Blue Mold
- Fruit Disease Focus - January, 1999
Penicillium spp.  
Blue mold
Lenticel infections caused by P. expansum


I. Introduction: Blue mold is the most important postharvest decay of stored apples in the United States. The losses from this disease can be significant but can be substantially reduced by following proper sanitation and control measures. The fungus, Penicillium expansum, not only causes fruit decay but also produces the carcinogenic mycotoxin patulin. This toxin may rise to unacceptable levels in fruit destined for processing and may also result in off flavors.

II. Symptoms:  The appearance of the decay caused by most species of Penicillium is very similar. The rotted areas are soft, watery and light brown in color. The surface of older lesions may be covered by bluish-green spores that initially are nearly snow white in color. The lesions are of varying shades of brown, being lighter on the yellow or green varieties and on the pale cheeks of colored varieties and showing dark shades of brown on the most highly colored varieties. The soft, watery consistency of the rotted tissue seems to be a more distinguishing feature than the color variations. Two characteristics are of importance in the recognition of P. expansum, the most common species, namely the musty odor and the formation of conidial tufts or coremia on the surface of well developed lesions. Under cold storage conditions, blue mold lesions (from wound infections) caused by the common species may be expected to be one to one and a quarter inches in diameter eight to ten weeks after infection. In rots caused by other Penicillium species, the decayed tissues are firmer, may lack surface growth under cold storage conditions, and are slow-growing compared with P. expansum. The importance of these species should not be ignored, since, it is probable that under certain conditions, they may assume considerable importance.

III. Disease Cycle: The Penicillium expansum type of blue mold has been the form most frequently reported, but a number of other less common species, which are also usually less aggressive, have been encountered. The positive identification of the different species that cause blue mold is only possible by means of laboratory cultures and microscopic examination, and even then positive determination is difficult because of the very slight differences encountered among species. All of the blue molds are primarily wound parasites, most frequently gaining entrance through fresh mechanical injuries such as stem punctures, bruises and insect injuries, finger-nail scratches by pickers, necrotic tissues of diverse origin or through normal stems or open calyx canals. Sometimes infections may occur through lenticels, especially when they are damaged by cracking after a sudden abundant supply of water following a period of dryness, or after bruising late in the storage season when fruit have been weakened by ripening and aging.

The blue mold spores are long-lived and may easily survive from season to season on contaminated bins, where the fungus can grow and produce copious amounts of spores. Contamination with these spores may come from various other sources including orchard soil carried on bins from the orchard, decaying fruit or air. Inoculation of the fruit going into storage is believed to occur mainly from the diphenylamine (DPA) drenching solution used for protection against superficial scald, where the spore concentrations increase with each successively drenched bin and may reach high levels if solutions are not changed regularly. Inoculation can also occur during fruit handling in water contaminated with the fungus in packinghouses. A single decayed fruit may contain enough spores to contaminate water on the entire packing line.

IV. Management: Postharvest treatment of fruit with fungicides has been traditionally the most common method of combating blue mold. However, this decay can be reduced by a variety of methods and procedures and many of them can be combined for overall improvement. Below are the various practices and methods that can reduce the severity of this disease.

1. General sanitation and avoidance of conditions favorable to infection. The general aim of sanitary practices is to reduce the available supply of fungal spores to the lowest possible point for any given environment. This includes reducing contamination of bins with orchard soil, which is a reservoir for the spores, sterilization of contaminated bins and packing machinery, and frequent changes of solutions and water used for drenching and handling fruit. Fruit should be picked at the proper maturity (not over-mature) and placed in cold storage as soon as possible. Picking wet fruit should be avoided. Bins containing harvested fruit in an orchard should be protected from rain so fruit will not become wet.

2. The avoidance of fruit injuries. Gentle handling of fruit by pickers during harvesting and care during the transportation of fruit from the orchard to the packinghouse may prevent many injuries. Attention should be given to mechanical features of the handling machinery in packinghouses to eliminate sources of injury from rough corners, unnecessary drops or gravity runs, or hard or unprotected receiving surfaces.

3. Chemical control. Killing spores in dump tanks, on bins, or in flume water with chlorine (100 ppm) or sodium O-phenylphenate (SOPP) (0.3 to 0.5%) has been effective in reducing the spore load and the resulting amount of decay. Chlorine and diphenylamine (DPA) are not compatible, so for fruit requiring DPA treatment (for superficial scald), chlorine must be allowed to dissipate before treatment with DPA. Ozone treatment, although not yet frequently used, can also be effective. However, as with chlorine, ozone has no eradicant or residual effect. Both of these treatments are most effective in conjunction with other sanitary measures that prevent exposure of fruit to reinoculation with fungal spores. The thiabendazoles (e.g. Mertect 16 fl oz/100gal) are the main fungicides available to combat decays in storage, and Captan 50W (2.5 lbs/100gal) is used to a lesser extent. Captan has been used mainly in combination with other fungicides because it acts as a protectant and has limited effectiveness. In areas where fungicide resistant strains of the fungus have developed, treatments with these fungicides may be unsuccessful.

4. Biological control. This new method of control shows great promise and is being studied by an increasing number of scientists worldwide. BioSave 110 TM and Aspire (recently renamed Decco I-182) are the two biofungicides currently available. The active ingredients are a bacterium and a yeast, respectively. These biocontrol agents exhibit only prophylactic activity and are not effective against existing infections. In years with high field infection rates, biocontrol will be most effective as a part of an integrated control strategy which can include reduced doses (one tenth of the recommended rate) of fungicides, calcium and heat treatments. Good sanitary practices are also very important as the efficacy of these methods decreases with increasing spore loads.

5. Integrated control. In addition to the already mentioned sanitary and preventive measures taken in the orchard, the control of blue mold may begin in the orchard with an application of calcium chloride (2 to 6 Lb/acre), which is known to reduce bitter pit but may also reduce decay. Rinsing fruit before putting them through a drencher to prevent buildup of fungal spores has been recommended. Calcium applied to fruit by infiltration after harvest consistently shows a significant reduction in fruit decay during storage. Treatment of apples with hot air (38 C for 4 days) reduces decay by killing fungal spores but it has very little or no residual effect. This treatment also improved Golden Delicious and Gala quality in storage. Holding fruit in hot dry air can also reduce lenticel infection; however, additional studies are needed before final recommendations can be made. Although postharvest apple treatment with calcium and heat awaits commercial application, these treatments have been shown to have additive effects in combination with biological control in controlling blue mold on apples.

Rapid cooling and storing of fruit under CA conditions reduces the development of blue mold. Cold temperatures have a much greater inhibiting effect on the initiation of infection than it has on decay development. Fruit under CA conditions maintain natural resistance to blue mold longer than fruit placed in regular cold storage. Maintaining proper storage temperatures and marketing fruit before decay develops may further reduce losses.

Text by Wojciech Janisiewicz, USDA Appalachian Fruit Research Station, Kearneysville, W.Va.

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