Biocontrol of Phytophthora cactorum in vitro with Enterobacter aerogenes

Abstract

Phytophthora cactorum causes significant mortality of mature trees in New Zealand pipfruit orchards. Replants of infected sites often succumb to this pathogen within 3 years. Enterobacter aerogenes, strain B8 was reported to control P. cactorum infection of apples in nursery soils in British Columbia. This strain was imported to New Zealand in 1991 and compared with indigenous isolates of E. aerogenes. The Canadian isolate differed from New Zealand isolates in utilisation of tartrate, tolerance of NaCl (5 cf. 7%), and inhibition of P. cactorum in vitro. Strain 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 of organelles, necrosis, vacuolation, breakdown of the cell wall, and hyphal death. Hyphae exposed to New Zealand isolates of E. aerogenes exhibited vacuolation of the outer cytoplasm and degradation of the cell wall and plasmalemma but not cell degradation and death.