SIVAN, J., Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
BARKAI‐GOLAN, R., Agricultural Research Organization, Volcani Center, Bet Dagan, Israel

Stemphylium botryosum f. lactucae, incitant of a leaf‐spot disease of stored lettuce, was found to be relatively restricted in its host range. Cross‐inoculations with spore suspension of this fungus failed to induce symptoms in any of the host plants tested, except carrot. Among isolates of S. botryosum from various hosts, only the isolate from carrot induced slight symptoms on lettuce. While mycelial growth of the lettuce isolate was confined to the range 13–37 oC spores germinated at more extreme temperatures. The optimum temperature for germination and for radial growth on PDA was found to be between 25 and 30 oC. Wet spores were quickly inactivated at 50 oC, whereas more than 40 % of dry spores withstood a 24 h exposure to that temperature. Only the outer leaves of lettuce responded readily to inoculation with a spore suspension, the required incubation period being 3 days at 25 oC. Symptoms developed less readily on bruised leaves. Relative humidity approaching saturation was necessary for prompt and typical infection, notably during the 24 h following inoculation. Short dry periods (60 % r.h.) interposed at a later stage, while somewhat inhibitory, did not prevent infection. Copyright © 1976, Wiley Blackwell. All rights reserved