Blue mold rot, early stage

Blue mold rot, moderate stage

Blue mold rot showing fungus spores

Blue mold rot, Golden Delicious

Blue mold rot, Granny Smith

Blue mold rot, Gala

Blue mold rot, Jonagold

Blue mold rot

Market Diseases of Apples, Pears, and Quinces
Blue Mold Rot

Penicillium expansum Thom

Occurrence and importance
Blue mold rot is the most common and usually the most destructive of all the rots found on fruits of 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.

Blue mold rot is not important as an orchard disease. Under conditions of abundant rainfall and summer or early-fall temperatures, it may occur on fruits fallen to the ground. 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.

Symptoms
The rot appears as soft, watery spots (top two photos). The decayed portions are sharply separated from the healthy tissues. 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. 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 appearance. It can readily be scooped out from the healthy tissue. Development of a surface growth of blue mold is determined to a greater extent by temperature and moisture conditions than by the size of the spot. In cool, dry air, surface mold rarely appears, even when the fruits are 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 surface (third photo). 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 superficial 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 occurs on mashed or overripe fruits around the packinghouse, where they become an important source of infection. Despite careful handling methods and packinghouse sanitation, most if not all of the fruits carry blue mold spores on their surface when they are packed. If conditions in storage or transit are favorable, this spore load can eventually give rise to fruit decay.

Blue mold is generally considered as a wound parasite, but it can penetrate through lenticels, particularly those on or near bruises. Late in the storage season, when fruits have become weakened by ripening and aging, most varieties have been found to be susceptible to lenticel infection by blue mold. This type of lenticel infection may readily result when rotted apples are carelessly handled during repacking. The main points of entry for blue mold, however, are through mechanical injuries, cuts, and bruises.

Factors that influence the development and spread of blue mold rot may be classed as follows: (1) The extent of the spore load on the fruits, (2) the condition of the fruits, and (3) environmental conditions. The presence of open lenticels and mechanical injuries, including bruises, has an important effect on susceptibility. Environmental conditions such as moisture, ventilation, and temperature directly influence the development of decay. The atmospheric moisture necessary to prevent apples from shriveling is sufficient for blue mold development. Lack of ventilation due to tight packing and to lack of air space in stowing increases the moisture around the fruits and slows the rate of cooling, making conditions usually favorable for rot development.

Blue mold develops more rapidly at temperatures higher than the usual storage temperature for apples. Apples that are delayed in going into storage, cooled slowly in storage, stored until late in the season, or held at warm temperatures after removal from storage are particularly subject to infection. The disease is not necessarily prevented or arrested even at 30° to 32°F. Rotten spots continue to enlarge, and new infections are initiated at these temperatures. Decay proceeds slowly in the early part of the storage season when the apples are firm and resistant, but during long periods of storage blue mold develops and causes the most serious losses to stored apples. Apples wounded by small pin punctures and inoculated by a spore-suspension dip developed small but visible lesions in 30 days at 31° to 32 °F. By the end of 60 days such lesions had enlarged to 3/4 to 1 inch in diameter.

Control measures
Some of the more important means of controlling blue mold rot are careful handling, packinghouse sanitation, and prompt cooling to cold-storage temperatures. Reduction of the spore load carried by the fruits is of great importance. This may best be accomplished by rigid packinghouse sanitation and by the use of a fungicidal wash.

Sodium orthophenylphenate (SOPP) is used as a fungicidal wash in the Northwest. Although treatments of this sort reduce blue mold rot, they are only supplements to careful fruit handling, packinghouse sanitation, prompt cooling of the fruits, and maintenance of cold storage temperatures in the recommended range.

Delay in cooling shipments caused by slow loading, belated icing of the cars, or loading of warm fruits gives an opportunity for the rot to develop and so increases the probability of loss. Delay in moving the cars also increases this probability by giving the fungus a longer time in which to work.