Fruit Susceptibility to Decay 



Pathologists at the Mid-Columbia Research 

 Station studied changes in susceptibility of fruit 

 to decay throughout the growing season. In 

 summary, fruit becomes most susceptibleto the 

 fungal decay organisms during the last month 

 before harvest. However, infection by the bull's- 

 eye rot organism can occur any time from petal 

 fall to harvest. 



Pears were wounded and inoculated 

 throughout the growing season with the differ- 

 ent decay-causing fungi. Fruit was harvested 

 and placed into storage for seven to eight 

 months. The decay was generally less than 10% 

 on fruit sprayed with fungal spores a month or 

 more before harvest (Table 1). Fruit treated 

 with decay organisms diuing the last few weeks 

 before harvest was seriously decayed during 

 storage. Consequently, growers should time 

 chemical control programs to cover fruit at least 

 two to three weeks before harvest, as it loses 

 its resistance to decay. 



Harvest maturity is critical. Studies on Bosc 

 pears have shown dramatically that more de- 

 cay occurs on later picked fruit. By delaying 

 harvest two weeks after commercial harvest, 

 there was a significant rise in the amount of 

 infection in nonwounded fruit that was sprayed 

 with the fungal suspensions. 



Sprays to Control the Diseases 



Three factors are of primary importance in 

 designing a fungicide spray program. These 

 factors include 1) when spores of a particular 

 disease organism are present in the greatest 

 quantity, 2) when fruit is most susceptible to 

 infection and decay, and 3) when environmen- 

 tal conditions most favor infection. 



Certain postharvest rots occur when infected 

 flower parts are trapped in the calyx end of the 

 fruit soon after bloom, i.e., caljrx-end infections 

 by Botrytis. A spray of Ziram, Manzate-200, or 

 Di thane M-45 within 10 days of petal fall helps 

 reduce infection. Growers in areas with bull's- 

 eye rot may need a second fungicidal spray if it 

 rains in August. Preharvest sprays of Ziram 

 also help reduce side rot incidence. 



Cultural Practices to Reduce Decay 



During the winter months, prune trees to 

 eliminate low hanging branches which might 

 set fruit in contact with cover crops or lie on the 

 ground. These fruit can easily come in contact 

 with soil-borne spores and become infected as a 

 result of the high hvunidity in the microclimate 

 of the cover crop. 



During the summer months, it is important 

 to keep weeds and grass under control. Spores 

 can be released from the cover crop, which also 

 provides high humidity for germination. In 

 particular. Gray Mold and other Botrytis spe- 

 cies grow well on weakened or dead plant ma- 

 terial in the orchard. Periods of rainy weather 

 or excessive irrigation promote the growth and 

 sporulation of these fungi, which account for a 

 general increase in the incidence of Gray Mold 

 in wet years. Conversely, too little water may 

 promote dusty conditions, which result in spread 

 of the soil-borne spores of Mucor, Penicillium, 

 and Botrytis. 



At harvest, growers can do a number of 

 things to reduce postharvest decay. Injury to 

 fruit during harvest and packing is probably the 

 most critical factor leading to postharvest de- 

 cay. Harvesting fruit at the proper maturity is 

 also extremely important. Pears harvested on 

 the immature side will abrade easily on the 

 packingline. Overmature fruit or fruit har- 

 vested late in the maturity range has reduced 

 storage life and is more susceptible to 

 postharvest diseases. Fruit is most susceptible 

 to diseases as it approaches maturity. 



Proper handling becomes critical in prevent- 

 ing decay and bruising. Pickers should not pick 

 up "grounders" (fallen fruit), since that fruit is 

 likely to be infected as well as ripening prema- 

 turely. Volatiles produced by these fruit stimu- 

 late the ripening of adjacent fruit and reduce 

 storage life. 



Avoid harvesting wet fruit, as it likely will 

 have spores adhering to the surface which may 

 germinate and infect. Allow fruit to dry before 

 harvesting. 



Most postharvest rot organisms are soil in- 

 habitants and can be picked up on the skids or 

 sides of bins. Mow the cover crop or use saw- 



Fruit Notes, Summer, 1995 



19 



