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Crown Rot and Collar Rot (Phytophthora cactorum, Phytophthora syringae)

 

Collar rot on trunk

Crown rot and collar rot are distinct diseases:

  • Collar rot is a disease of the scion which usually only attacks mature trees >10 years old and mainly Cox.
  • Crown rot a disease of the rootstock which, in the UK, is mainly a disease of young trees in the first two years of establishment.

Susceptibility of varieties and rootstocks  varies, with Cox, James Grieve, MM.104 and MM.106 being the most susceptible.

Both diseases are caused mainly by P. cactorum and are favoured by wet weather. P. cactorum is soil borne and can overwinter and survive in the absence of apple as oospores (resting spores).  These germinate to release zoospores which move in soil moisture to infect the roots/root crown or scion through cracks, damage or lenticels.

The first symptoms of crown or collar rot may be poor growth, leaf yellowing or premature autumn colours.

  • In the rootstock below ground the presence of typical orange/red-brown rot under the bark is characteristic of crown rot.
  • A water-soaked, weeping area on the trunk which has a distinct orange/red-brown rot under the bark is  characteristic of collar rot.

 

Crown rot on rootstock

Both problems are sporadic and therefore difficult to monitor and predict.

However, mature Cox orchards at risk from collar rot should be checked in late June for trunk lesions, particularly where conditions in May or the previous May were wet.

Control

Effective control of both problems requires an integrated approach based on cultural methods such as avoiding wet sites for new orchards, good soil drainage, using the correct rootstock for the site and avoiding damage to the trunk or rootstock.

  • For crown rot, trees showing foliar symptoms are usually too badly damaged to save.  These should be grubbed and burnt.
  • The replanted tree and the trees in the rest of the orchard can be treated with the chemical fungicide Cuprokylt FL (copper oxychloride) or Fubol Gold WG (mancozeb + metalaxyl).
  • Cuprokylt must be used no later than bud burst and can be applied by a paint brush, knapsack, hydraulic ground sprayer or air assisted ground sprayer.
  • Fubol Gold should be applied after harvest, but before green cluster stage.
  • Collar rot can be effectively treated provided the trunk lesions are spotted early.
  • The lesion should be either cut out completely back to sapwood ensuring the cutting passes through the graft union or a groove cut down to the sapwood, surrounding the lesion.
  • The groove or the whole lesions are then painted with Cuprokylt FL according to the instructions on the current EAMU approval.

Control in organic orchards

  • Control in organic production must be based on cultural control measures , particularly selection of the correct site and scion/rootstock combination.

 

Fungicides for control of Phytophthora bark diseases- Efficacy

Active ingredient Trade names Fungicide group Safety to Typhs Use Disease controlled
copper oxychloride Cuprokylt FL inorganic safe ground spray, pruning paint, knapsack application collar rot

 

mancozeb + metalaxyl-M Fubol Gold WG dithiocarbamate + phenylamide safe ground spray applied to orchard floor Phytophthora diseases

Choice of fungicides for control of  Phytophthora bark diseases – Safety factors

 

 

Active ingredient

Hazards  

Harvest interval (days)

 

Max. no sprays per year

 

Other restrictions (Max conc/dose)

Buffer zone
human fish + aquatic life bees width

(m)

 

copper oxychloride h t u Pruning paint – bb 1 1litre/4 litres water u
copper oxychloride h t u Knapsack, hydraulic ground, air assisted ground sprayer – bb 3 Varies with product sm
mancozeb + metalaxyl M h t safe bb u 1.5 kg/ha sm

d = dangerous; h = harmful; ir = irritating, a = may cause allergic reaction, t = toxic

PH = post harvest; Pre bb = pre-bud burst, sm=statutory minimum of 5 m for broadcast air-assisted sprayers

u=uncategorised/unclassified/unspecified, c=closed cab required for air assisted sprayers

 

 Further reading