Penicillium rot – additional information

Symptoms and recognition

Penicillium rot commonly occurs on all apple varieties at low levels either alone or as a secondary infection on other rots. The symptoms are similar on all varieties.

This fungus invades fruit via wounds, bruises or cracks anywhere on the fruit surface or, on over-mature fruit, can invade via the lenticels.
The rot is rarely seen in the orchard except occasionally on rotting fruit on the orchard floor.
The fungus causes a pale green to dark brown circular soft rot which spreads rapidly over the fruit surface and into the flesh, forming a sharp interface between the healthy and rotted tissue, such that the rot can be scooped out.
Sometimes after grading only the sound fruit with the rot cavity remains, the actual rotted tissue having dropped out.
Open-eyed varieties such as Bramley may have Penicillium rot in the fruit centre, where the fungus has gained access via the drench solution during post-harvest treatment.
Mature lesions are covered with brilliant white pustules which quickly turn blue, and give the rot its common name of blue mould.
Penicillium rot can be spread by contact forming nests of soft disintegrating rots but can also rapidly colonise other rots such as brown rot and Phytophthora rot.

Disease cycle and epidemiology

At least 11 species of Penicillium have been isolated from naturally infected apples with Penicillium type rots but P. expansum is by far the most common and economically important species.

Penicillium species known to cause apple rots can be isolated from most orchard soil, but the disease is rare in the orchard except occasionally on fruit that has fallen to the ground.
The fungus can be found on decaying flower parts at blossom time and can also be isolated occasionally from the cores of fruitlets collected in the orchard.
Most fruit infections occur during harvesting of fruit when airborne or waterborne spores enter wounds on fruit damaged during harvesting and handling.
The fungi survive on decayed mummified fruit or fruit bits stuck on bins or lying around the storage or packhouse area.
However, most wound infections in storage result from waterborne spores in post-harvest drench solutions or in water flumes used to float fruit onto packing lines.
The drenches and water flumes become contaminated from dirty bins or dirty fruit from the orchard.
Pear is also susceptible to Penicillium rot which also causes rots of other fruits such as grape and strawberry.

Cultural control and prevention

Successful prevention and control of Penicillium rots are dependent on good crop handling and hygiene.

At harvest and during fruit thinning throw discards into the alleyway where they can be macerated and more rapidly broken down.
Similarly, push fruit dropped under the tree into the alleyway for more rapid breakdown.
Dirty bulk bins are an important source of Penicillium inoculum, particularly in the fruit drencher and grading line.
Bins must be cleaned immediately after downloading onto the grader.
Physically remove fruit and rots remaining and hose out.
Particularly dirty bins should be set aside for special cleaning with soapy water, scrubbing brush and hose.
Spraying bins with disinfectant is not effective and not a substitute for scrubbing and hosing.
Clean up packhouse stores and drencher areas so that old rotting fruit is not left where it can provide Penicillium inoculum for the new crop.
Do not dump rot discards from grading back into orchards. These can act as sources of rot inoculum for the next crop.
Dispose of in a suitable dump/hole where it can be soil covered.
Supervise packers at harvest to minimise damage to fruit.
If using post-harvest anti-scald agents, ensure the drencher solution is regularly changed to prevent the build-up of Penicillium spores.
Ensure that only good quality fruit of the correct mineral status is stored long-term. Fruit of good calcium status is more resistant to rotting.

Biological control

Research in other countries, particularly USA, has identified various microbial antagonists of Penicillium which have been developed as biocontrol agents for use as post-harvest treatments for control of Penicillium rot.

These have included both yeasts and bacteria some of which are commercially available elsewhere, although not in the UK, e.g., Yield Plus (yeast – South Africa).
However, while these products appear to be effective against wound pathogens such as Penicillium or Botrytis, they are ineffective against the orchard fungi such as brown rot which are responsible for most rotting in store.
Therefore at present these products are not worth exploring in apple.

Chemical control

Pre-harvest fungicide sprays such as cyprodinil + fludioxonil (Switch) and pyraclostrobin + boscalid (Bellis) used for control of other storage rots may give some control of Penicillium rot.
Treatment of water with chlorine (calcium hypochlorite) has been used in the USA and South Africa, either alone or as a pre-wash prior to fungicide application, to reduce inoculum levels of Penicillium spores and other wound pathogens, present on the fruit surface and in water in flotation tanks and hydro-coolers.
Trials at East Malling confirmed that active chlorine (100 ppm at pH 7.5) killed fungal spores, bacteria, yeast cells present in the drench tank, with consequent reduction in rotting.
However, such treatment is ineffective against fungal rots which are already present in the fruit as latent infections.
Specialist application equipment is needed to ensure that the effective concentration is maintained as chlorine is quickly mopped up from solution by any organic debris such as leaves.
Chlorine is also corrosive to metal surfaces.

Avoiding fungicide resistance

The resistance status of P. expansum to cyprodinil + fludioxonil (Switch) and pyraclostrobin + boscalid (Bellis) is not known. The most effective strategy is to ensure cultural measures are used effectively and not rely on fungicides for control.

Apple Best Practice Guide