Good sanitation at both the orchard and the packing house is key to minimizing fruit rots, Dr. Robert Spotts emphasized during Washington State University’s Apple Quality meeting in August. "To get zero decay, you have to get down to zero spores," he said.
Decay begins in the orchard, so spore reduction needs to begin with orchard sanitation. Fruit that fall to the ground are food for fungi, and fungal spores on the bottom of the bins get into the packing house, where they can be washed into the drencher or flumes. That initial source of spores magnifies from then on, said Spotts, a plant pathologist at Oregon State University’s Mid-Columbia Agricultural Research and Extension Center.
In a low-tech study, Spotts and his colleagues brought bins into a small packing house at the Hood River, Oregon, station and compared the number of Penicillium spores on the bins before and after washing with a garden hose. The unwashed bins had more than three times as many spores on them as the washed bins.
In a commercial situation, the spore load in the dump tank water increases during the packing season. By spring, apples or pears that have rotted in storage can have millions of spores on them that wash off into the water as they’re brought out to be packed. By March, the number of Penicillium spores can be very high.
"It becomes a challenge as the season progresses to keep the water clean," Spotts said.
In other studies, Spotts compared the efficacy of several treatments on wooden and plastic bins. A chlorine treatment at 100 parts per million killed 80 percent of the Penicillium spores on plastic bins and 47 percent of spores on wooden bins. Quaternary ammonium killed 97 percent of the spores on both types of bins, and a 10-second steam treatment killed 100 percent of the spores on plastic bins and 97 percent on wooden bins.
"Spores don’t like steam at all," he reported.
Neither do they like hot water. In experiments at the research station, a hot water pressure system gave good control of gray mold and mucor rot in d’Anjou pears and also killed insects and insect eggs. Spotts said the system could be scaled up for commercial use as part of an integrated decay control strategy.
Filtration has been tested as a way of removing decay spores. Penicillium spores, which are the shape of a squashed golf ball, are only about three microns in diameter.
In tests, a filter with a 10-micron sieve removed 73 percent of Penicillium spores, a 1-micron sieve removed 92 percent, and a 0.45-micron sieve removed 99 percent. A sand filter did not remove any spores, but it removed dirt and debris so that less chlorine was required. The filters need to be cleaned out regularly so that the collected debris doesn’t become a source of spores, Spotts said.
Filtration can be combined with ozone or chlorination. Chlorine is an effective disinfestant for blue mold and mucor, but it’s important to keep the pH level in the range of 6.5 to 7.5, Spotts said. Chlorine dioxide also works, but is difficult to keep at an effective level. Ozone is effective, particularly in combination with filtration and chlorine, and can reduce the amount of chlorine required.
Spores can attach themselves to the walls and ceilings of packing houses. By the end of the packing season, there can be up to 2,000 spores per square centimeter. Spotts described one where fungus growth on ceiling beams looked like barnacles. Cleaning, scrubbing, and fumigating can make a huge difference in the spore load, he said.