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 or Fubol Gold WG (mancozeb + metalaxyl). At the time of up-dating this information in January 2024, copper was not approved for use on apple, but various products have been authorised for use as 'emergency authorisations' in the past, so growers and agronomists should continually check the latest status of copper products.
- 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 (EAMU 2119/21) | 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 width (m) | ||
|---|---|---|---|---|---|---|---|
| human | fish + aquatic life | bees | |||||
| 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 | |||||||