In the past decade, great strides have been made in understanding the biology and epidemiology of Peronospora belbahrii, the causal agent of basil downy mildew. Researchers have been busy assessing the efficacy of both cultural and chemical management techniques in the greenhouse and in the field, and some new tools are now available. Plant breeders have produced a few sweet basil cultivars with reduced susceptibility to the pathogen, and continue to work toward the development of true genetic resistance. While much progress has been made, downy mildew management remains an issue of concern for basil growers.
What follows is an updated review of current management practices for basil downy mildew.
The most important environmental factors favoring disease development are high humidity and extended leaf wetness. These factors can be reduced by:
- Heating and venting the greenhouse toward evening, especially when warm days are followed by cool nights.
- Improving horizontal air flow by the use of fans.
- Running fans at night. Fans may be connected to sensors that will turn them on when the RH reaches 70% and turn them off when RH drops below 65%.
- Reducing plant canopy density by spacing to speed leaf drying.
- Watering in the morning, or using drip irrigation rather than overhead wtarering.
- In the field, planting in well drained sites with good air drainage and orienting rows with the prevailing winds.
- Controlling weeds. Weeds can lead to increased humidity levels.
- Harvesting early. The risk of downy mildew development increases as the season progresses.
- Growing tolerant cultivars. Classic sweet (Genovese) types of basil are highly susceptible to downy mildew. No truly resistant cultivars are yet available; ‘Eleonora’, ‘Everleaf’ (AKA ‘Pesto Party’), ‘Tuscany’, and ‘Caesar’ are tolerant.
- Exposing infected plants to red light (λ 575-660 nm) for several hours at night to inhibit spore production. While sporangiophores can be formed in light or darkness, spore production requires a period of darkness. Some crop loss may be incurred, as sporangiophores can still emerge from the leaves; however, the reduction in spore production can help prevent the pathogen from spreading to uninfected plants. This is most effective on young plants as direct leaf exposure to light is needed.
Few fungicides are labeled for herb plants and there are differences in registrations for field grown plants versus greenhouse plants. Always check the labels on individual products. Currently labeled products are listed below. Ranman and Revus have the advantage of being labeled for use against downy mildew on both basil and spinach. Many organic products in addition to those listed here are labeled for use on basil, but efficacy data is lacking. Milstop and Actinovate are among the best performing OMRI labeled products in research trials, though efficacy can be inconsistent. Both conventional and organic products should be used in a preventative manner to protect plants from infection, as few have any curative properties. Chemical management must be used in conjunction with cultural management techniques for best results. Rotate active ingredients (FRAC groups) to prevent resistance development. For further information, consult the New England Vegetable Management Guide at http://nevegetable.org/
- Cyazofamid (Ranman, FRAC group 21): labeled for field and greenhouse use.
- Mandipropamid (40): Revus is labeled for field use, Micora for greenhouses with permanent flooring.
- Phosphorus acids (K-Phite, Fosphite, 33): recommended use at a low rate in combination with a conventional fungicide.
- Oxathiapiprolin (Orondis Ultra B, U15): labeled for basil downy mildew in field and greenhouse.
- Azoxystrobin (Quadris, 11): registered for field use on basil, though not for downy mildew specifically.
- Fenamidone (Reason, 11): now labeled specifically for basil downy mildew in field and greenhouse; note that it is in the same FRAC group as azoxystrobin.
- Copper (Cueva et al, M1): greenhouse and field.
- Potassium bicarbonate (MilStop): greenhouse and field.
- Streptomyces lydicus (Actinovate): greenhouse and field.
Angela Madeiras, Extension Educator and Diagnostician, UMass Extension Plant Diagnostic Lab