Control of Fruit Rot with Yeast

Two yeasts, Pichia gulliermondii or US-7, and Aureobasidium pullulans , and two bacteria, Pseudomonas cepacia and Pseudomonas syringae, were tested to compare their effectiveness against green mold (Penicillium digitatum) and blue mold (Penicillium italicum ) on citrus. US-7 was not the most effective of the four, but it is still highly effective and has the greatest chance of gaining public acceptance because it occurs naturally on citrus and in some dairy products, is nonpathogenic, and does not create antibiotics.

US-7 promises to be an effective biological control of post-harvest fruit rot, and should be available within two years. A team of scientists at the USDA-ARS Appalachian Fruit Research Station in Kearneysville, W.Va., led by plant pathologist Charles Wilson, have found US-7 effective against all major citrus rots, and for certain rots on apples, grapes, and tomatoes. Wilson and Edo Chalutz, a scientist with the Institute for Technology and Storage of Agricultural Products in Israel, discovered and patented the yeast.

Through nutrient competition, US-7 effectively controls the following pathogens: diplodia rot, sour rot, green mold, and blue mold on citrus; Penicillium and Botrytis rots on apples; Rhizopus, Alternaria, and gray mold on tomatoes; and Rhizopus and Botrytis on grapes. Yeasts like US-7 can rapidly use available nutrients and multiply, thus excluding pathogens. They can also colonize the surfaces of fruits and vegetables for long periods of time.

A biological rot control is greatly needed now because the most widely used group of fungicides, ethylene bisdithiocarbamates (EBDCs) was recently banned by the EPA. Benomyl, used on pears and apples, was also withdrawn from the market. With the exception of sulphur, little is left to combat post-harvest rot, which accounts for about a fourth of the world’s fruit harvest losses.

The Experiment

The antagonists, or organisms that fight fruit pathogens, were isolated from the surface of unsprayed lemons. The lemons were put in a beaker of sterilized water and shaken. A small sample of wash water was spread on an agar plate and incubated. In this way, over 100 isolates were obtained and then refrigerated.

Cultures to be tested were grown on a nutrient yeast dextrose agar medium for at least three days. Cultures were then multiplied and maintained through repeated sampling and incubation.

Grapefruits and lemons were washed in a 2% hypochloride solution and wounded 6 times with a dissecting needle. The wounds were filled with broth containing the test organism. When dried, each wound was challenged with the Penicillium. The fruit was incubated for 4 days at room temperature in plastic containers. The wounds were observed for 14 days.

The best antagonists were tested with pathogens on grapefruits with 1 wound per fruit, and incubated for 3-15 days. Each experiment involved 5 fruits with 2 wounds per isolate and was repeated 4 times. The percentage of noninfected fruit was recorded.

Results

Of the 122 isolates, the two yeasts and two bacteria mentioned above inhibited infection by 50% or more, 11 days after inoculation. Protection by the bacterial antagonists had a tendency to last longer than the two yeasts.

P. cepacia provided the best protection against the two molds by creating antibiotic zones of inhibition against the Penicillium. However, because P. cepacia appears to control mold by antibiosis and may lead to antibiotic resistance in humans, the marketing of this bacteria was not pursued by the team. The other three antagonists were as effective for the first week, but less so after 13 days.

The most likely antagonist to gain commercial public acceptance is P. gulliermondii, which commonly occurs in cheese and milk, is nonpathogenic, and controls Penicillium rot without antibiotic production. Although the last two antagonists were effective, they will not be marketed because A. pullulans is not commonly associated with food, while P. syringae is actually a plant pathogen.

For more information:

Dr. Charles L. Wilson
USDA-ARS Appalachian Fruit Research Station
45 Wiltshire Road
Kearneysville, W.Va. 25430 U.S.A.