EFFECTS OF INOCULUM CONCENTRATION, LEAF AGE AND WETNESS PERIOD ON THEDEVELOPMENT OF DARK LEAF AND POD SPOT (ALTERNARIA-BRASSICAE) ON OILSEED RAPE (BRASSICA-NAPUS)

Experiments were done under controlled environment and glasshouse cond itions to study the effects of inoculum concentration, leaf age and we tness period on the development of dark leaf and pod spot (Alternaria brassicae) on oilseed rape (Brassica napus). On leaves of potted oilse ed rape plants (cv. Bienvenu) inoculated with A. brassicae conidial su spensions, the severity (number of lesions cm(-2)) of dark leaf spot i ncreased as inoculum concentration increased from 80 to 660 Spores ml( -1) and as leaf age increased from 4 to 14 days. On pods on detached r acemes of spring oilseed rape (cv. Starlight), the incidence of dark p od spot (% of pods diseased) increased as inoculum concentration incre ased from 80 to 10(4) spores ml(-1). Increasing inoculum concentration above 10(4) spores ml(-1) did not increase the incidence but did incr ease the severity of dark pod spot. A minimum wetness period of 4 h wa s needed for infection of oilseed rape leaves (cv. Envol) by A. brassi cae at 18 degrees C and disease severity increased with increasing wet ness period up to 12 h. The length of dry interruptions after 3-8 h of initial wetness affected the severity of dark leaf spot. A second wet ness period increased the severity of dark leaf spot if the dry interr uption was less than or equal to 6 h and if the first wetness period w as less than or equal to 8 h. The incubation period of A. brassicae de creased from 3.5 to 2.5 days as inoculum concentration increased from 80 to 660 spores ml(-1) on leaves (cv. Bienvenu) at 17-25 degrees C an d from 3.8 to 1.0 day as inoculum concentration increased from 80 to g reater than or equal to 2 x 10(3) spores ml(-1) on pods (cv. Starlight ) at 18 degrees C.