• Labonte, George A
  University of Maine
  

Studies to determine the organisms responsible for the damping-off of coniferous seedlings from two nurseries in Maine revealed that Fugarium spp., a Pythium species, and Rhizoctonia solant were the primary causal agents, in order of importance, at both nurseries. In addition, Cylindrocladium sconarium was found in about the same proportion as R. solant from seedlings grown in the greenhouse in soil from the Orono nursery, but this fungus was not isolated from seedlings grown in soil from the Greenbush nusery.

In conjunction with the determination of the causal organisms, studies were made to evaluate the effect of thiran, captain, and PGNB in the control of these pathogens. The results obtained from greenhouse experiments indicated that 50% captan was safe to use as a seed treatment with Norway pine at the maximum adherent dosage. Treatment of the seed with 65% thiram at the maximum adherent dosage gave indications of being phytotoxic by significantly reducing emergence in six out of ten trials.

Treatment of the soil with thiram and PCNB at two nurseries gave conflicting results, presumably because of the differences in soil type and in the severity of the disease at the two locations. Treatment of the soil with thiram at the rate of 1.5 gram per square foot at the Greenbush nursery showed detrimental effects by reducing emergence and by significantly increasing post-emergence mortality of Norway pine as compared with the untreated plots. Thiram applied to the soil at the rate of 2.0 grams per square foot at the Orono nursery gave slightly better emergence of Norway pine then that observed in the untreated plots. PCNB was also beneficial at the Orono nursery when applied at the rate of 0.5 cc. of the 24% emulsion in one quart of water per square foot, but at the rate of 0.15 gram of the 75% dust per square foot it caused a reduction in the emergence of white and Norway pine.

Treating the seed with captan and sowing on soil treated with either PCNB or thiram resulted in a reduction in the emergence of white pine and Norway pine seedlings at the Greenbush nursery and in the reduced effectiveness of those chemicals at the Orono Nursery. The captan seed treatment alone was not detrimental.

Phytotoxicity studies with thiram and captan indicated that treating Norway pine seed with these chemicals at the rate of 2 grams of captan or 3 grams of thiram per pound of seed did not reduce emergence nor did it reduce seedling growth significantly. However, captan used at the rate of 137 grams or thiram at the rate of 235 grams per pound of seed significantly reduced both emergence and growth of seedlings.

It was concluded that these chemicals could be sued successfully to control damping-off if used at the proper rate of application. The rates of application should be gauged by the soil type and by the expected severity of the disease. However, chemical treatments are not recommended unless substantial losses are expected, because of possible phytotoxicity.