Quick facts
- Basil downy mildew can spread rapidly and result in complete yield loss.
- Infection starts on lower leaves and moves up the plant.
- The pathogen that causes basil downy mildew can transmit on seed, transplants or fresh leaves.
- There are no resistant varieties of sweet basil available.
- Monitor all seedlings and transplants closely for yellowing leaves and gray downy growth on the lower surface of the leaf.
- Certain fungicides can protect plants from basil downy mildew.
Basil downy mildew was first officially identified in Minnesota in 2012. Under the right weather conditions, basil downy mildew can spread rapidly and result in complete yield loss.
Although Peronospora belbahrii, the pathogen that causes basil downy mildew, cannot survive Minnesota’s winters, it can return on infected seed or transplants or by windblown spores.
Identifying basil downy mildew symptoms
- Infection starts on lower leaves and moves up the plant.
- Infected leaves first turn yellow in areas restricted by major veins. With time, the entire leaf turns yellow.
- Irregular black spots appear on infected leaves as they age.
- Fluffy gray spores grow on the underside of infected leaves.
Peronospora belbahrii, the pathogen that causes basil downy mildew, can transmit on seed, transplants or fresh leaves. Infected transplants and leaves may not show symptoms if maintained in cool, dry conditions. Spores of P. belbahrii can also travel long distances on moist air currents.
Peronospora belbahrii tolerates cool weather and can infect and produce spores in temperatures as low as 59°F. The pathogen, however, thrives in warm, humid conditions. As a result, the most devastating damage is often in late summer.
Peronospora belbahrii needs two different mating types to produce tough resting spores known as oospores. Only one mating type is present in the USA. As a result, no oospores form and the pathogen will not be able to survive Minnesota's harsh winters. This may change if the second mating type comes to the country.
Preventing and managing the disease
- There are no resistant varieties of sweet basil (Ocimum basilicum) available.
- Commercially popular varieties are highly susceptible.
- Lower disease levels have occurred in red leaf basil varieties (O. basilicum purpurescens) and in lemon flavored varieties (O. citridorum).
- Only varieties of O. americanum, have shown no symptoms or sporulation when inoculated with downy mildew.
Varieties with no to low disease are not necessarily good substitutes for susceptible sweet basil varieties. They often have different leaf color and flavor, dramatically affecting the final product.
Growers should choose the most resistant variety that is acceptable to their market.
Breeders are working to combine the flavor and other characteristics of sweet basil with the resistance found in other species of Ocimum.
Susceptibility of basil varieties to basil downy mildew
| Level of resistance | Scientific name | Varieties |
|---|---|---|
| Highly susceptible | Ocimum basilicum | Genovese, Nufar, Italian Large Leaf, Queenette, Superbo, Poppy Joe's and many others |
| Moderately susceptible | O. basilicum purpurescens | Red Rubin, Red Leaf |
| Moderately susceptible | O citridorum | Lemon std., Mrs. Burn's Lemon, Lemona & Lime |
| Less susceptible | O. americanum | Blue Spice, Spice & Blue, Spice F1 |
- Seed carries Peronospora belbahrii.
- Monitor all seedlings and transplants closely for yellowing leaves and gray downy growth on the lower surface of the leaf.
- If any plant has basil downy mildew, remove and destroy the plant immediately.
Increase row width
Increase row width and distance between plants to provide good air movement between plants to allow leaves to dry quickly after rain, dew or irrigation.
Use drip irrigation
If sprinkler irrigation is the only option, water deeply and infrequently early on a sunny day so leaves dry quickly in the sun. In greenhouse production, adjust ventilation to reduce humidity.
Till under diseased plants
Promptly till under diseased plants that are from past harvests to reduce the spread of the pathogen from one plant to another through spores produced on infected leaves.
Certain fungicides can protect plants from basil downy mildew. Begin sprays before infection occurs to be effective. Peronospora belbahrii is not a true fungus but rather a member of the Oomycota. As a result, many common fungicides provide no control against downy mildew.
In one study, extreme periods of rainy, wet weather resulted in no control by any fungicide combination. Read and follow all label instructions whenever applying a fungicide. Use tank mixes and rotation between fungicides to reduce the risk of fungicide resistance.
70-90% disease reduction when applied every 7 days
| Trade name | Active ingredient | FRAC code |
|---|---|---|
| Quadris | Azoxystrobin | 11 |
| Ridomil Gold SL+ | Mifenoxam | 4 |
| Fosphite or ProPhyt | Phosphorous acid | 33 |
2-45% disease reduction if applied at label recommended intervals
| Trade name | Active ingredient | FRAC code |
|---|---|---|
| Milstop | Potassium Bicarbonate | NC |
| Actinovate | Streptomyces lydicus | NC |
| Oxidate | Hydrogen Dioxide | NC |
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Be sure that the plant you wish to treat is listed on the label of the pesticide you intend to use. And observe the number of days between pesticide application and when you can harvest your crop. Remember, the label is the law.
Reviewed in 2018