Pathogens such as Fusarium, Pythium, Phytophthora, Thielaviopsis and Rhizoctonia that cause pre- and post-emergent damping-off can cause serious problems in organic transplant production.
Remember, Phytophthora and Pythium are more likely to cause damping-off in cool, wet soils. Conversely, Rhizoctonia and Fusarium are more likely to cause damping-off under warmer, drier conditions. In general, Pythium tends to kill seedlings before they emerge whereas Rhizoctonia and Fusarium tend to kill seedlings after emergence. There are exceptions to the rules, but none the less, all damping-off pathogens can cause serious losses if not identified and controlled properly. Remember seeds or transplants that sit in cold, wet soils for prolonged periods of time are more prone to damping-off. Outside weather conditions also play an important role in potential disease development in spring transplant production. Most importantly, daily watering schedules need to be monitored and/or adjusted so as not to overwater during cool, cloudy periods or underwater during bright, warm, sunny days. Always do your watering early enough in the day so leaves are dry going into the overnight.
Taking preventative measures to mitigate potential problems caused by damping-off pathogens is the best approach; and is one everyone needs to consider prior to and during the organic transplant production season. There are a number of OMRI-approved biological controls that can be incorporated into the soil media prior to seeding, as a seed treatment, or as a drench. Remember biological control agents can be fungi or bacteria that work by various mechanisms which include antibiosis, parasitism, induction of host-plant resistance, and competition. Trichoderma virens (SoilGard 12G, Certis USA) colonize host roots and is antagonistic to Pythium and Rhizoctonia. Plantshield HC and Rootshield WP (Trichoderma harzianum, Bioworks, Inc.) also colonize roots and provide protection against root pathogens such as Pythium, Rhizoctonia, Fusarium, Cylindrocladium and Thielaviopsis. Actinovate (Streptomyces lydicus, Natural Industries, Inc.) is a bacterium labeled for Pythium, Phytophthora, Fusarium, Rhizoctonia, and Verticillium. Mycostop (Streptomyces griseoviridis, Agbio, Inc.) also colonizes roots and is labeled for control or suppression of many root rot and wilt pathogenic fungi such as Pythium, Fusarium, Rhizoctonia, and Phytophthora. All of these products work best if they are incorporated or applied before any damping-off occurs. This means incorporating them into the media mix prior to seeding, or applying them as a seed treatment, or as a drench shortly after seeding and continuing with follow-up treatments during the remaining transplant production season. The key to controlling and/or suppressing damping-off pathogens with biological controls is keeping the biological populations high and continually present on root surfaces of the host and by following good cultural practices. For more information on the products mentioned above please see Table E-15 on pages E46-E48 of the 2015 Commercial Vegetable Production Recommendations. Applications of the products mentioned above should be done according to the manufacturer’s label.