HOOD RIVER - Plant pathologists at Oregon State University's Mid-Columbia Agricultural Research and Extension Center have invented a new, more environmentally benign way to prevent significant spoilage of apples, pears and cherries in cold-storage.
Using strains of yeast microflora naturally found on fruit, they have been able to combat fungal diseases in stored fruit, reducing the need for chemical fungicides by up to 99 percent.
"Decay caused by fungal diseases such as gray molds, blue molds, mucor rot, bull's eye, side rot and brown rot cause millions of dollars of loss to fruit growers in the Pacific Northwest alone each year," explained Bob Spotts, professor of plant pathology at the OSU center in Hood River.
Spotts and colleague Tara Chand-Goyal isolated, tested and patented three strains of yeast that effectively control post-harvest decay - not by toxic means, but by out-competing the disease organisms that cause fruit rot.
"The yeasts, if sprayed on the fruit after harvest, out-competes the disease-causing organisms for nutrients," explained Spotts. "It works strictly by competition in the post-harvest cold storage environment. The yeast uses up all the resources that the fungal pathogens would need to thrive."
The yeasts were carefully collected off fruit, identified, then put through a series of experiments in conditions similar to those found in fruit storage facilities. They chose yeast organisms that didn't harm the fruit, but were able to out-compete disease organisms. They screened them in a low nutrient medium, because the surface of fruits is not naturally rich in nutrients.
The OSU researchers also grew them out in cold conditions, like those in fruit storage. Only those yeasts that survived the cold, nutrient-poor conditions of cold storage were isolated and tested further.
Spotts and Chand-Goyal mixed the spores of six types of molds or rots with yeasts and applied them to fruit wounds, the perfect site for fungal disease development.
Instead of disease outbreaks, the yeasts multiplied on the inoculated fruit, on the fruit wounds, where there were nutrients, they said. The yeasts consumed the fruit sugars, leaving little food for the fungal pathogens that cause fruit to rot. With the yeast applications, just a tiny fraction of the normally needed fungicide was necessary to keep the pathogens in check.
"We found we could prevent diseases using the yeasts and 97 to 99 percent less fungicide than normally used by fruit producers," said Chand-Goyal.
About one-fourth of all the produce grown in the United States rots while in storage. Losses world-wide average 50 percent. The main way these diseases are squelched is by chemical fungicides, applied after harvest but before storage.
But fungicides are losing their effectiveness, the researchers said.
"Fungicide-tolerant strains of disease organisms are now present in most packing houses, rendering chemical fungicides less effective or totally ineffective," said Spotts. "Also, very few fungicides are available for post-harvest use by the fruit industry. The yeast will control fungicide-tolerant as well as fungicide-sensitive organisms."
Spotts and Chand-Goyal are planning to commercialize these three yeast strains so they will be available to industry. They term their new method as the "next generation" of biological control products for fungal diseases in stored fruit.