By Geraldine Warner
A South African researcher will be working in Oregon's Hood River district for the next three years to help the pear industry figure out how, when, and why pears become infected by gray mold.
Gray mold, caused by the fungus Botrytis cinerea, is a major cause of decay in stored pears. Some pears have to be repacked toward the end of the season.
Dr. Robert Spotts, plant pathologist with Oregon State University's Mid-Columbia Agriculture Research and Extension Center, Hood River, said a survey of packing houses in the Mid- Columbia district during the 1994-95 season showed that 45% of the decay of d'Anjou pears was gray mold and 41% was blue mold (Penicillium sp.)
Although Penicillium is showing significant resistance to the postharvest fungicide TBZ (thiabendazole), Botrytis is showing only a low level of resistance, in the range of 5 to 10%, Spotts said. However, gray mold continues to be a problem. He said it is possible that infections begin in the orchard during the growing season or at harvest, and TBZ is not systemic enough to stop them.
Spotts is heading a three-year research project that aims to find out when and why pears become infected with gray mold. Working on the project is Dr. Cheryl Lennox, who was formerly at the University of Stellenbosch, South Africa, where research is in progress on gray mold of grapes and stone fruits. She will work in the Hood River area during the summers and go to South Africa during the winters to do similar studies there. This will enable her to study gray mold during two growing seasons each year. The project is supported by the Winter Pear Control Committee.
Lennox said gray mold can infect either the stem end of the fruit or the calyx end, or penetrate the fruit through puncture wounds. Stem end decay appears to be the worst problem, and she hopes the study will reveal why the stem end of the fruit becomes infected and why it is so severe.
The other key question she hopes to answer is when gray mold infections begin. She says it is likely that calyx end infections begin soon after bloom. If infections in the orchard can be related to rot during storage, it may ultimately be possible for the industry to identify before harvest which lots of fruit have the best potential for long-term storage and which should be sold quickly, she said.
Another potential benefit from identifying when infections become established in the field is that fungicides could be applied when the fruit is most vulnerable. She will sample orchards monthly for Botrytis and test sensitivity to fungicides.
Botrytis spores thrive on dead and dying plant material in the orchard, especially during cool, moist weather. The spores are formed in clusters and, because they are relatively light, can be dispersed by the wind. Spotts said it is possible that the spores infect the pear stems around harvest time, and as the pears come into contact with each other, the stems wound other pears and inject the pathogen into them.
Spotts said high levels of decay in recent seasons may be linked with weather during the growing season and fruit maturity. Wet spring weather favors the build-up of fungi, and overmature fruit is more susceptible to decay.
Dr. Peter Sanderson, researcher with the Washington Tree Fruit Research Commission, said blue mold used to be considered the Washington pear industry's number one postharvest decay problem, and a survey he did two years ago showed that most of the decay in repacked fruit was blue mold. However, during the 1995-96 season, gray mold was a significant problem.
Penicillium spores are found in great numbers in dump tanks, and Sanderson said packers have probably improved their dump tank sanitation by using chlorine or SOPP (sodium ortho phenyl phenol). Botrytis, on the other hand, is not often found in water systems, such as dump tanks, and so is not affected by those sanitation measures.
Sanderson and Spotts said gray mold of pears has not been studied extensively before. Sanderson noted that blue mold has been a more alarming problem, because it is a soft decay that turns a pear into a mushy, ugly piece of fruit, whereas gray mold is a firm decay, with fungus growing on the fruit. However, gray mold can be serious because it spreads from fruit to fruit. It is sometimes called nest rot.
Sanderson said though fruit may be colonized by Botrytis spores while in the orchard, infections do not seem to develop until the fruit is in storage. The rot thrives in warm temperatures, although it will also grow in cool temperatures. What it must have is high humidity, and the humid conditions it needs are created when pears are packed in polyliners, which are designed to prevent shrivel.
Dr. Rodney Roberts, research leader with the U.S. Department of Agriculture in Wenatchee, Washington, believes latent infections begin in the orchard, and that could be one of the reasons they are not controlled by postharvest applications of TBZ.
Some organisms, including fungi, colonize plants and establish themselves, though there may be no evidence of disease at first. Botrytis needs some kind of opening to infect the fruit. It lives on dead tissue and possibly colonizes the dead flower parts at the calyx end of the fruit after bloom, he said.
Roberts believes stem end infections begin at harvest when the open ends of the stems come into contact with inoculum. The infection then spreads down the stem to the fruit.
Roberts said he visited an orchard during d'Anjou harvest and saw decaying Bartlett pears on the ground in the middle of the rows. In warm, moist conditions, pears on the ground are readily colonized by soil-borne pathogens, such as Botrytis, and they can release a lot of inocula into the air.
Roberts is completing an experiment that involved harvesting pears, sterilizing the surface, and packing them in boxes without any kind of postharvest treatment or handling. After long-term storage, the amount of gray mold was to be assessed and related to the amount of Botrytis found in the field.
This, he said, should give a good indication of the extent that preharvest infections might be responsible for development of gray mold in storage.
Copyright © 1996, Good Fruit Grower
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