Many factors affect the susceptibilty of the tree to canker. These include climate, variety, rootstock, soil type, water content, pruning and fertilizer regime.
- Applications of excess nitrogen fertilizer, especially farm yard manure, increase the canker risk.
- A high water table and heavy clay acid soil will also increase the incidence of canker.
- Factors that put the tree under stress, including water stress on light sandy or thin chalky soils, also appear to increase the canker risk.
- Shoot growth also appears to influence canker development, such that when shoot growth is rapid canker development is increased.
Varietal susceptibility
All varieties are apparently susceptible to canker to some degree and a variety can vary in its degree of resistance between localities. Rootstock can also influence the susceptibility of the scion variety.
- Trees on less vigorous rootstocks such as M.9 tend to be more canker susceptible.
- Cameo, Gala, Spartan, Fiesta, Discovery, Mackintosh, Ida Red, Red Delicious are considered very susceptible.
- Cox, Bramley, Jonagold are moderately susceptible and Grenadier has low susceptibility.
- Varietal susceptibility to N. ditissima may be linked to the size of the xylem or water conducting vessels in the wood.
- More susceptible varieties such as Spartan tend to have larger vessels.
Susceptibility of some apple varieties to fungal diseases
| Variety | Scab | Powdery mildew | Nectria canker | Blossom wilt |
|---|---|---|---|---|
| Braeburn | m | m | m | ? |
| Bramley | h | m | m | vl |
| Boskoop | m | m | ? | ? |
| Cameo | h | m | h | ? |
| Cox | h | h | m | m |
| Crispin | vh | h | h | vl |
| Ceeval | m | m | ? | ? |
| Discovery | vl (pg) | vl (pg) | h | vl |
| Early Victoria | l | l | ? | ? |
| Edward VII | l | vl | ? | ? |
| Egremont Russet | m | m | m | vl |
| Elstar | h | m | m | ? |
| Falstaff | m | l | l | vl |
| Fiesta | h | m | h | vl |
| Florina | l (vf) | l | ? | ? |
| Gala | vh | m | h | vl |
| Greensleeves | m | l | ? | ? |
| Grenadier | vl (pg) | vl (pg) | vl | vl |
| Golden Delicious | h | h | m | vl |
| Idared | h | h | m | vl |
| James Grieve | m | m | m | vh |
| Jonagold | h | m | m | vl |
| Judeline | l | l | ? | ? |
| Kent | h | m | h | vl |
| Liberty | m (vf) | m | ? | ? |
| Lord Derby | l | m | m | vh |
| Lord Lambourne | l | vl | ? | l |
| Laxton Superb | m | m | m | vl |
| Red Charles Ross | m | m | ? | ? |
| Saturn | vl (vf) | m | ? | ? |
| Spartan | m | m | vh | vl |
| St Edmunds Pippin | m | m | m | m |
| Topaz | vl (vf) | m | l | ? |
| Vanda | vl (vf) | h | l | ? |
| Winston | h | ? | ? | ? |
vl = very low, pg = polygenic resistance, vf = major gene resistance
l = low susceptibility, m = moderate, h = high, vh = very high
Canker in new apple orchards
Canker on trees in newly planted apple orchards can arise from two sources. Either from the nursery of tree origin as symptomless infection, which can take up to 2 or 3 years to express itself or spread in from existing canker in an adjacent orchard.
- The relative importance of these inoculum sources varies according to site.
- A new theory on canker in young orchards questioned the accepted view, but recent research using DNA fingerprinting techniques has confirmed that canker in new orchards may have spread in from adjacent cankered orchards and/or be introduced from the nursery as symptomless infection on young trees.