BIOCONTROL OF PHYTOPHTHORA-CACTORUM IN-VITRO WITH ENTEROBACTER-AEROGENES

Phytophthora cactorum causes significant mortality of mature trees in New Zealand pipfruit orchards. Replants of infected sites often succum b to this pathogen within 3 years. Enterobacter aerogenes, strain Bs w as reported to control P. cactorum infection of apples in nursery soil s in British Columbia. This strain was imported to New Zealand in 1991 and compared with indigenous isolates off. aerogenes. The Canadian is olate differed from New Zealand isolates in utilisation of tartrate, t olerance of NaCl (5 cf. 7%), and inhibition of P. cactorum in vitro. S train B8 was fungicidal whereas New Zealand isolates were fungistatic. Electron microscopy of mycelium exposed to strain B8 on agar revealed extensive disruption of cell membranes, intermixing and aggregation o f organelles, necrosis, vacuolation, breakdown of the cell wall, and h yphal death. Hyphae exposed to New Zealand isolates of E. aerogenes ex hibited vacuolation of the outer cytoplasm and degradation of the cell wall and plasmalemma but not cell degradation and death.