Dave Rosenberger described how he tested the effectiveness of low-volume nonrecycling drenches for fruit going into storage. His audience included New York fruit growers and International Fruit Tree Association members on tour during the fruit field day at Cornell Agricultural Research Station at Geneva, New York.
The practice of drenching fruit before placing it into extended storage is one that storage operators would love to eliminate, for several reasons. Question is, can they?
Some storage operators in western New York have done it successfully. Instead of drenching bins of fruit and then recovering and recycling the liquid, they spray about 2.5 quarts of solution over a bin of apples, covering them without runoff.
The solution contains DPA (diphenylamine) for control of storage scald and carbon dioxide injury, especially in the Empire variety, and sometimes fungicides—captan and/or Scholar (fludioxonil) or Penbotec (pyrimethanil)—are included to control decay in storage.
Last year, Dr. Dave Rosenberger, a plant pathologist at Cornell University, and his research team tested the new practice, which he called nonrecycling drench, with the conventional method of drenching bins of fruit with a high volume of recycling drench.
The practice of drenching fruit before storage has several shortcomings.
—These recycling drenches, he said, are often thought to spread more disease spores than they eliminate. Bins are known to harbor fruit rot organisms and carry them over from season to season.
—Disposal of used drench water can be a problem.
—Fungicidal drenches are not allowed for fruit destined for some export markets.
—Now, with the increasing emphasis on food safety, these drench solutions might actually be a vehicle for introducing animal feces onto apples from bin bottoms. “While apples are not considered a very good host for human pathogens, drench water gets dirty and could spread deer scat from bins to apples,” Rosenberger said.
—In addition, these recycling drenches are a bottleneck at best in the process of getting fruit into storage.
—The practice of drenching is also more difficult to carry out for smaller operations that may not have the best facilities.
The fungicides used to kill the decay organisms are expensive, and they also represent one additional source of pesticide residue on fruit.
“Either way you look at it, nonrecycling drenches, with or without fungicide, are more economical,” he said . “We wanted to verify that they worked.”
In one of the research treatments, Rosenberger and his colleagues at Cornell’s Hudson Valley Laboratory at Highland, New York, set up an experiment in which specially made bins—of normal depth but less volume—were filled with Cortland apples wounded with nine nail punctures in each and then inoculated with high levels of Penicillium expansum, the organism that causes rot on fruit.
The bins were then treated either with a recycling or a nonrecycling drench, and each drench treatment contained DPA only, or DPA plus fungicides.
There was more decay in the recycling drench treatments when fungicides were not used in the drench solution. “The recycling drench water picked up the spores that we had misted over the fruit and effectively inoculated other fruit in the bins, whereas that occurred to a much less extent in the nonrecycling drench treatment,” he said. In further verification of that, decay levels were greater in later-treated bins than in those treated first, indicating the drench water does indeed become more heavily laden with spores.
“Just switching away from the recycling drench to the nonrecycling drench treatment system in the absence of any fungicide provided a 65 percent reduction in disease incidence,” he said. “The fact that nonrecycling drench applications do not accumulate and recirculate spores gave it a distinct advantage over recycling drench applications.”
Looking at the distribution of decayed fruit, however, there was more decay at the bottoms of bins than higher up, indicating that fruit received less even fungicide coverage in the nonrecycling drench system. When Rosenberger increased the amount of liquid applied to those bins beyond the 2.5-quart rate, the additional solution ran out the bottom and didn’t provide better coverage.
“Where fungicides were applied via a nonrecycling drench, the incidence of decay was three to five times greater in fruit at the bottom of the bin than in fruit located near the tops of the bins,” he said. Still, he added:
“Although the nonrecycling drench treatments were not as effective as the recycling drench treatments, the nonrecycling drench treatments might be effective enough to provide acceptable decay control under commercial conditions where fruit would be exposed to lower levels of inoculum than those used for this trial and relatively few fruit would have wounds.”
Was a fungicide needed at all? Both Penbotec and Scholar reduced decay levels compared to a control. But Rosenberger thinks more research is needed. It is possible that fungicide treatments could be eliminated if DPA could be applied as a nonrecycling drench under low-inoculum conditions. In ordinary conditions, apples have fewer injury points susceptible to disease invasion.
In the trials evaluating scald, there was no difference in incidence whether DPA was applied as a recycling or nonrecycling drench. “It seems likely that the volatility of DPA allows DPA vapors to suppress scald on the portions of the fruit that may escape direct contact with the solution when DPA is applied as a nonrecycling drench,” Rosenberger said.
DPA is available as a storage treatment using thermofogging. If fungicide treatment is not needed, thermofogging can eliminate the need for any drench of any kind.