Both P. syringae and P. cactorum are generally widespread in apple orchard soils.
- They overwinter as resting spores (zoospores) which are formed in fallen apple fruits and leaves and released onto soil when these rot.
- P. syringae is active at mean air temperatures of 0-16oC (optimum 10-14oC), P. cactorum is active at 8-18oC (optimum 12-18oC).
- Thus fruit rot epidemics are associated with high rainfall in cool weather for P. syringae and in warm weather for P. cactorum.
- P. syringae is thus mainly responsible for fruit rots during UK apple harvest.
- During rain resting spores germinate in the soil to release swimming spores (zoospores) which are splashed onto low hanging fruit or on fruit in contact with soil.
- Infection occurs via lenticels.
- Fruit, which are infected 2-3 weeks before harvest, rot and are therefore discarded at picking time and not stored.
- However, if infection occurs near or at harvest, then symptomless infected fruit are stored and these develop into rots in store and spread to healthy fruit by mycelial contact causing large nests of rotted fruit since P. syringae can grow in fruit at storage temperatures of 3-4oC.