “There are slight differences in epidemiological effects of pathogens in different locations.”
Researchers are developing a predictive model for timing of fungicide applications and pruning. “Studies in Virginia and Connecticut have shown that mulching significantly reduced infection from soil-borne inoculum,” said LaMondia. “Mulching also reduced plant-to-plant spread within a landscape planting. The mulch acts as a physical barrier – spores and leaves drop down into mulch, which reduces splash dispersal and makes it less likely the disease will move farther.”
Sanitation is a key factor in reducing the spread of boxwood blight. The challenge is effectively killing conidia, which is the stage at which disease is most likely to spread. LaMondia tested many sanitizers and found conidia were sensitive to all those tested.
Pruning in dry conditions is not sufficient to prevent disease spread. “Think about disinfecting equipment and using fungicides before or after,” he said. “The DMI fungicides inhibit sporulation, so even if there is infection, if they are not producing spores, they won’t spread.”
Avoiding susceptible cultivars is an important management method. “Resistance is often the most effective disease control,” said LaMondia. “We’ve found species and cultivars differ in susceptibility, and some differences are striking.”
Buxus species and accessions vary in susceptibility to boxwood blight. Differences in infection efficiency, number of lesions from a certain number of spores on the same plants, length of the latent period, time until sporulation, lesion size and number of conidia produced are all factors in susceptibility. Plant best management practices include exclusion, not introducing infected plants, good sanitation, using cultural and fungicide controls, plant spacing, mulch, reducing overhead irrigation, appropriate use of fungicides and using advanced diagnostics to determine which plants are infected.
“We have some plant resistance now,” said LaMondia. “I think it’s only going to get better over time. When we combine all of this with the epidemiological risk models, we’ll know when we need to use management tactics.”
More information on the BBIG and its efforts are available at boxwoodhealth.org/objectives-and-approach.
by Sally Colby