Development and physiology of infection by the downy mildew fungus Peronospora parasitica (Pers. ex Fr.) Fr. in susceptible and resistant Brassica species

Abstract

The growth of two isolates of the downy mOdew fungus Peronospora parasitiea, one obtained from cauliflower (Brassica oleracea) and the other from oilseed rape (B. napus) was assessed in their respective hosts of origin, and also in the alternative combination. Both isolates were capable of infecting either host, but there were marked contrasts in the time course and extent of mycelial development, the amounts of associated host-cell necrosis, and eventual intensity of sporulation. Oilseed rape, which was partially resistant to the isolate from cauliflower, exhibited extensive necrosis of mesophyll cells, in conjunction with reduced mycelial development, and delayed and reduced sporulatioo by the pathogen. The isolate from, oilseed rape was virulent on both host species. Pathogenesis in the susceptible combinations was accompanied by large increases in electrolyte leakage, and increased activity of the enzymes /3-glucosidase, ribonuclease, and perO'Xidase. Effects OD chlorophyll content were variable and activities of acid phosphatase and acid phosphodiesterase were unaffected. Electrophoretic analyses of extracts from fungal sporangia and infected seedlings indicated that the large increases in j3-glucosi,dase were of pathogen origin, while evidence from inhibitor studies suggested that enhanced ribonuclease activity was due to a new post-infectional fonn of the enzyme. The significance of these findings is discussed in relation to pathogenesis and host resistance mechanisms.

Monograph on downy mildew of crucifers.

“…Similar cytological events occur in uninduced hosts inoculated with incompatible isolates of P. parasitica (Kluczewski and Lucas, 1982 (Natti et al, 1967). However, in a later study, Hoser-Krause et al (1991) found that in broccoli, (B. (Dickson and Petzoldt, 1996).…”

“…56) and peroxidase (Fig. 57) (Kluczewski and Lucas, 1982). The large increase in /?.-glucosidase were of pathogen origin while enhanced ribonuclease activity was due to a new post-infectional form of the enzyme.…”

“…Pathogenesis in a susceptible combination is accompanied by large increases in electrolyte leakage, and increased activity of the enzymes, B-glucosidase, ribonuclease and peroxidase (Kluczewski and Lucas, 1982). The large increase in B-glucosidase originates from the pathogen and the enhanced ribonuclease activity is due to a new post infectional form of the enzyme.…”

“…The growth of two isolates of P. parasitica obtained from cauliflower and oilseed rape (B. napus) was assessed in their respective hosts of origin and also in the alternative combination by Kluczewski and Lucas (1982). Both isolates were capable of infecting either host, but there were marked contrasts in the time course and extent of mycelial development, the amounts of associated host cell necrosis, and eventual intensity of sporulation (Figs.…”