Abstract:
The effects of metiram (Polyram 80 DF) on the growth of Venturia inaequalis, cause of
apple scab, and the degradation of metiram were examined in culture media. Samples of V.
inaequalis conidia were collected from nine orchards in 1998 and six orchards in 1999 and tested
for sensitivity. Samples were plated on water agar amended with metiram or mancozeb. Mean
EC50 values (effective concentration of fungicide required to inhibit germination of half the
conidia) for each population were calculated. The mean EC50 values for metiram ranged from
0.26 - 1.20 ^ig metiram a.i./ml, with differences (Student Newman Keul's Test (SNK), a=0.05)
between populations. EC50 values for mancozeb ranged from 0.06 - 0.58 which were also
different (SNK, a=0.05). Five of these populations were examined for mycelial growth
sensitivity to metiram by testing 30 monoconidial isolates from each population on metiram
amended potato dextrose agar. Mean EC50 values for populations were calculated and ranged
from 3.44-5.94 |ig metiram/ml, and showed differences (Friedman Test, a=0.05). As the EC50
values obtained are far less than the concentrations applied in the field, results indicate that
Ontario populations of V. inaequalis are still sensitive to metiram and mancozeb.
The stability of metiram in PDA at 22°C was studied over a 10-day period. The initial
concentration of metiram decreased by approximately 50% within the first day, and continued to
decline slowly, to approximately 20% of the initial concentration. The factors possibly affecting
initial metiram degradation, including agar, heat, and the use of glass or polystyrene Petri dish
composition were examined. The effects from the polystyrene in the Petri dish composition were
negligible, however more studies must be done to examine metiram degradation during the first
24 hours of preparation.
