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 is 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.

 

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 a copper product (check current status of copper authorisations before use) or Fubol Gold WG (mancozeb + metalaxyl).
• 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.

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
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)
mancozeb + metalaxyl M	h	t	safe	Pre bb	u	3 kg/ha/yr	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