EFFECT OF INOCULUM CONCENTRATION, TEMPERATURE, DEW PERIOD, AND PLANT-GROWTH STAGE ON DISEASE OF ROUND-LEAVED MALLOW AND VELVETLEAF BY COLLETOTRICHUM-GLOEOSPORIOIDES F-SP MALVAE

In greenhouse and controlled-environment tests, disease development on round-leaved mallow (Malva pusilla) and velvetleaf (Abutilon theophrasti) of Colletotrichum gloeosporioides f. sp. malvae was affected by inoculum concentration and plant growth stage at time of inoculation, postinoculation air temperature, and dew-period duration and temperature. The highest levels of control were obtained with an inoculum concentration of 2 X 10(6) spores per milliliter for M. pusilla and 4 X 10(6) spores per milliliter for A. theophrasti. All growth stages were susceptible, although younger seedlings were less susceptible than older plants. Postinoculation air temperatures of 30 C for M. pusilla and >15 C for A. theophrasti reduced disease development significantly. A minimum dew period of 20 h at 20-25 C was required to achieve satisfactory control of M. pusilla, whereas a dew period of 48 h was required to achieve a comparable level of control of A. theophrasti with the same inoculum concentration. Shorter repetitive dew periods of 16 h, which resemble field conditions more than a long, single dew period, also adequately controlled M. pusilla. However, infection on A. theophrasti was not affected by any of the repetitive dew periods. C. g. malvae meets the epidemiological characteristics of a good bioherbicide for control of M. pusilla, whereas its potential for control of A. theophrasti is questionable and remains to be determined.