Botrytis rot caused by the fungus Botrytis cinerea is one of the most common causes of rotting in stored apples.
- The disease develops more rapidly at cold storage temperatures than any other rot.
- Losses can be significant with up to 12% in untreated fruit.
- The rot occurs mainly in store both as a wound rot and as a primary rot.
- It is rarely seen in the orchard as an extensive rot pre-harvest, but occurs as a blemish or slight rot at the calyx end of the fruit which dries to form dry-eye rot.
- Botrytis cinerea has a wide host range and is capable of attacking a wide range of plant species, weeds, ornamentals, arable and causing significant problems on most horticultural crops especially fruit.
- There appears to be little or no host specialisation.
- In the UK it is the main cause of rotting on stored pears and on strawberry, raspberry and blackcurrant.
- All apple varieties are attacked.
- Botrytis cinerea is ubiquitous, present in all orchards and areas of the UK and most of the world.
- It causes problems as a fruit rot on apple wherever apples are grown especially USA, South Africa, Australia and Europe.
In the orchard
Botrytis fruit rot is rarely seen in orchards as a rot. On apple it may be visible as dry-eye rot at the calyx end of the fruit.
- The symptoms range from a slight skin red blemish on one side of the calyx to a distinct one-sided rot which has dried and shrunk to form the typical dry-eye rot.
- The presence of such symptoms in the orchard usually bears no relationship to the subsequent incidence of Botrytis rot in store.
On Cox the symptoms are very variable depending on the source of infection.
- Botrytis rot associated with wounds tends to be regular in shape, firmish, pale to mid-brown in colour, often with a darker area around the calyx and lenticels (some times reddish spots), giving the fruit a freckled appearance.
- Botrytis rot associated with calyx (eye) infection varies in colour from pale to dark brown and is irregular in shape, often appearing as fingers of rot extending down from the calyx.
- This irregular rotting serves to distinguish Botrytis from other causes of eye rots such as Nectria which are usually circular and sunken.
- Similar rotting may also originate from the stalk end or on the cheek, which may suggest a core rot origin.
On other apple varieties Botrytis rot is mainly mid-brown.
- Infected fruit initially remain moderately firm becoming softer with time.
- Mycelium with grey spore masses may be visible particularly on the calyx or around the wound.
- Once out of store, these spore masses become more abundant, and are a useful aid to identification.
- Very occasionally on apple, large black resting bodies (sclerotia) may be seen, particularly at the wound where the rot originated.
- Botrytis rot spreads in store by contact and nests of rot may therefore be visible in later stored fruit.
Other problems that may be confused with Botrytis rot
Botrytis rot is most easily confused with brown rot or Phytophthora rot.
- On Cox, Phytophthora rot can be usually distinguished by its marbled appearance.
- On other varieties, particularly Gala, Egremont Russet and Jonagold, the symptoms are very similar and can only surely be distinguished by laboratory examination.
Botrytis cinerea is ubiquitous in the orchard being present as sclerotia in soil on plant debris, weeds, grass mowings, windbreak trees, mummified fruits and bark.
- In wet, windy, weather at most times of the year the sclerotia sporulate and the spores (conidia) are spread by wind and rain.
- At blossom time spores will infect dying blossom and remain as latent infections in the remains of the flower parts still attached to the developing fruits, or become established as latent infections in the calyx.
- Occasionally the fungus continues to develop and form a small rot or blemish around the calyx.
- This does not usually progress far and then dries forming the dry-eye rot lesion.
- Usually though the fungus does not start to rot fruit from these blossom infections until the fruit has been in store for several months, usually December onwards.
- Then the fungus will invade the fruit at the calyx end forming the typical calyx end rot with irregular fingers of rotting spreading down from the calyx.
- Once developed the fungus can spread to healthy fruit in the bin by contact spread, forming large nests of rotted fruit. Research has shown that most apples become symptomlessly infected with Botrytis during flowering.
- However, not all infected fruit subsequently rot in store.
- The factors that affect development of Botrytis eye rot in store are not fully understood.
- Controlled atmosphere storage, especially low oxygen, appears to encourage rot development but further research is needed to determine other factors that may be involved.
Botrytis may also act as a wound pathogen, where it behaves more like Penicillium rot.
- Fruit becomes infected via wounds sustained during harvesting and handling, particularly from Botrytis spores contaminating drench tanks and water flumes on grading machines.
- Botrytis that invades via wounds starts rotting immediately in store and the rot readily spreads to healthy fruit in the bin causing extensive nesting of rots.
In the orchard
Since the rot is not usually visible in the orchard and the inoculum ubiquitous and not a limiting factor, disease monitoring as a basis for decisions is not possible.
- The incidence of dry-eye rot in the orchard is not related to subsequent rotting in store.
- Forecasting methods are being developed for Botrytis rot on other fruit and flower crops such as strawberry.
- These systems are not appropriate for Botrytis as a wound pathogen and currently unlikely to be applicable to Botrytis eye rot until the factors affecting rot development in store are identified.
Rot risk assessment
Since some orchards appear to be more prone to Botrytis eye rot in store, it is possible to obtain some idea of the risk of rotting in store based on:
- History of Botrytis eye rot (from packhouse records)
- Rainfall June to harvest (average or >average during this period indicates a possible risk)
Decisions on the risk of Botrytis eye rot on Cox
To minimise losses from Botrytis eye rot developing in store fruit should be scheduled for earlier marketing where:
- There is a risk of Botrytis eye rot identified from rot history (moderate to high incidence).
- The rainfall during summer has been average or greater than average.
Since Botrytis is ubiquitous in orchards, elimination of inoculum sources is impossible and cultural methods of control are not appropriate for control of Botrytis eye rot.
- However, successful prevention and control of Botrytis as a wound rot, like Penicillium rot is dependent on good crop handling and hygiene.
- In the orchard, throw discarded fruit into the alleyway where they can be macerated and more rapidly broken down.
- Remove old rotted fruit from bulk bins and scrub and clean as they come off the grader.
- Keep packhouse areas clean to minimise contamination of water flumes in packhouses.
- Supervise pickers at harvest to minimise fruit damage and ensure damaged fruit is not stored.
- Ensure that only fruit of the correct mineral status is stored long term.
Research in other countries, particularly USA has identified various microbial antagonists of Botrytis which have been developed as biocontrol agents for use as post-harvest treatments.
- These are generally also active against Penicillium rot. Examples include Yield Plus (yeasts ex South Africa).
- These appear to be effective against wound fungi such as Botrytis and Penicillium, but not against orchard fungi, or Botrytis rot arising from latent infections of the calyx.
- Currently there are no commercially available biocontrol agents approved for use on apples.
Pre-harvest orchard sprays
In Europe and USA, fungicide sprays applied at blossom time have given some control of Botrytis eye rot in store. In the UK, in trials, similarly timed sprays were ineffective.
- A pre-harvest spray of Bellis (pyraclostrobin + boscalid) or Switch (cyprodinil + fludioxonil) will give some control of the Botrytis wound rot.
- On Bramley, the use of diphenylamine for control of scald will also give some protection against Botrytis rot.
- Treatment of water with chlorine (see under Penicillium rot) will reduce inoculum levels of Botrytis present in the drench tank water and reduce wound infections due to B. cinerea.
- It will not control Botrytis present as latent infection.
Avoiding fungicide resistance
- Tests have shown that 60% of B. cinerea isolates from apple are resistant to benzimidazole fungicides.
- It is not known whether there are isolates resistant to Bellis (pyraclostrobin + boscalid) or Switch (cyprodonil + fludioxonil).