Usually P. cactorum and occasionally P. syringae are the fungi responsible for crown rot and collar rot in the UK and Europe, but more recently, particularly in other parts of the world, other species of Phytophthora have also been implicated.
- P. cactorum and P. syringae are soil-borne and frequently present in apple orchard soils.
- Primary inoculum may also be introduced into an orchard with infected nursery stock or possibly in contaminated irrigation water.
- Once introduced Phytophthora spp. survives primarily as oospores (resting spores) in plant debris or soil.
- In the absence of the host, oospores may survive in the soil for several years depending on conditions.
- The fungus may also multiply in the soil by colonising fallen apple leaves or fruits on the ground or other plant parts, or even colonising some other plant species, such as certain weed species.
- In wet soil conditions the oospores germinate to produce fruiting bodies (sporangia) which, in turn, release zoospores which move in soil moisture to infect apple bark on roots or scion.
- Disease incidence and development is affected by availability of water above ground and in the soil, soil type, tree age, the height of the graft union, the interaction between scion and rootstock and soil management techniques.
- There is also seasonable variability in the extent to which apple bark tissues are colonised by Phytophthora spp.
- Colonisation by P. cactorum is greatest between the pink bud stage of tree growth and the beginning of extension growth, i.e. when the tree is actively growing in spring/early summer, so wet conditions during this period are conducive to attack by P. cactorum. P. syringae however, is more active in the autumn/winter months when the tree is dormant.
- Infection of apple tissue takes place mainly at the soil surface and tissues at greatest risk of infection are those in contact with the soil or within the range of soil splash.
- Infection of soft-tissue structure such as fine roots can occur directly, but usually wounds are needed for bark infection.
- These may be from frost cracks, cultural operations, or age cracks appearing at the graft union. Infection of root burrs may also lead to crown rot.
- Soil moisture is the most critical factor in crown rot and collar rot and these are therefore more likely to be prevalent on sites with poor drainage.
- Also drainage may be locally poor when trees have been badly planted in post hole bores.