A fungus that emits gaseous compounds is being tested as a fumigant for quarantine treatments of apples. Dr. Lerry Lacey, entomologist with the U.S. Department of Agriculture in Yakima, believes the fungus Muscador albus has potential as a biofumigant that would be safer and more environmentally benign than methyl bromide fumigation. It could also be a component of a systems approach for preventing export of live codling moth with fruit. Detections of live codling moth in apples shipped to Taiwan can close the market.
M. albus grows on the limbs of cinnamon trees and produces a mixture of volatile compounds that are toxic to some plant pathogens, rot-causing organisms, plant parasitic nematodes, and insects. Lacey has been testing the effects of the fungus on codling moth adults and larvae in simulated storage conditions.
For his research, he used a formulation of the fungus developed by AgraQuest, Inc., of California.
Newly emerged codling moth adults were placed in chambers where they were exposed to M. albus for three, four, or five days. On average, the M. albus treatment resulted in 93 percent mortality, versus 26 percent in the control. All the codling moth adults were dead after the five-day treatment, although many of the untreated moths also died because of their naturally short life span and the possible stress associated with handling and confinement.
In experiments with newly hatched codling moth larvae, 87 percent of the M. albus-treated larvae died after three days of exposure, compared with only 9 percent of untreated larvae.
Larvae that had burrowed into apples were slightly less susceptible to the biofumigant. After three days of treatment, 71 percent of the treated codling moths were dead, versus 9 percent of the control larvae.
Cocooned diapausing larvae were exposed to the fumigant for seven or 14 days. The seven-day treatment resulted in 31 percent mortality. None of the larvae survived the 14-day treatment.
Lacey concluded that fumigation with M. albus during the first few days of storage at a relatively warm temperature (24°C, 75°F) might be adequate to control adults and newly hatched larvae, but longer treatments would be required for larvae infesting apples and diapausing larvae.
As the fungus grows, it generates carbon dioxide. In all the M. albus treatments, carbon dioxide levels increased and oxygen levels decreased in the chambers as the fungus grew over time, whereas levels in the control treatments were unchanged. Lacey said that M. albus was clearly the primary cause of mortality of adults and young larvae. However, in the longer treatments for the diapausing adults, the contributing effect of high carbon-dioxide levels cannot be ruled out.
If M. albus is to be used to control codling moth in stored or packed fruit, more research is needed on the effect of prolonged exposure to high concentrations of carbon dioxide and the M. albus volatiles on apple quality in sealed cartons at shipping temperatures (between 2 and 7°C, 35.6 to 44.6°F), Lacey said.
Lacey reports that it’s not yet known which of the fungus’s volatile compounds (a mixture of alcohols, esters, ketons, acids, and lipids) are responsible for its insecticidal activity. If the active volatiles can be identified, the use of one or more of the most active compounds against insects would enable control of codling moth without growing the fungus and generating carbon dioxide, he believes.
The systems approach to achieving quarantine security of exported apples capitalizes on cumulative mortality from multiple types of control. Lacey believes that fumigation with M. albus could be a component of this strategy to prevent live codling moths from being exported. In addition, M. albus could contribute to the control of disease organisms that happen to be present.
In the experiments with diapausing larvae, a significant amount of the Penicillium fungus, which causes blue mold in apples, grew in the control chambers. No Penicillium growth was seen in the chambers treated with M. albus. Previous research by Dr. Ali-Akbar Ramin in Iran and Dr. Wendy Schotsmans in New Zealand showed that M. albus could control Botrytis cinerea (dry eye rot, or blossom end rot) and blue mold in apples, with no effect on fruit quality.
Research on the effect of temperature and dosage on the efficacy of M. albus for control of gray mold on table grapes has been conducted in California. Dr. Franka Mlikota Gabler used two temperatures—5°C and 20°C (41°F and 68°F)—in her experiments. The biofumigant was more active at the higher temperature, but higher doses and longer exposure at 5°C still provided control.
Dr. Guido Schnabel at Clemson University, South Carolina, and Julien Mercier with AgraQuest, Inc., did research showing that packed peaches could be protected from brown rot during storage by using a pack containing M. albus in the bagged shipping cartons.
Lacey said that diapausing larvae in fruit bins can be a source of codling moth in storage and can introduce codling moth into orchards. M. albus could offer an alternative approach for controlling diapausing codling moth in bins as well as fruit cartons.
Jonathan Margolis, senior vice president for research and development at AgraQuest, said that because of the promising results of M. albus in trials on a wide range of products, the company registered it with the U.S. Environmental Protection Agency. Subsequently, however, it proved to be prohibitively expensive to make and distribute the formulation that was registered.
The fungus was grown on rye grain and typically placed in a sachet, similar to a teabag. The sachets could be stored frozen or dry and then rehydrated by dipping in water to reactivate the fungus so it produced volatiles.
Margolis said the company is actively working to develop a new and cheaper formulation for commercial use. He said he could not elaborate because of intellectual property concerns. •
Dr. Lawrence Lacey is an entomologist with the U.S. Department of Agriculture’s Agricultural Research Laboratory in Yakima, Washington.