Growers and landscapers are concerned about boxwood blight and are determined to prevent its spread.
Dr. Nina Shiskoff, Foreign Disease Weed Science Research Unit, USDA-ARS, admitted managing boxwood blight is complicated. “The pathogen can be spread on tools, by water splash, wreaths, vehicles, boots, infested litter or animals,” she said. “Another complication is that if you want to control disease, you have to control all the life stages of the pathogen.”
Shishkoff explained that the infective organism can grow as mycelia, or tiny tubes of protoplasm that spread through plant tissue. It also develops microsclerotia – thick-walled clumps of cells embedded in leaves, roots and stems. Boxwood blight also spreads via spores that form on the surface of diseased plant tissue that can detach and spread. Discarded litter can contain all three stages of the organism.
“Some pathways of spread are dominated by the microsclerotia, others by spores,” said Shishkoff. “There are limited circumstances where mycelium is important, particularly when propagating cuttings when symptomless plant tissue may be present without you being able to tell.”
Another potential complication in managing the spread of this disease is distinguishing between the two species of fungi that cause boxwood blight: Calonectria pseudonaviculata, the pathogen currently in the U.S., and C. henricotiae, which is currently in Europe but could be detected in the U.S. at any time. Shishkoff’s research focuses on the behavior of both pathogen species in order to be prepared if C. henricotiae enters America.
Shishkoff was interested in what happens to the pathogen under natural conditions. Previous research showed that spores persisted for three to four weeks in soil, while microsclerotia persisted at high levels for up to 40 weeks. Pathogen survival is influenced by soil depth and sun exposure.
“The pathogen was knocked down pretty well if it was on the surface of field soil in full sun,” said Shishkoff. “Sanitation is necessary to get rid of the pathogen.”
When an extensive greenhouse renovation took place at the plant laboratory, Shiskoff used that time to determine whether thorough steam cleaning would eliminate experimental boxwood blight pathogens from the greenhouse. She placed leaves and twigs of samples infected with both pathogen species and isolated microsclerotia in the bench and gravel area to be steamed.
“We steamed from 149º to 210º for 24 hours and both pathogens were killed at all depths in the gravel,” she said. “Steam treatment can be very effective.”
Another project exposed spores of both species to various temperatures. At 113º, spore levels were reduced within 10 minutes. At 122º to 126º, six minutes of exposure was enough to kill spores.
“That’s fine if you’re looking at isolated microsclerotia,” said Shishkoff. “The data show an infested cutting with spores sticking to the surface would be cleaned up with hot water treatment. The real question is whether an infected cutting could be cured of the pathogen.”
To answer this question, four boxwood cultivars were exposed to 113º and 118º, which Shishkoff said are typical temperatures for hot water treatments. After three months, root production was studied to see how many roots were produced by cuttings.
Boxwood blight. Photo courtesy of David L. Clement, University of Maryland, Bugwood.org
“If cuttings were treated for 30 minutes at these temps, there was not a great effect on three of the four on the amount of root production compared to controls,” said Shishkoff. “But in 60 minutes, the plants were pretty much cooked. More work is necessary to determine how much time cuttings can tolerate higher temperatures that would affect the pathogen.”
For chemical sanitation, Shishkoff focused on microsclerotia. “I looked at different modes of action for sanitizers to see if any would be effective against microsclerotia,” she said. “I isolated microsclerotia, placed them on filter paper and cut the paper into squares, then put them in mesh bags and dipped the bags in the sterilant for different times. I rinsed them well to remove the sterilant, then plated them on media to see how many survived treatment.”
Shishkoff found that Green Shield® used at the labeled rate was ineffective after over an hour of exposure. SaniDate® and bleach were somewhat effective, but 70% ethanol and ZeroTol® were highly effective. ZeroTol was used at the maximum labeled rate for unclean surfaces and work areas.
Shishkoff said 60 minutes is too long to use a sanitizer, so she looked at ZeroTol for 15 minutes at the labeled rate for pots and tools. She also included Lysol, which was completely ineffective over 15 minutes. Bleach and SaniDate were slightly effective, but ethanol killed microsclerotia within five minutes.
“Peroxides are tricky because different forms have different efficacies,” said Shishkoff. “We tried various vaporized hydrogen peroxide generators, and they would not kill microsclerotia under the instructions for using the machine. Some peroxides under some formulations work, others do not.”
Alcohols as disinfectants have both positives and negatives. “They don’t leave any residue on treated items,” said Shishkoff. “An aqueous solution of ethanol can be used on skin; it’s dehydrating but not toxic. It can be used on work surfaces and benches. However, alcohols are volatile and flammable, cannot be used near open flames and must be stored properly to prevent them from volatilizing.”
When used with caution, alcohols are good disinfectants for most surfaces but can harden rubber and dissolve some glues.
What about diseased litter on equipment and boots? “Create a cleaning station away from plants where you can rinse off boots and tools,” said Shishkoff. “Make sure leftover debris isn’t going to continue to produce spores. After rinsing boots and tools, you can spray with an alcohol solution or another effective sanitizer to be doubly sure it’s sanitized. When doing landscape work or in a nursery, you can wear a Tyvek suit and properly dispose of it or spray it down if it must be reused.” Litter can be torched, composted and raked, then properly disposed.
Infected litter in compost must reach a killing temperature to be effective. “The temperature is what releases ammonium,” said Shishkoff. “There should be a tarp over the compost to keep the ammonium in.”
For litter under boxwood actively producing spores and potentially splashing spores onto plants, there’s no good sanitation method. However, mulch under boxwood will bury infested weeds and litter and prevent splash.
For current information on boxwood blight management, visit boxwoodhealth.org.
by Sally Colby
