Dr. Robert Spotts, a plant pathologist at Oregon State University in Hood River, said there is an urgent need for a new control as an alternative or supplement to the standard fungicide, TBZ (thiabendazole), which appears to be losing its effectiveness against some pathogens because of resistance.
Dr. Eugene Kupferman, postharvest Extension specialist with Washington State University, has estimated that postharvest losses are costing the Pacific Northwest winter pear industry about $2.5 million a year.
The three yeasts Spotts has tested are effective against gray mold, blue mold, bull's-eye rot, and side rot on apples and pears, and have some effect against mucor rot, which is not controlled by TBZ. They also are effective against brown rot of sweet cherries, Spotts said.
One biocontrol for postharvest decays, patented by Dr. Charles Wilson with the U.S. Department of Agriculture at Kearneysville, West Virginia, has already been commercialized by Ecogen, Inc., and is being marketed by Elf Atochem under the trade name Aspire. The active ingredient is the yeast Candida oleophila, which Wilson discovered on tomatoes. Aspire is registered for use on citrus as well as pome fruits.
The EcoScience Corporation also has two products on the market for control of postharvest diseases, one for apples and one for pears. EcoScience is developing another product, based on the yeast Cryptococcus laurentii, which was identified as a potential biocontrol for mucor rot, gray mold, and blue mold by Dr. Rodney Roberts, research leader at the USDA lab in Wenatchee, Washington.
But Spotts said the three yeasts he is working with have proven more effective than the others in side-by-side comparisons, both when applied alone and when combined with a low rate of TBZ.
"We have worked with Aspire, and we have compared that with our yeasts, with and without TBZ, and Aspire is just not very effective on both apples and pears," he said. "Even with the reduced rate of TBZ, we don't get a level of control that I think is commercially acceptable."
One of the three yeasts Spotts has been working with is a strain of Cryptococcus laurentii, the yeast that Roberts identified as a potential biocontrol, but he maintains it is significantly different in a number of ways.
"We've documented they are different," he said. "We need to convince the patent office of that, and whether or not we will succeed, I don't know."
Dr. David Sugar, with OSU's Southern Oregon Experiment Station in Medford, has compared the two strains and found they both work well against blue mold, but Spotts' HRA5 strain was slightly more effective than Roberts' 87-108 strain against side rot.
Alone, the Cryptococcus strains provided about 80% control of blue mold. When combined with a low rate of TBZ, they gave 95% control.
Sugar said what is more significant is that Spotts and Roberts' yeasts were far more effective in the first year of tests than the registered biocontrol products. The Bio-Save products gave about 50% control and Aspire about 20% when used alone.
Sugar said he is testing the biocontrols because he is interested in seeing them developed as alternative controls for growers. "These biocontrols offer a far broader spectrum of control than TBZ. They offer some relief from the resistance problem, and they can be combined or used in sequence with TBZ to improve the level of control," he said.
Spotts' three yeasts, Cryptococcus laurentii strain HRA5, Cryptococcus infirmo-miniatus, and Rhodotorula glutinis, were among a collection of more than 30 yeast strains that he and Dr. Tara Chand-Goyal, a microbial ecologist, evaluated. They were isolated three years ago from pears taken from unsprayed orchards in five locations in Washington and Oregon.
Spotts said he considers the three yeasts as a second generation of biocontrol agents, because they were discovered and screened using newer techniques than those already developed. For example, they were grown in a nutrient-poor medium, rather than in conditions that would favor optimal growth, and screened at cool temperatures.
Previously, biocontrols were screened by making a wound in the fruit, putting the biocontrol in the wound, and then applying the pathogen later.
"It doesn't happen that way in nature," said Spotts, who combined the pathogen and biocontrol and put them in the fruit at the same time to simulate more closely what happens in the packing shed. Often, pathogens in dump tank water infect the fruit through wounds.
All three yeasts controlled blue mold and gray mold on pears in his tests. C. infirmo-miniatus was most effective for control of mucor rot. D'Anjous were used for most of the pear tests, but control of side rot was evaluated on Bosc, a particularly susceptible variety. All three yeasts controlled side rot on Bosc pears, whereas TBZ was ineffective.
On sweet cherry, C. infirmo-miniatus or C. laurentii, combined with a low dose of iprodione (Rovral), controlled brown rot as well as a high dose of iprodione.
The yeasts continue to look promising in larger-scale tests, Spotts said. Last year, he tested them on 32 tons of apples and pears at a commercial packing house. He said the yeasts are very effective against TBZ-resistant strains of the blue mold pathogen as well as sensitive strains.
Even though the yeasts work well alone, they are designed to be used with a reduced rate of fungicide.
An agricultural chemical company is interested in commercializing the biocontrols, Spotts said. A patent has been applied for, and Spotts hoped to have some formulated product to test this fall.
Copyright © 1996, Good Fruit Grower
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