Blue mold is a common postharvest disease on apples and pears worldwide. This disease is an economic concern not only to the fresh-fruit industry but also to the fruit-processing industry because some strains of Penicillium expansum produce the mycotoxin patulin, which can rise to unacceptable levels and thus affect the quality of apple juice.
Blue mold originates primarily from infection of wounds such as punctures, bruises and limb rubs on the fruit. Blue mold can also originate from infection at the stem of fruit. Stem-end blue mold is commonly seen on d’Anjou pears, but it also occurs on apples such as Red Delicious (Fig. 3). Calyx-end blue mold occurs on Red Delicious apples but is usually associated with fruit that are drenched prior to storage.
The decayed area appears light tan to dark brown. The decayed tissue is soft and watery and the lesion has a very sharp margin between diseased and healthy tissues. Decayed tissue can be readily separated from the healthy tissue, leaving it like a “bowl”. Blue or blue-green spore masses may appear on the decayed area, starting at the infection site. Decayed fruit has an earthy, musty odor. The presence of blue-green spore masses at the decayed area and associated musty odor are the positive diagnostic indication of blue mold. Without the presence of spore masses of blue mold, blue mold can be misdiagnosed as Mucor rot, but a sweet odor is commonly associated with Mucor rot (see the comparison between the two diseases in Table 3).
Penicillium expansum Link is the primary cause of blue mold of apples and pears. Several other Penicillium species, including P. solitum, P. commune, P. verrucosum, P. chrysogenum and P. regulosum, have also been reported to cause decay on apples and pears. P. expansum can grow at temperatures as low as -3ºC and conidia can germinate at 0ºC.
In the orchard, Penicillium spp. survives in organic debris on the orchard floor, in the soil, and perhaps on dead bark on the trees. Conidia are also present in the air and on the surface of fruit. In the packinghouse facility, DPA- or fungicide-drench solutions, flume water and dump-tank water are common sources of Penicillium spores for fruit infection during the handling and packing processes. Spores of P. expansum are also commonly present in the air and on the walls of storage rooms.
P. expansum is essentially a wound pathogen. Wounds on the fruit skin such as punctures and bruises that are created at harvest or during the postharvest handling process are the major avenue of invasion by the fungus. Fruit with wounds can be inoculated with spores of thiabendazole-resistant isolates of P. expansum during postharvest drenching with diphenylamine and Mertect (thiabendazole, TBZ). Fruit may also be inoculated with Penicillium during the packing process. P. expansum can also cause decay through infection at lenticels, but this type of infection usually occurs on over-mature fruit or when lenticels have been injured. More than 50% of the P. expansum isolates recovered from decayed fruit collected from packinghouses in Washington State are resistant to thiabendazole, whereas only approximately 3% of the P. expansum isolates from apple orchards are resistant to TBZ. A prestorage application of TBZ is likely the major source of TBZ-resistant strains.
Orchard sanitation to remove decayed fruit and organic debris on the orchard floor helps reduce inoculum levels of Penicillium spp. in the orchard. Good harvest and handling management to minimize punctures and bruises on the fruit helps prevent the fruit from infection at wounds by P. expansum and other Penicillium species.
Thiabendazole is commonly used as either a prestorage drench treatment or a line spray to control gray mold and blue mold. TBZ is effective to control gray mold but is not effective to control TBZ-resistant Penicillium. Two new postharvest fungicides, fludioxonil (Scholar) and pyrimethanil (Penbotec), can be used as drenches, dips or line sprays and have been reported to be effective to control blue mold originating from wound infections.
Biocontrol agent BioSave 110 (Pseudomonas syringae) applied on the packing line helps control blue mold from infection of wounds.
Sanitizing dump-tank and flume water is an essential practice to reduce infection of fruit by Penicillium spp. during the packing process. Fruit bins and storage rooms can harbor TBZ-resistant isolates. Bin and storage room sanitation may be beneficial in reducing TBZ-resistant populations in the packing facility.
A: Decayed area brown, soft and watery, with sharp margin; mostly originating from infection of wounds; blue-green spore masses
G: Stem-end blue mold commonly seen on d’Anjou pears, particularly after an extended period of storage
Table 3. Comparison between blue mold and Mucor rot
Texture: soft, watery
Color of decayed area: light tan to dark brown
Signs of pathogen: white mycelium, blue or blue-green spore masses
Color of internal flesh: brown
Odor: earthy, musty
Texture: very soft, juicy
Color of decayed area: light brown to brown
Signs of pathogen: gray mycelium with dark sporangia
Color of internal flesh: light brown to brown
Sugar, D., and Spotts, R. A. 1999. Control of postharvest decay in pear by four laboratory-grown yeasts and two registered biocontrol products. Plant Dis. 83:155-158.