Identifying Basil Downy Mildew Based on Symptoms
The diagnostic symptom of this disease is the dusty to fuzzy appearing growth of the pathogen on the lower surface of leaves. Typically it is dark brown. It can be a lighter almost gray color on some spice types of basil. This growth is the pathogens spores (sporangia) plus the structures the spores form on (sporangiophores). Sometimes it is on the upper surface. Morning is the best time to see this sporulation, following a night with dew or relative humidity near 100%, because under these conditions the pathogen produces spores. During the day spores are dispersed by wind. Sporangiophores that remain are not pigmented like the spores and thus are not readily visible.
Leaves affected by downy mildew usually are yellow. Yellow tissue often occurs in bands defined by major veins reflecting the fact the pathogen as it grows through leaf tissue cannot progress through large veins. Affected leaves can be yellow with no signs of sporulation when plants are not exposed to dark, humid conditions. To determine if the yellowing is due to downy mildew infection, sporulation can be encouraged by putting yellow leaves upside down on wet paper towel in a ziplock bag in dark overnight or putting potted plants in a closed plastic bag with wet paper towel overnight.
Photographs of symptoms at this web page were submitted by reporters to confirm downy mildew on their basil.
The following four photographs of characteristic symptoms on lower and upper leaf surfaces followed by images of the pathogen’s spores through a dissecting microscope and then a compound microscope were taken by Rebecca Melanson, Mississippi State University.
Managing Downy Mildew in Gardens and Commercial Plantings
Recommendations for gardeners.
1. Select resistant varieties. See following section for growers. Note that some varieties are not available yet for other than commercial growers.
2. Minimize plant exposure to conditions favorable for infection and disease development by growing plants in pots and bringing them indoors overnight and on rainy days. Additional information.
Growing basil in a sunny location where there is good air movement during the day might seem like a good management practice, but these plants will not be protected from rain or dew wetting the leaves thereby creating favorable conditions for the pathogen.
3. When symptoms are first found and there are many symptom-free leaves, harvest these leaves for preserving (dry, freeze, make pesto, etc.) before the pathogen has a chance to spread to them. Affected leaves are not harmful to consume, but they quickly turn yellow and die.
4. Promptly destroy affected plants that have become unusable to minimize the amount of spores in the environment that could be dispersed by wind to healthy basil plants elsewhere.
Contact fungicides available for gardeners to use are not considered sufficiently effective based on evaluations of organic fungicides.
The pathogen causing basil downy mildew cannot survive in the soil, including on affected, now dead plant tissue, as some other plant pathogens can, therefore removing affected plant tissue and planting in a different area the following year are not practices for managing basil downy mildew as they are for other diseases.
Recommendations for commercial growers (greenhouse and field).
1. Select resistant varieties. Devotion, Obsession, Passion, and Thunderstruck are available from VanDrunen Specialty Seeds (some as organic seed). Prospera is being marketed by Siegers and Johnny’s Selected Seeds (organic). Amazel is being sold as cuttings primarily for producing plants for the home garden market because its seed is sterile. Eleonora, Emma and Everleaf (aka Basil Pesto Party) have limited level of resistance. To minimize selection pressure on the pathogen to overcome host resistance and to achieve acceptable control, all resistant varieties need to be used with other management practices, in particular fungicides, due to very low tolerance for symptoms in herbs especially when used fresh.
2. For greenhouse crops produced during the off-season for field production (e.g. winter), select seed that has been tested for the pathogen or steam-treated. Basil seed when submerged in water exudes a gelatinous material making the seed difficult to handle, thus hot water seed treatment is not a viable practice. Spores from affected basil growing outdoors can be moved by air currents into greenhouses through open vents.
3. Apply fungicides on a preventive schedule; alternate among chemistry:
- Greenhouse: Ranman, Revus, and phosphorous acid (phosphanate) fungicides (later recommended tank-mixed with other fungicides rather than used alone especially when conditions are favorable).
- Field: above plus Quadris.
- Greenhouse-grown plants for retail sale to consumers can also be treated with: Micora, Heritage, Segovis, and Subdue MAXX. Only Heritage and Subdue MAXX are permitted used on plants to be marketed as fresh herbs in grocery stores.
4. Make greenhouse conditions unfavorable for downy mildew development:
- Use base heating, dehumidifier, wide plant spacing, base watering, circulating fans, and lights to keep humidity below 85% in the plant canopy. Set up sensors in the plant canopy to monitor humidity to ensure implementing practices are sufficient.
- Direct greenhouse fans toward plants so that leaves move, which prevents water depositing on leaves when humidity is high. Wet leaves are favorable for infection.
- Turn lights on during the first hours of night so that there is no more than 6 hours of darkness before daytime. The pathogen makes spores when it is dark. Red light is most inhibitory. Treatment is effective only for leaves directly exposed to light.
5. Monitor plants routinely for symptoms. Put plants or detached leaves suspected of being infected but lacking spores in a closed plastic bag with wet paper towel in a dark location overnight to induce spores to form.
6. Heat treatment can be used to cure affected plants in a greenhouse. It is recommended as a rescue treatment when other practices were not adequate rather than as a routine practice. It is recommended done at first sign of downy mildew and over 3 consecutive days with 3 – 4 hours exposure to 104 – 113 F. High temperature can be achieved using solar heating on sunny, summer days by closing vents or using a transparent IR polyethylene sheet covering. Closely monitor temperature to ensure it remains close to 113 F and does not rise higher, which can kill plants.
7. Promptly destroy unmarketable affected plants to eliminate this source of inoculum for other plantings. Do not seed another crop in a greenhouse complex, including in a separate room, until after affected plants are gone.
Grower feedback on performance of downy mildew resistant varieties in terms of disease suppression, yield, flavor, appearance, marketability, etc. would be greatly appreciated.
Please Note: The specific directions on fungicide labels must be adhered to — they supersede these recommendations, if there is a conflict. Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.