Colletotrichum gloeosporioides f. sp. malvae (C. g. malvae) is effective in controlling round-leaved mallow (Malva pusilla), a common weed in the Canadian prairies. To confirm host range experiments, the latent period and penetration of C. g. malvae was determined under controlled environmental conditions on field crops (wheat, flax, lentil, mustard, rape seed, sugar beet, sunflower, safflower) and on crops in the Malvaceae (okra and cotton) as well as on round-leaved mallow, from which C. g. malvae was originally isolated. In non-target crop plants, very little germination, appressoria formation, or penetration of C. g. malvae were observed compared with the infection occurring on round-leaved mallow. Of the non-target species tested, most penetration was observed on safflower, with only 5·1% penetration form the total appressoria formed compared with 17% on round-leaved mallow. C. g. malvae was re-isolated from all crop cvs tested, but only from inoculated stems and leaves. The recovery of C. g. malvae significantly decreased with time of isolation of plant material for most cvs. Of the millions of conidia applied to non-target crops, only a few were present after 72 h, few appressoria were produced, and only minimal penetration occurred. The behaviour of C. g. malvae conidia on the surface of leaf material of non-target plants supports the visual disease ratings observed in the experiments of crop tolerance under controlled and field condition and host-range tests.