Basil Downy Mildew

Peronospora belbahrii

Downy mildew 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 and we anticipate basil downy mildew will be a major disease of basil in the US in the foreseeable future.

It 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 where basil production occurs year round, such as in Florida. 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. There is also evidence that the disease can be spread by contaminated seed, though we do not yet understand how this occurs and how important contaminated seed is as a source of primary inoculum. Long distance transport from FL to MA might be explained by aerial dispersal of spores but rapid transcontinental transport probably occurred via infested seed sold internationally. Air-borne dissemination from infected plants is more likely. 

Crop Injury:

Early symptoms can easily be mistaken for a nutritional deficiency. Infected leaves develop diffuse, but vein-delimited yellowing on the top of the leaf and a characteristic fuzzy, dark gray growth on the underside of the leaves, which may be mistaken for soil splashed onto the leaf under-surface, however, close inspection with a hand lens will show the sporangia. Leaf yellowing is often the first symptom of basil downy mildew. Yellowed areas are usually bordered by leaf veins. 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 be seen in severe cases. Symptoms of downy mildew on basil can easily be mistaken for a nutritional deficiency. The fuzzy growth of spores on the underside of the leaf may look as if soil had been splashed onto the leaf under-surface.

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.

Cultural Controls & Prevention:

The most important environmental factors favoring disease development are high humidity and extended leaf wetness. These factors can be reduced by:

• 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%.
• Reduce plant canopy density by spacing to speed leaf drying.
• Water in the morning, if practical, or use drip irrigation rather than overhead.
• While sporangiophores can be formed in light or darkness, spore production requires a period of darkness. Exposing infected plants to red light (λ 575-660 nm) at night inhibits spore production. 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.
• In the field, plant in well drained sites with good air drainage and orient rows with the prevailing winds.
• Control weeds and space plants to enhance leaf drying.
• Grow tolerant cultivars. Classic Genovese types of basil are highly susceptible to downy mildew. No truly resistant cultivars are yet available; ‘Eleonora’, ‘Everleaf’, “Tuscany’, and ‘Caesar’ are tolerant.

Purchase seed or transplants from reliable sources. We know that the pathogen may come in with seed, though the frequency and importance of seed as a source of primary inoculum are not well understood, and testing of seed is difficult. Therefore, our recommendation is to buy seed from a trusted source. Talk to your seed supplier about how the seed was produced, if it has been tested, and also if the variety exhibits any resistance to the pathogen. 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 keep a careful eye on them. Basil DM has repeatedly been traced back to transplants grown out of state so buying in transplants should be avoided. If you do buy transplants, inspect them carefully before purchasing and if you find any signs of disease report it to the store manager or call your local Extension service. Inspect plants regularly by looking on the undersides of leaves for sporulation (see leaf photos).

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/.

Plant resistant varieties. There are now several varieties that provide good suppression of BDM, but not full immunity (see list below). Use resistant varieties for later plantings. It is recommended to use an integrated management program that includes applying fungicides to resistant varieties to ensure effective control. Meg McGrath at Cornell University has been conducting evaluations of resistant varieties as well as research on fungicide efficacy and is interested in hearing growers’ feedback on occurrence of the disease and performance of these new varieties. Send feedback to Meg at mtm3@ cornell.edu.

• 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 organicallyproduced 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). 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.

Relative susceptibility of basil types: Field trials conducted in southern New Jersey in 2009 determined that commonly-grown sweet basil (Ocimum basilicum) cultivars such as 'Poppy Joe' and 'Nufar' were the most susceptible to downy mildew. The least susceptible basils included the lemon and spice types such as O. x citriodorum and O. americanum cultivars , ‘Lemon Std’, ‘Lemon’, ‘Lime’, ‘Spice’, ‘Blue Spice’ and ‘Blue Spice Fil’.

Chemical Controls & Pesticides:

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 2021 from Cornell’s Basil Downy Mildew Management page.

Updated by Angela Madeiras