Basil Downy Mildew

Peronospora belbahrii

Downy mildew of basil is caused by the oomycete Peronospora belbahrii. The disease was first reported on basil in Uganda in 1930, but did not attract international attention until it appeared in several new locations; Italy (2003), France (2004) South Africa (2005), Iran (2006) United States (Florida, 2007) and Argentina (2008). During 2008 and 2009, the disease occurred throughout the east coast in epidemic proportions both in the field and in greenhouses. Considerable economic losses occurred in Massachusetts during that time.

Crop Injury

Leaf yellowing is often the first symptom of basil downy mildew. The yellow areas are typically bordered by leaf veins. This may at first be mistaken for nutrient deficiency. When spores are produced, a characteristic fuzzy, dark gray to purple growth on the underside of the leaves is evident. Sporulation on the upper surfaces of leaves may also be seen in severe cases. The fuzzy growth of spores on the underside of the leaf may look as if soil had been splashed onto the leaf undersurface.

Disease Cycle

Spores are produced in darkness before sunrise. Sporulation occurs at 50-78°F and >85% relative humidity (RH). Spores are borne at the ends of tiny tree-like structures called sporangiophores which grow out of the stomata. The spores are dispersed by wind and splashing water. On wet leaves at temperatures from 41 to 83°F, spores germinate within 2 hours and infect plant tissue within 4 hours. Five to 10 days may elapse between infection and the appearance of symptoms; the duration of the latency period depends upon temperature and light exposure. Basil downy mildew is unable to survive winter temperatures in the northeastern US.

The pathogen is an obligate parasite, meaning that it cannot survive outside of a living host. It does not produce overwintering oospores but survives from year to year on living plants in places where basil production occurs year-round. From these sites the pathogen spreads via wind-dispersed sporangia that can travel great distances due to their dark pigmentation, which protects them from UV radiation. Although the downy mildew pathogen has been detected in basil seed, seed transmission is considered to be an uncommon event. Air-borne dissemination from infected plants is more likely. 

Cultural Controls & Prevention

The most important environmental factors favoring disease development are high humidity and extended leaf wetness. 

• Toward evening, heat and vent the greenhouse, especially when warm days are followed by cool nights.
• Improve horizontal air flow by the use of fans.
• Run fans at night. Fans may be connected to sensors that will turn them on when the RH reaches 70% and turn them off when it drops below 65%.
• In the field, plant in well drained sites with good air circulation and orient rows with the prevailing winds.
• Space plants in a way that allows good air circulation to facilitate drying of leaves.
• Manage weeds.
• Water in the morning, if practical, or use drip irrigation rather than overhead.
• Expose plants to red light (λ 575-660 nm) at night to inhibit sporangia production in infected plants. Studies have shown that exposing plants to red light for a period of 10 minutes every few hours during the night can significantly reduce sporulation; however, the shading of leaves by other leaves negates this effect, so it is most effective when plants are smaller.
• Remove any symptomatic plants and dispose of them in the trash. Keep a trash bin with a lid close to you as you work to avoid carrying sporangia through the greenhouse or field.

Purchase seed or transplants from reliable sources. Basil seeds are not amenable to hot water seed treatment as they produce a gelatinous exudate when in water, though some seed companies are starting to use steam seed treatment.

Grow your own transplants and scout regularly for signs and symptoms. If you buy transplants, inspect them carefully before purchasing, before bringing them into the greenhouse, or before planting.

Plan to plant and harvest early. The pathogen tends to arrive in MA around mid-July, though in some years it can be earlier. Keep track of where the disease is being found via Pest Alerts in Veg Notes and via the basil downy mildew monitoring program here: https://basil.agpestmonitor.org/map/

Grow tolerant or resistant cultivars, such as:

• Obsession DMR, Devotion DMR, Thunderstruck DMR, Passion DMR: Developed by the Rutgers University breeding program, these are all sweet basil varieties available from several seed companies, including organically produced options. These have shown high levels of resistance in trials, though more symptoms were observed in 2020 than in previous years.
• Prospera series (CG1, ILL2, PL4, PS5, Red DMR). Available from several seed companies, these have performed well in Cornell disease trials over the past three years.
• Amazel is a Proven Winners variety. It is seed sterile and sold as cuttings primarily for producing plants for the home garden market
• Pesto Besto is another Proven Winners variety with the same source of genetic resistance as Amazel; sold by seed.
• Eleonora, Emma, and Everleaf (aka Basil Pesto Party) are older varieties that have demonstrated limited to moderate resistance in recent trials.
• Other varieties that tend to have fewer symptoms are the non-sweet types, including red leaf, Thai, lemon, lime, and spice types.
• Organic options are available from some companies.

Chemical Management

Excellent control of downy mildew can be achieved with conventional fungicides applied weekly on a preventive schedule, but control is greatly reduced when applications are started after disease detection. If the symptoms are widespread and severe, destroy the crop immediately to stop spread of the disease to other plantings on your farm.

Pay close attention to labels. Basil is a minor crop and is not always found on pesticide labels, and there are differences in registrations for use in field versus greenhouse production. Some products have supplemental labels for use on basil. Labels, including supplemental labels, can be found at www.cdms.net/Label-Database.

Research trials have shown that the phosphite fungicides (eg. K-Phite, Prophyt, Fungi-phite) are among the most effective 7 chemical controls. Other effective materials include mandipropamid (eg. Revus), cyazofamid (eg. Ranman), and azoxystrobin (eg. Quadris). All of these except Quadris can be used in both field and greenhouse in MA—Quadris is labeled for field use only.

Fungicides with targeted activity are prone to resistance development due to their single-site mode of action and thus need to be used within a fungicide resistance management program. Resistance to mefenoxam (Ridomil) developed quickly in Israel, demonstrating the capacity of this pathogen to develop resistance. See here for example fungicide programs.

While several OMRI-listed products are labeled for downy mildew on basil or herbs, none have been found to be effective in controlling the disease. Cornell’s results from evaluations of several of these products (as well as conventional products) can be found here.

It is the grower’s responsibility to read and follow label instructions. Be sure that a particular product is registered for use in your state. The label is the law and any recommendations made here are superseded by the label.

Crops that are affected by this disease:

• Basil

Variety information updated for 2024 from Cornell’s Basil Downy Mildew Management page.

Updated by Angela Madeiras