EFFECTS OF DIMETHOMORPH ON THE MORPHOLOGY AND ULTRASTRUCTURE OF PHYTOPHTHORA

Dimethomorph at a concentration of 1.0-mu-M had profound effects on the morphology and ultrastructure of Phytophthora. The principal changes in ultrastructure were associated with extensive proliferation and aberrant deposition of cell-wall material. In actively growing regions of P. infestans, abnormal wall ingrowths (pegs) were laid down within 2 h exposure, some giving rise to false septa that occluded hyphae. The location of false septa in this species and in P. erythroseptica and P. cactorum coincided with the constrictions in hyphae that gave treated colonies a beaded morphology. Altered wall biogenesis was also manifested as thickening and in the development of more than one cell wall layer. Aberrant wall depositions contained ground and membranous cytoplasmic inclusions. Other ultrastructural changes induced by dimethomorph were the accumulation of fingerprint vacuoles, dense bodies and membrane debris (whorls). It is concluded that in the presence of dimethomorph there is loss of control of the biochemical processes involved in normal cell wall biogenesis.