Notes on Apple Diseases
Blue mould and grey mould in stored apples
Identification
Blue mould, caused by Penicillium spp., especially Penicillium expansum,
and grey mould, caused by Botrytis cinerea, are the principal postharvest
diseases of apples in storage.
When apples are infected by blue mould, the rotted areas on the fruit
are soft, watery and light brown in colour. Bluish-green spores cover
the surface of older lesions. Fortunately, this fungus will not move from
an infected fruit to an adjacent fruit unless the fruit is bruised or
punctured in some way.
When apples are infected by grey mould, the texture of the decay on the
fruit is firm and the skin is tight and tough. The lesions are pale tan
to brown and the surfaces of older lesions are covered with greyish mycelium
and dark brown spores. Unfortunately, grey mould can move from fruit to
fruit and whole bins or boxes of fruit can become infected.
Period of activity
The apples become infected in the orchard and disease can develop throughout
the storage period. Infection occurs at harvest and spores enter primarily
through wounds such as stem punctures, bruises and insect injuries. The
fungi survive between seasons on contaminated bins and can grow and produce
large amounts of spores. These fungi can be transferred from contaminated
bins to water used for dipping or drenching as well as grading equipment.
Spores can be carried through air currents or by water.
Thresholds
There is no economic threshold for these diseases. A single decayed fruit
may contain enough spores to contaminate water on the entire packing line.
Management notes
Handle fruit carefully during and after harvest to minimize stem punctures,
skin breaks and bruises, which act as infection sites for the fungus.
During storage and transportation, operators must move the bins with a
minimum of jolts and drops. Refrigerate the fruit as quickly as possible.
Adequate refrigeration capacity for each storage room is required. Modern
refrigeration facilities and controlled atmospheres also reduce fruit
rot.
Sanitation in the handling and grading area is important. Remove rotted
fruit from the premises and the property. Change the water used for dipping
or drenching frequently.
Preharvest treatment of Scala SC for postharvest control of blue mould
and grey mould has been recently registered in Canada.
Postharvest treatment with a fungicide thiabendazole (TBZ) can reduce
disease incidence. Mertect SC is registered in Ontario to control the
development of blue and grey mould on stored apples and pears. Mertect
SC can be used as a postharvest dip or drench. Certain precautions must
be followed:
- Constant agitation is required to maintain suspension during treatment.
- Mertect must be in direct contact with the skin of the fruit. Do
not apply wax prior to Mertect treatment.
- Consult the label for additional instructions.
Resistant strains of these fungi are common and are not controlled by
treatment with thiabendazole. The addition of diphenylamine (DPA) to reduce
superficial scald in the postharvest treatment is reported to control
some TBZ-resistant strains of Penicillium and Botrytis. Recently however,
these pathogens have also developed resistance to diphenylamine (DPA).
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