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Basil Downy Mildew

Updated July 2020. (Originally prepared for Vegetable MD Online.)

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Downy mildew has been the most common and most important (damaging) disease of basil in the United States (US) since it was first reported in south Florida in October 2007. In 2008 it was confirmed in both field- and greenhouse-grown basil crops, as well as home gardens across the eastern US and Canada. Downy mildew is caused by the Oomycete Peronospora belbaharii. The pathogen’s abundantly produced spores can be readily wind-dispersed. This is the main dispersal mechanism enabling the pathogen to disperse throughout the eastern US every summer, similar to its well-known relative causing cucurbit downy mildewP. belbaharii is also seed-borne. Leaves with any symptoms are rendered unmarketable especially for sale as fresh herb. Downy mildew has caused complete crop loss.

Symptoms and Signs

The first step in preparing for basil downy mildew is learning the symptoms (Fig. 1-6). The most noticeable symptom is chlorosis which can resemble a nutritional deficiency (Fig. 1, below), thus when downy mildew first occurred growers generally did not recognize the presence of a disease. Later, the downy-appearing sporulation of the pathogen develops almost exclusively on lower leaf surfaces (Fig. 4-5). Observing spores on the underside of leaves is key to diagnosis. Because spores are produced during the dark night period, early morning is the best time to inspect basil for downy mildew. Leaves with yellowing resembling downy mildew but lacking spores can be placed upside down on a wet paper towel in a closed plastic bag in dark for a day to induce spore production, if the pathogen is present. Visit the Downy Mildew on Basil photo gallery (LIHREC) for additional images.

Additional possible causes of leaf chlorosis and necrosis in basil should be considered when making a diagnosis. Physiological injuries and insect herbivory (Fig. 7-10) can cause similar symptoms, and it is important to distinguish the causal agents.

Fig. 1: Leaves of basil affected by downy mildew are yellow (chlorotic) with affected tissue eventually dying and becoming brown (necrotic).

diseased basil in the field

basil plants

Fig. 2: Yellowing of the upper surface of affected basil leaves often occurs in sections of the leaf delineated by veins because the downy mildew pathogen cannot grow past major veins in leaves.

upper side of basil leaves

Fig. 3: Purplish gray spores of the downy mildew pathogen only develop on the lower surface of leaves. These are the same leaves in Fig. 2. Sporulation coincides with yellowing on the opposite side of the leaf.

the lower side of basil leaves

Fig. 4: Close-up views of sporulation in sections on the lower surface of leaves.

close-up of a basil leaf

close-up of a basil leaf

Fig. 5: Yellowing, necrosis, and sporulation on basil leaves and flower bracts.

basil plant

Fig. 6: Basil is susceptible to downy mildew from emergence.

basil seedling with symptoms of downy mildew

Other problems that could be confused for basil downy mildew

Fig. 7: Injury to basil leaves from sunburn or other physiological cause resembles downy mildew but lacks the angular appearance and characteristic sporulation on the lower surface.

damaged basil leaves

Fig. 8: A close-up of damage demonstrates the lack of angular lesions and characteristic sporulation.

close-up of a damaged basil leaf

Fig. 9: Thrips frass and injury from a severe infestation could be mistaken for downy mildew due to its dark color. Note that it lacks the dusty, fuzzy appearance of downy mildew pathogen sporulation.

basil leaves with thrips damage

Fig. 10: A close-up of thrips damage demonstrates the lack of the fuzzy appearance of downy mildew sporulation.

close-up of thrips damage on a basil leaf

Management

Using seed not infested with the basil downy mildew pathogen, selecting a less susceptible variety, and applying fungicides are the primary management practices for downy mildew. Pathogen-free seed is most important for plantings not expected to be exposed to wind-dispersed spores, such as greenhouse crops when too cold outdoors for basil to survive. Minimizing leaf wetness and reducing humidity to obtain conditions unfavorable for disease development may suppress downy mildew, especially in greenhouses. An article focused on management in greenhouses is available.

Seed Tests

Recent efforts to develop a seed test have resulted in a genetic-based procedure specifically for Peronospora belbahrii that is now being validated (suspected contaminated seeds are needed for this step, see information about submitting samples below). And Eurofins STA Laboratories in Colorado now tests basil seed for Peronospora spp. It is sufficient to test only at the genus level with this pathogen since it is the only species of Peronospora that would be associated with basil seed.

Seed Treatment

Seed companies (including High Mowing Organic Seeds) are starting to steam treat basil seed.  It is not amenable to hot-water treatment because while in water the seed produces a gelatinous exudate.

Varieties

Good suppression of downy mildew can be obtained with new resistant varieties that started to be marketed in 2018. They are the fruition of several years of conventional breeding by breeders working separately on this goal. It can take many crosses to obtain a plant with resistance plus all the desired horticultural traits that are in a susceptible variety, which include for sweet basil large, smooth, dark green, downward cupped leaves with good classic sweet basil flavor.

Devotion, Obsession, Passion, and Thunderstruck are the first resistant varieties released from the Rutgers University basil breeding program. They are marketed by VanDrunen Specialty Seeds. Organically-produced seed is available. Results from evaluations at Cornell (LIHREC) along with evaluations conducted earlier that documented high level of resistance of Rutgers experimental crosses are also available. Some symptoms did develop on the resistant varieties which is typical because disease resistance is rarely immunity.

Prospera is being marketed as organic seed by Johnny’s Selected Seeds. No symptoms were found on plants in the evaluation conducted at Cornell in 2018. Few symptoms were found in 2019. In some evaluations conducted elsewhere there were some plants that became severely affected which could be due to seed mix up.

Amazel is a Proven Winners variety that also did not develop symptoms in the Cornell 2018 variety evaluation (LIHREC) and had some symptoms in 2019. Its seed is sterile and thus sold as cuttings primarily for producing plants for the home garden market.

While above resistant varieties suppressed downy mildew very well in the Cornell evaluations, there have been a few reports of poor suppression elsewhere suggesting the pathogen has evolved to overcome resistance. Downy mildew appeared unsuppressed on Amazel plants in greenhouses on Long Island in spring 2019. Pathogen isolate from these plants was found to be genetically different from other US isolates examined. It is recommended to use an integrated management program that includes applying fungicides to resistant varieties to ensure effective control. Please email mtm3@cornell.edu if you observe poor suppression of downy mildew on any resistant varieties listed above; a sample of your plants may be wanted to examine the pathogen.

Eleonora, the first commercially available resistant variety, demonstrates limited suppression.

Emma and Everleaf -aka Basil Pesto Party and M4828Z when evaluated at Cornell (LIHREC)- also have moderate resistance.

  • Grower feedback on performance of downy mildew resistant varieties in terms of disease suppression, yield, flavor, appearance, marketability, etc. would be greatly appreciated. Please email mtm3@cornell.edu.

Research on resistance started with looking to see if there are inherent differences in susceptibility among varieties and species of basil, and to identify sources of genetic resistance. Commonly grown sweet basil (Ocimum basilicum) was shown to be more susceptible than some of the other exotic, spice, and ornamental basils such as O. citriodorum and O. americanum. All sweet basil varieties evaluated before breeding for resistance was started were found to be very susceptible. Other basil varieties with fewer symptoms in evaluations include red types (including ‘Red Leaf’ and ‘Red Rubin’), Thai basil (Queenette’), lemon basil (‘Lemon’, ‘Lemon Mrs. Burns’, ‘Sweet Dani Lemon Basil’), lime basil (‘Lime’), and spice types (‘Spice’, ‘Blue Spice’, ‘Blue Spice Fil’, ‘Cinnamon’). An article on this study is available as a PDF. Similar results were obtained in trials conducted in Illinois and Florida. Fewer symptoms were observed only on those basils that are different from sweet basil being distinct in visual appearance, aroma and flavor, and which have quite limited markets. The challenge in breeding for resistance to basil downy mildew is developing improved resistant varieties that still look, taste, grow and pack-out as a traditional high quality sweet basil.

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.

Fungicides

To control downy mildew effectively with fungicides, it is considered necessary to start before first symptoms and to make applications frequently. Many of the fungicides currently labeled for this new disease, plus others not registered yet, have provided limited suppression in some fungicide evaluations, demonstrating the difficulty in controlling this disease, especially in a research setting with applications made with a backpack sprayer, and thus the importance of starting before disease onset. Part of the challenge of controlling downy mildew is the need for blemish-free herbs when marketed as fresh sprigs.

To determine when to initiate a fungicide program and when to consider harvesting early to avoid losses to downy mildew, growers should not only sign up to receive alerts and routinely check the map of reported occurrences to determine when downy mildew is occurring on basil nearby, but also regularly inspect their crop for symptoms. The cucurbit downy mildew forecasting web site might be useful for predicting when conditions are favorable for basil downy mildew since both pathogens likely have similar requirements for successful wind dispersal long distances (e.g. overcast skies) and subsequent infection (e.g. wet leaves or high humidity). Summer is not a time to forget about this disease: unlike most other downy mildew pathogens, e.g. the ones affecting lettuce and cruciferous crops, which stop developing in summer, the basil downy mildew pathogen seems to develop best under moderate to warm temperatures while also tolerating cool temperatures. Don’t forget to report occurrence of downy mildew as soon as possible at the monitoring page or via e-mail to mtm3@cornell.edu.

Organic Fungicides

There is a 2(ee) recommendation allowing use of the new botanical fungicide EcoSwing (extract of Swinglea glutinosa) for downy mildew in field-grown basil. Basil has been added to the list of herbs on the label for Cueva (10% copper octanoate). Downy mildew is in the list of foliar diseases controlled by this OMRI-listed product. Procidic (3.5% citric acid) is specifically labeled for basil downy mildew. It is exempt from EPA registration under FIFRA and thus does not need to be registered in NY. It was reviewed and determined to be NOP compliant by Washington State Dept of Ag. Actinovate AG (active ingredient is Streptomyces lydicus), Double Nickel 55 (Bacillus amyloliquefaciens), MilStop (potassium bicarbonate), OxiDate (hydrogen dioxide), Regalia (extract of Reynoutria sachalinensis), TerraNeem EC (cold pressed neem oil),  Trilogy (neem oil), and Zonix (rhamnolipid biosurfactant) are OMRI-listed fungicides labeled for use on herbs and for suppressing foliar diseases including downy mildew. MilStop, Regalia, and OxiDate are labeled for use outdoors and in greenhouses. The Actinovate, Cueva, Double Nickel, Procidic, TerraNeem, Trilogy, and Zonix labels do not have a statement prohibiting use in greenhouses. Double Nickel label has directions for greenhouse use for soil-borne pathogens. OxiDate has limited residual activity and thus if used should be combined with or followed by another product. Evaluations of most of these products conducted at Cornell (LIHREC) document the challenge of controlling downy mildew organically.

Conventional Fungicides for Field-Grown Basil

Revus (mandipropamid; FRAC code 40), Ranman (cyazofamid; FRAC 21), Presidio (fluopicolide; FRAC 43), Reason (fenamidone; FRAC 11), Quadris (azoxystrobin; FRAC 11), Armicarb (potassium bicarbonate), and phosphorous acid fungicides (FRAC P 07) can be used in conventional production of basil, in addition to the organic fungicides listed above. Quadris and Presidio are the only ones not permitted to be used in a greenhouse. Reason is the only one not permitted used on Long Island. Quadris can be applied 6-15  times depending on rate with no more than 2 consecutive applications for resistance management. Reason can be applied 4 times with no consecutive applications of it or another FRAC code 11 fungicide (e.g. Quadris). Ranman was the first product labeled with targeted activity for oomycetes, the group of pathogens that includes those causing downy mildews. Ranman can be applied 9 times with no more than 3 consecutive applications which must be followed by the same number of applications of other fungicides. Revus can be applied 4 times with no more than 2 consecutive applications. Resistance management restrictions for Presidio require it be tank mixed with another fungicide labeled for this disease that has a different mode of action; rotation with other chemistry is recommended in the general resistance management section of its label. There are several phosphorous acid (phosphanate) fungicides labeled for this disease with no use restrictions, including ProPhyt, Fosphite, Fungi-Phite, Rampart, pHorsepHite, and K-Phite. Quadris is labeled for use on basil but not specifically for downy mildew; it also has been shown to be effective for this downy mildew. In states like NY where the target disease is required to be specified on the label, Quadris cannot be used without an approved FIFRA 2(ee) recommendation, which the applicator must possess. These 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. Ranman, Revus, and Reason were registered as a result of assistance from the IR-4 program. Results from research conducted at Cornell (LIHREC) documenting efficacy are available.

Conventional Fungicides for Greenhouse Basil

Ranman, Revus and phosphorous acid fungicides are permitted to be used in a greenhouse. See previous paragraph for use information and resistance concerns. Heritage SC (azoxystrobin; FRAC 11), Micora (mandipropamid; FRAC 40), Segovis (oxathiapiprolin; FRAC 50, previously U15), and Subdue MAXX (mefenoxam; FRAC 4) are additional fungicides that can be used in greenhouse-grown plants for retail sale to consumers. It is important to use a fungicide resistance management program including alternation among as many chemistries based on FRAC code as possible. Heritage can be applied once to plants at each production stage, plug and finish. It must be applied in alternation with another fungicide. Micora and Segovis also can be applied at most twice to a crop. Micora can only be used in an enclosed greenhouse with permanent floor. Segovis can also be used in outdoor nurseries. Subdue MAXX can be applied once to foliage and must be tank-mixed with another fungicide labeled for this use and applied at full label rate. Phosphorous acid fungicides can be tank-mixed with any of these greenhouse fungicides. Micora, Segovis, and Subdue MAXX can only be applied to foliage of plants for retail sale as transplants; they are not permitted used on plants to be marketed as fresh herbs in grocery stores.

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.

Other Practices

Practices that minimize leaf wetness and reduce humidity may contribute to control. These include planting where there is full sunlight and good air movement with rows parallel to the prevailing wind direction, maximizing plant spacing, and using drip irrigation. However, these practices will not suppress dew formation or high humidity over night creating very favorable conditions for infection and spore formation. Gardeners can avoid by bringing potted plants indoors (LIHREC). Humidity can be lowered in greenhouses by using circulating fans and lights, by increasing temperature, and with dehumidification. Humidity needs to be below 85% within the plant canopy to suppress downy mildew. Observations of downy mildew occurrence in field and greenhouse basil plantings suggest that environmental conditions can significantly affect severity of downy mildew.

Fanning is a practice developed and being used in Israel for basil grown in protected culture. It entails directing greenhouse fans toward plants so that leaves move. This prevents water depositing on leaves when humidity is high.

Illuminating either leaf surface of plants growing under protected conditions during nighttime was shown to effectively suppress downy mildew by inhibiting spore production through a study conducted in Israel. Light was supplied in high tunnel-like structures with 20W Day Light fluorescent bulbs each equipped with a white metal reflector (30 cm diameter), with one bulb per meter row. Spores formed on leaf tissue shaded by other leaves, thus this procedure is most effective when plants are small. Initial experiments were done with illumination throughout night. Recent research has revealed light exposure is most important during the first 6 hours of night. Red light was shown to be the most inhibitory under laboratory conditions. Downy mildew also developed slower on basil under red light in a greenhouse experiment conducted at the University of Florida.

High temperature is lethal to the pathogen. Maximum temperatures for infection, colonization, and spore production are 80 – 88 F. Research conducted in growth chambers demonstrated that temperatures up to 113 F kill spores and mycelium of the pathogen in affected plants, with length of effective exposure decreasing with higher temperature range, least being 6 – 9 hours at 104 – 113 F. Subsequently solar heating has been used to cure plants in Israel by closing greenhouse vents or using a transparent IR polyethylene sheet covering during sunny days. It is recommended done at first sign of downy mildew and over 3 consecutive days with 3 – 4 hours exposure. It necessitates routine monitoring to ensure temperature reaches effective range while not rising high enough to kill plants. If temperature does not go about 95 F, treating for a fourth day is recommended.

Basil crops should be disked under or otherwise destroyed as soon as possible after last harvest, or when abandoned because of disease, to eliminate this source of inoculum for other plantings. A sunny day is the best time to physically destroy an affected crop because the disturbed spores will be killed by UV radiation.

More information/prepared by:

Margaret Tuttle McGrath
Associate Professor
Long Island Horticultural Research and Extension Center (LIHREC)
Plant Pathology and Plant-Microbe Biology Section
School of Integrative Plant Science
College of Agriculture and Life Sciences
Cornell University
mtm3@cornell.edu

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