MAEKET DISEASES OF APPLES, PEARS, QUINCES 13 



the size of the spot. In dry, cool air the growth rarely appears, 

 even when the fruit is totally decayed. In air that is moist and 

 warm the growth is almost sure to appear on spots of any size. 

 Usually small tufts or bunches of fungus spores appear on the 

 surface. These are white at first and bluish green later and look 

 very much like minute bundles of bound grain (pi. 1, A). The 

 blue-green color is due to production of spores. In all the pome 

 fruits a musty odor can be detected when the rot becomes well 

 advanced and spore production is heavy. The decayed flesh also 

 has a musty taste. 



CAUSAL FACTORS 



Blue mold {Penicillium expansum) sometimes produces a super- 

 ficial growth where it is not causing rot; but where rot exists the 

 fungus can be found in the rotted tissues, even in the smai^est spots; 

 later on it may become evident externally in the form and color 

 already described. 



Blue-mold rot is not important as a field disease. Under condi- 

 tions of abundant rainfall and summer or early fall temperatures it 

 may occur on fruits fallen to the ground or on mashed or overripe 

 specimens around the packing house, where it becomes important 

 as a source of infection. It is almost never found on fruits hanging 

 on the tree, unless they have been injured by insects, hail, or other 

 agencies that produce skin breaks. 



Most of the damage from this rot takes place in transit and 

 storage, although even here the occurrence of such damage is no 

 proof that the fruit was generally diseased when it left the packing 

 house. There may have been a few slightly but visibly rotted fruits 

 and a few in which the rot existed but not at such a stage that it 

 was discernible. It is certain, however, that despite careful handling 

 methods and packing-house sanitation (p. 3) most if not all of the 

 fruit bore blue-mold spores on its surface when it was packed ; con- 

 sequently, if conditions in transit or storage were favorable, the 

 rotten spots already present, whether visible or not, became larger 

 and new ones could be formed by the germination of spores lodged 

 on the fruit. 



It was thought until recently that blue mold is unable to pene- 

 trate the unbroken skin of apples. Investigations in the Pacific 

 Northwest {6) have indicated, however, that the fungus is able to 

 enter apples through lenticels and that lenticel infection may be 

 responsible for a large part of the average annual loss from blue- 

 mold rot. There is evidence that infection can occur through only a 

 few of the lenticels on apples. Nothing is known of the factors that 

 tend to increase the number of infectible lenticels, but the possibility 

 that such lenticels may occur makes it highly important that har- 

 vesting, cleaning, and packing methods be such as will reduce the 

 spore load on the fruit. 



Conditions that favor the development and spread of blue-mold 

 rot in transit and storage are moisture, moderately high temperature, 

 and skin breaks and lenticels through which the fungus can enter. 

 Moisture may be present on fruit picked during times of rain or 

 heavy dew or on fruit not thoroughly dried after washing; it may 

 also condense on fruit because of a damp atmosphere in a storage 



