Good Fruit Grower

April 1

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44 APRIL 1, 2015 GOOD FRUIT GROWER S cientists in Spain have shown that microwav- ing peaches and nectarines on the packing line could be an effective way to control brown rot. Brown rot (Monilinia sp.) is a serious post- harvest disease of stone fruits worldwide. No fungicides are registered in the European Union for post- harvest treatment of stone fruit, so producers have to rely on preharvest fungicide applications and good sanita- tion practices and avoid wounding the fruit. However, brown rot can still be a problem, reports Dr. Josep Usall, Scientists in Spain showed that microwaving peaches and nectarines prevents rot without harming the fruit. by Geraldine Warner PHOTOS COURTESY OF JOSEP USALL No preharvest fungicide applications are allowed in Europe to control brown rot on nectarines and peaches. postharvest specialist with the Institute for Food and Agricultural Technology (IRTA) in Leida, Catalonia, Spain, who directed the microwave research. "We knew that this microorganism is sensitive to temperature," he said. "If the peach has been infected in the fi eld and arrives at the fruit processing plant without rotting, it will rot within four days. But if you subject it to this treatment, it will not rot, and the process does not damage the fruit." Microwave treatments are used commercially to control pests in grains and stored products, but have not yet been used to control postharvest diseases. Research has previously been done on controlling Botrytis and Penicillium expansum (blue mold) in peaches, but it was done using a household microwave oven. Researchers from IRTA and the University of Leida used an industrial microwave tunnel with a continuous conveyor belt for their research, in which they showed that brown rot could be controlled without affecting fruit quality. The tunnel was 6.5 meters (almost 22 feet) long. Fruit were microwaved at powers between 5 and 20 kilowatts for periods ranging from 34 to 120 seconds. Treatment at 5 kW provided no brown rot control, even with the lon- gest exposure time. However, treatment at 10 kW for 100 seconds or 17.5 kW for 50 seconds provided complete control. Fruit quality The scientists went on to evaluate the effects of two microwave treatments (10 kW for 95 seconds and 17.5 kW for 50 seconds) on inoculated fruit of different weights and maturity levels and on fruit with natural brown rot infections. They also evaluated the effect of the treatments on fruit quality. Brown rot control in the largest fruit (weighing about 260 grams, or 9 ounces) was lower than in the smaller fruit. This could be due to the different fi nal tempera- tures reached by fruit of different weights, with smaller fruit becoming hotter, the researchers report. Before the experiment, the peaches and nectarines were held at room temperature for one, two, or three days, so that there was a range in maturity, but maturity level did not appear to affect how well the microwave treatment controlled brown rot. When they evaluated fruit quality, the scientists found that fruit that had been microwaved was fi rmer than the fruit that had not. Scientists believe that heating the fruit might inactivate hydrolytic enzymes in the cell walls or delay peak production of the ripening hormone ethylene. Appearance of the fruit was not affected by any of the microwave treatments they evaluated. However, small Microw control aves BROWN ROT

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