Penicillium rot or blue mould is one of the most common and easily recognised post-harvest rots of apple, but is not necessarily responsible for large losses. Its significance has increased in recent years because it produces a mycotoxin, patulin, which occurs in Penicillium-rotted fruit and subsequently in fruit juice produced from reject fruit.
The life cycle and epidemiology involves airborne or waterborne spores invading fruit through wounds, bruises or cracks anywhere on the fruit surface and is often a secondary invader of other rots.
Penicillium rot is rarely seen in the orchard apart from occasionally on fallen fruit under the tree. Consequently there are no forecasting methods developed or monitoring systems applicable to it. The fungus is ubiquitous and infection will always occur if fruit is damaged or not handled correctly.
- All apple varieties are susceptible but it is most often seen on Bramley in store.
- 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.
- Mature lesions are covered in brilliant white pustules which quickly turn blue.
- P. expansum survives on mummified fruit or fruit bits stuck on bulk bins or lying around storage or packhouse areas
- Most wound infections in storage result from water borne spores in post-harvest drench solutions (e.g. anti-scald agents) or in water flumes used to grade fruit.
Control or prevention of Penicillium rot is mainly dependent on cultural methods based on good hygiene, particularly of bins, and of good supervision at harvest to minimise damage to fruit.
- Pre-harvest fungicide treatment is generally ineffective against Penicillium as rot incidence is related to fruit damage.
- However, both Switch (cyprodinil + fludioxonil) and Bellis (pyraclostrobin + boscalid) are active against P. expansum and may give some control.
- Most isolates of P. expansum are resistant to benzimidazole fungicides.
- Treatment of water with chlorine both may give some control of Penicillium.
Control in organic orchards
- Cultural methods of control are equally applicable and effective in organic production, provided only best quality fruit is stored.