Powdery mildew occurs sporadically on tomatoes grown in high tunnels because the pathogen usually is not present. Best organic fungicide program based on my knowledge of efficacy of organic products plus feed-back I got from growers in the northeast region:
- Start with sulfur before fruit start to ripen. It is the most effective product for powdery mildew, but leaves a residue which is undesirable on fruit.
- Switch to a biopesticide like Double Nickel for 2 weeks.
- Apply a mineral oil such as JMS Stylet oil through the harvest period. Oils do not leave a residue. A break is needed between sulfur and oil applications because if there is sulfur residue on leaves when oil is applied, it can cause sulfur to be moved into leaves resulting in phytotoxicity.
Copper is not as effective for powdery mildews.
Conventional fungicides with targeted activity for powdery mildew that do not have a statement prohibiting their use in greenhouses include Inspire Super, Torino, and Vivando. Also Switch in upstate New York. And Fontelis in other states.
It is valuable to determine the source of the pathogen for a powdery mildew outbreak. If the plants were purchased as seedlings, consider contacting the producer to determine if they saw powdery mildew while producing the seedlings or in the past in their greenhouse. If this is not a feasible source of the pathogen, look to see if powdery mildew is present on weeds or another plant being grown in the high tunnel.
Page created June 2024 by Margaret T. McGrath