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Phytophthora rot – additional information

Disease status

Phytophthora fruit rot was relatively unimportant until the 1970s when it emerged as an important cause of rotting in stored fruit in many countries in north-west Europe, with some batches of stored Cox with up to 88% rotting due to P. syringae in the 1974 UK harvest.

  • Its emergence as an important storage rot followed the adoption of intensive systems of apple production using dwarfing rootstocks resulting in low-hanging fruit.
  • The tree row in these systems is maintained weed-free by use of herbicide (herbicide strip), thus exposing the low-hanging fruit to the risk of soil splash.
  • The rot is of sporadic occurrence associated with wet harvests but losses can be significant when weather conditions are favourable.

Other hosts

  • Pear is also susceptible to Phytophthora fruit rot.

Varietal susceptibility

  • All apple varieties appear to be susceptible to infection.

Distribution

  • Both P. cactorum and P. syringae are responsible for fruit rot.
  • P. cactorum is often responsible for rotting of fruit in summer and in warm climates, being favoured by warmer temperatures.
  • P. syringae generally causes rotting in northern apple growing regions and, in the UK, is mainly responsible for rotting in store.
  • P. syringae is favoured by cooler temperatures.
  • Phytophthora fruit rot is reported in most parts of the world where apples are produced.
  • Its relative importance is dependent on the incidence of rainfall pre-harvest.

Symptoms and recognition

Phytophthora rot appears in the orchard and in store and symptoms are similar.  Symptoms vary according to the variety.  The rot is usually firm and the skin easily peeled away.

Phytophthora rot on Cox

  • Cox – A firm rot ranging in colour from pale green to mid-brown often with a marbled appearance.
  • Bramley, Gala, Jonagold and Egremont Russet – Phytophthora rot usually appears as a firm, dark brown rot.  Very occasionally, rots may be pale green.  On Bramley the rotting is often blotchy.
  • Cox and Gala – The rot may also appear as a small brown/grey firm cheek rot, which, if the fruit is cut, is associated with extensive internal rotting.  Such symptoms are usually found in fruit which have been treated post-harvest.

Phytophthora rot on Gala

  • A vinegary aroma is often associated with Phytophthora rot.
  • The rot spreads in store by contact and so nests of rot may be present in late stored fruit.

Other rots that may be confused with Phytophthora rot

  • Phytophthora rot is usually readily distinguishable on Cox and Bramley but on Gala, Jonagold and Egremont Russet may be easily confused with brown rot (Monilinia fructigena).
  • Differences may only be distinguishable by a specialist and may require culturing on to media to be certain.

Disease cycle and epidemiology

Both P. syringae and P. cactorum are generally widespread in apple orchard soils.

  • They overwinter as resting spores (oospores) which are formed in fallen apple fruits and leaves and released onto soil when these rot.
  • P. syringae is active at mean air temperatures of 0-16oC (optimum 10-14oC), P. cactorum is active at 8-18oC (optimum 12-18oC).
  • Thus fruit rot epidemics are associated with high rainfall in cool weather for P. syringae and in warm weather for P. cactorum.
  • P. syringae is thus mainly responsible for fruit rots during UK apple harvest.
  • During rain resting spores germinate in the soil to release swimming spores (zoospores) which are splashed onto low hanging fruit or on fruit in contact with soil.
  • Infection occurs via lenticels.
  • Fruit, which are infected 2-3 weeks before harvest, rot and are therefore discarded at picking time and not stored.
  • However, if infection occurs near or at harvest, then symptomless infected fruit are stored and these develop into rots in store and spread to healthy fruit by mycelial contact causing large nests of rotted fruit since P. syringae can grow in fruit at storage temperatures of 3-4oC.

Disease monitoring and forecasting

No forecasting methods have been developed for Phytophthora fruit rot.  However, the risk of rotting in store due to Phytophthora can be assessed pre-harvest based on orchard rot history, the amount of bare ground in the orchard, % crop <½ metre (knee height) from the ground and the accumulated rainfall in the 15 days prior to harvest.

  • Identify orchards at risk from Phytophthora rot in July so that rot management strategies can be formulated.
  • At risk orchards can be identified based on orchard rot history, amount of bare ground and likely incidence of low hanging fruit.
  • Reassess these factors nearer harvest in August / September.
  • Estimate the bare ground under trees, taking into account mulch and weed cover:

100% bare ground (overall herbicide) = high risk.

0% bare ground (overall grass or mulch or weed cover) = low risk.

20% or >bare ground (herbicide strip) = moderate-high risk.

  • Multi-row bed orchards are most at risk.
  • Select 20 trees at random and assess the percentage of the crop <½ metre (knee height) from the ground:

15% or >of crop = risk.

  • Monitor rainfall in the 15 days before harvest:

20 mm or >accumulated rain = risk.

Procedures for assessment of risk of Phytophthora rot pre-harvest

 

Orchard factor Assessment procedure Risk criteria
1. Bare ground Inspect orchard in July

Estimate bare ground under trees, taking into account mulch and weed cover

100% bareground

= overall herbicide

= high risk

overall grass or mulch

= 0% bareground

= no risk

20% or more bare ground in

herbicide strip

= moderate/high risk

 

2. % crop <½ metre (knee height) from soil splash Select 20 trees at random and assess % crop <½ metre from soil

Make assessment in July to identify at-risk orchards.

Reassess near harvest

 

15% or >= risk
3. Rainfall Monitor rainfall in 15 days up to harvest

 

20 mm or > = risk

Cultural control

  • Throw dropped fruit into alleyway and macerate to encourage rapid rotting.
  • Apply 5% urea spray prior to leaf fall and after leaf fall macerate up leaves in the orchard to encourage rapid breakdown and reduce the risk of colonisation by Phytophthora.
  • Mulch soil surface to reduce risk of soil splash.
  • Avoid overall herbicide in orchards.
  • Selectively harvest fruit where a risk has been determined so that only fruit picked above ½ metre from the ground, and therefore less likely to be soil splashed, are stored.
  • Avoid placing bulk bins in mud at harvest time.
  • Put mud flaps on tractors to avoid mud splash on to fruit during transport.
  • Where a Phytophthora risk has been determined and no fungicide treatment applied, schedule fruit for earlier marketing to minimise losses in store.

Biological control

  • No biocontrol methods have been developed.

Chemical control

Pre-harvest

  • In orchards at risk from Phytophthora rot, particularly where orchard rot history has indicated a significant incidence of Phytophthora rot in store following a wet harvest, two fungicide sprays of captan or Bellis (pyraclostrobin + boscalid) to low hanging fruit, one month and 14 days pre-harvest will give some protection to fruit from Phytophthora rot.
  • Alternatively Fubol Gold WG (mancozeb+metalaxyl-M) has off-label approval for use as a spray to the orchard floor for control of Phytophthora fruit rot, up to two treatments per season applied after leaf fall but before green cluster.
  • Captan or Bellis are the preferred orchard treatment at present.
  • Use of mancozeb + metalaxyl-M in this way has a high risk of Phytophthora species developing resistance to it.
  • This would affect not only control of Phytophthora fruit rot but also of other Phytophthora diseases of apple such as crown and root rot.

Avoiding fungicide resistance

  • The risk of resistance to captan is very low, however the risk of resistance to metalaxyl-M is high.
  • Monitoring for resistance in P. syringae isolates has detected isolates which are slightly less sensitive, however there was no evidence of reduced control of the fruit rot.
  • Resistance of P. cactorum to metalaxyl has been demonstrated in other crops such as strawberry crown rot but none has been detected in apple.
  • The resistance status of Phytophthora spp. to Bellis (pyraclostrobin + boscalid) is not known.