16 MISC. PUBLICATION 168, U. S. DEPT. OF AGRICULTURE 



Blue Mold Rot 



(Penicillium expansum Thorn) 



OCCURRENCE AND SYMPTOMS 



Blue mold rot is the most common and usually the most destruc- 

 tive of all the rots found on apple, pear, and quince in transit, 

 in storage, and on the market. It occurs on all varieties of these 

 fruits grown in all parts of the country. 



The rot appears as soft, watery spots. The decayed portions are 

 sharply separated from the healthy tissue. If the margin of the 

 rot is rubbed lightly, a sharp margin of healthy flesh can readily 

 be detected. The spots range in color from brown to pale straw 

 and show all possible variations in size (pi. 6, C-E). They may 

 occur on any part of the fruit. The spots are shallow at first, but 

 they extend deeper very rapidly — in fact, just about as rapidly 

 as they increase in diameter on the surface — so that by the time 

 the rot reaches the core it has involved a third or more of the fruit. 

 Internally the decayed tissue is watery and has a glassy appear- 

 ance. It can readily be scooped out from the healthy tissue. 

 Whether a surface growth of blue mold develops depends very 

 largely on temperature and moisture conditions and very little on 

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

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

 warm surface mold is almost sure to appear on spots of any size. 

 Usually small spore-bearing fungus tufts appear on the sur- 

 face. These are white at first and bluish green later. The blue- 

 green color is due to the 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 su- 

 perficial growth even where it is not causing rot. Where rot exists 

 the fungus can be found in the rotted tissues, even in the smallest 

 spots; later the fungus may become evident externally in the 

 form and color already described. 



Blue mold rot is not important as an orchard disease. Under 

 conditions of abundant rainfall and summer or early-fall tem- 

 peratures it may occur on fruits fallen to the ground or on 

 mashed or overripe ones 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 agents that produce skin breaks. 



Despite careful handling methods and packing-house sanitation, 

 most if not all of the fruit carries blue mold spores on its surface 

 when it is packed. If conditions in storage or transit are favorable, 

 this spore load can eventually give rise to fruit decay. 



In the past blue mold was generally regarded as a wound para- 

 site. It was shown in 1932 (5), however, that blue mold can pene- 

 trate uninjured apples through lenticels, such infection having 

 been observed on eight varieties of apple. More recent investiga- 

 tions in the Pacific Northwest (37) confirmed the fact that the 

 fungus can enter apples through open lenticels. These investiga- 



