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Brown rot – additional information

Disease status

  • Brown rot is one of the most important causes of rotting in stored apples and also causes significant losses in the orchard pre-harvest.
  • The fungus attacks fruit and also causes spur cankers.
  • If not controlled, the disease can build up to a significant incidence over several seasons.
  • Monilinia fructigena is not restricted to apple and also attacks pears and occasionally plums, cherries and quince.

Varietal susceptibility

  • All apple varieties are susceptible to brown rot.
  • In store and in the orchard it is the most frequent rot found on all varieties.
  • In the orchard, losses can be greatest on early varieties such as Discovery and Grenadier which are not usually stored.

Distribution

  • Brown rot is widespread and common in UK apple orchards and in Europe.

Symptoms and recognition

Orchard – fruit rot

  • Affected fruit show a pale brown/mid brown circular rot usually associated with a wound.
  • The rot rapidly becomes covered with buff-coloured pustules, usually in concentric rings.
  • Although the initial infection is always through a wound, the brown rot fungus can then spread to other fruit in a cluster by contact.

Orchard – cankers and mummies

 Brown rot canker and mummified fruit in orchard

  • Brown rot overwinters in the orchard as cankers usually at the base of dead fruiting spurs, often referred to as foot cankers.
  • Water-marking may be apparent on such cankers.
  • Buff-coloured pustules appear on these cankers in early summer.
  • The fungus also overwinters as mummified fruit either stuck on the tree or on the orchard floor.
  • These fruits are shrivelled almost black and develop buff-coloured pustules in summer after rain.

Store – fruit rot

  • Infection in store begins as a small brown spot on a wound or where a healthy fruit has been in contact with an infected fruit.
  • It rapidly invades the entire fruit forming a mid to dark brown almost black, usually evenly shaped firm rot.
  • In Cox and Bramley, the rot surface is often covered with white fungal growth and black resting bodies (sclerotia).
  • This symptom is less common on Gala, Jonagold and Egremont Russet.
  • After prolonged storage the whole fruit may become hard, black and mummified.
  • Symptoms of the rot in store do not resemble those observed in the orchard.
  • Brown rot spreads in store by contact.
  • Nests of brown rotted fruit may therefore be observed in later stored fruit.

Other diseases that may be confused with brown rot

In the orchard

Brown rot cankers can be confused with other cankers, especially those caused by blossom wilt (see Diagnosis of cankers, in Apple Canker section [hyperlink ?]).

  • Mummified fruit are usually distinctive especially when sporing.
  • Similarly, brown rot in the orchard is easily distinguished from other rots by the buff-coloured pustules.
  • Botryosphaeria obtusa can also cause a brown rot on apples before harvest, usually at the eye end of the fruit and associated with a wound.
  • This rot can be distinguished from brown rot by the absence of buff-coloured pustules, the possible presence of pin head-sized fruiting bodies, and the fact that it is usually very firm.

In store

Distinguishing brown rot from other rots may be more difficult since many are brown in colour.

  • Brown rot may be confused with Botrytis rot, Phytophthora rot, particularly on Gala, Jonagold and Egremont Russet.
  • The identity of the rot may only be determined by a specialist and may require culturing onto artificial media in the laboratory to be certain.

Disease cycle and epidemiology

  • Monilinia fructigena overwinters in the orchard either on cankers or as mummified fruit on the tree or ground under the tree.
  • These produce spores after rain in the summer sometimes as early as May but more usually in June, depending on temperature.
  • The spores are spread by wind to infect young fruitlets through wounds.
  • The fungus can only infect through wounds, but once in, can then spread from infected to healthy fruit in a cluster by mycelial spread.
  • Wounds can be caused by insects (especially codling moth), russet cracks, scab, growth cracks.
  • Free water is required for spore germination but once in the wound, further development is not dependent on rain.
  • Wounds are most susceptible when fresh and susceptibility declines with age.
  • Conversely, fruit is most resistant when young and susceptibility increases as the fruit matures.
  • Infected fruit rapidly become covered with buff-coloured pustules which serve as inoculum for other fruit, and are spread by wind and by insects attracted to the juicy rotting fruit.
  • Fruit infected near harvest remain symptomless and are harvested along with healthy fruit and stored.
  • The rot subsequently develops in store and spreads by contact to healthy fruit.
  • In the orchard, the fungus can spread from the rotted fruit into the fruiting spur forming a canker.
  • Rotted fruit may drop and form mummies on the ground or remain attached to the tree and mummify in situ.
  • The sexual state of the brown rot fungus occurs on overwintered mummified fruit, but is very rare in the UK and is not important in the epidemiology of the disease.
  • The fungus is favoured by warm humid weather.
  • Rain is essential to initiate sporulation, but not essential for fruit infection as fresh wound surfaces are moist.

Disease monitoring and forecasting

Assessment of rot incidence during fruit grading will give an indication of the problem in the orchard and the risk of rotting in store due to brown rot can be assessed pre-harvest to decide on storage potential.

  • In late August or as near harvest as possible, inspect 20 trees at random in the orchard and record the numbers of fruit infected with brown rot.  Include dropped fruit as well.
  • Record total fruit on about five trees to give an estimate of numbers of fruit per tree.
  • Estimate % fruit with brown rot/tree.
  • Numbers >1% brown rot indicate a significant risk of brown rot in store.
  • Orchards with early varieties such as Discovery or Grenadier as pollinators, which have a high incidence of brown rot, will increase the risk of brown rot.
  • No forecasting methods have been developed for brown rot of apple.

Cultural control

  • Prune out brown rot cankers during winter pruning.
  • Similarly, remove mummies from the tree during pruning and, together with those under the tree, throw into the grass alley where they can be macerated by the mower.
  • During summer remove infected fruit as soon as they appear and throw into the alley where they can be macerated.
  • Remove waste fruit from early pollinator varieties and throw in the alley to be macerated so they are not a source of inoculum for the main orchard variety. Alternatively, when planning orchards, avoid using early varieties as pollinators.
  • At harvest selectively pick fruit so only sound fruit is placed in the bin. This will significantly reduce the incidence of brown rot in store.
  • Avoid damage to fruit, especially at harvest.
  • Cultural control is an important aspect of integrated control, but is only one contributory factor.

Biological control

  • This is not an option at present.
  • Research in other countries indicates that fungal antagonists exist which may suppress canker development and could form the basis of alternative control.

Fungicide control

Ensure good control of apple scab and apple pests, especially codling moth, which may provide entry points for the brown rot fungus.

  • Pre-harvest fungicide sprays with captan, Switch (cyprodinil + fludioxonil) or Bellis (pyraclostrobin + boscalid) are only partially effective as the fungus invades fruit through damage.
  • Where there is a significant risk of brown rot in store, market the fruit early

Avoiding fungicide resistance

  • The risk of resistance is generally low as intensive fungicide programmes are not used.
  • Resistance to benzimidazole fungicides is common in the closely related species M. fructicola but none has been recorded in M. fructigena.

Further reading

Byrde and Willets, 1977. The brown rot fungi: their biology and control.