For over a decade, basil downy mildew (BDM) has caused significant losses in basil grown in organic and conventional field and greenhouse production across the United States. At the time of its introduction, there were very few fungicides labeled for its control making it nearly impossible to grow a successful crop in many areas of the country. The pathogen, Peronospora belbahrii, is an obligate parasite, meaning it needs a living host in order to survive. Thus, in more northern regions of the country that experience a freeze (i.e., winter), the pathogen will die when the host freezes during the fall. Because of this, the pathogen must be re-introduced the following spring or summer from southern regions of the country. This is similar to cucurbit downy mildew, where the pathogen can survive on the host that is growing in the field during the winter months (e.g., southern Florida or Mexico). The exact timing of when basil downy mildew may show up in your geographic region depends on a number of factors. The more southern you are located in the continental US, the more likely the pathogen will show up earlier in the spring or summer. In New Jersey the pathogen has been reported as early as 12 June and as late as 2 August. The first step in mitigating losses to basil downy mildew is in your selection of the best varieties. In recent years, there have been a number of new commercial sweet basil varieties released with a high level of resistance to basil downy mildew. Sweet basil varieties without BDM resistance should always be grown prior to the expected arrival of the pathogen in your region. There is a BDM monitoring website, led by Cornell University, which tracks the movement of the pathogen across the country each year. Growers can use the website to see where BDM has been reported across the country. Once BDM has been detected in your area you can expect it to remain active until the end of the production season. BDM resistant sweet basil varieties should always be grown after BDM has been detected in your region to help mitigate losses due to the disease. If you are located in the southern US, the easiest approach would be to use BDM resistant sweet basils the entire production season. All basil growers must remember that any of the new BDM resistant sweet basils are not “immune” to the disease. If disease pressure becomes extremely high or environmental conditions become highly conducive for disease development over a long period of time BDM resistance will break down for that season. Thus, it is extremely important to still initiate a fungicide program when using any DMR resistant sweet basil, especially if disease pressure is expected to be high.

For several years, the IR-4 Project has been working diligently with stakeholders and registrants to facilitate the registrations for a number of fungicide products (conventional, biopesticide, and organic) to control basil downy mildew. These efficacy studies have been done by Extension personnel at many Universities across the country. The following is a comprehensive list of conventional, organic, and biopesticides currently labeled for the control of BDM in the US.

Conventional fungicides currently labeled for basil downy mildew control:

• Ranman 400 SC, FMC Agricultural Products
  • cyazofamid, FRAC Group 21
  • Can be used in a greenhouse, 0-day PHI
• Revus, Syngenta Crop Protection,
  • mandipropamid, FRAC Group 40
  • Micora labeled for use in the greenhouse; 1-day PHI
• Ridomil Gold, Syngenta Crop Protection
  • mefenoxam, FRAC Group 4
  • Field use only; 21-day PHI
• Orondis Ultra, Syngenta Crop Protection (not yet approved by EPA)
  • oxathiapiprolin (FRAC Group 49) + mandipropamid (FRAC Group 40)
  • Field use only (foliar); 0-day PHI
• Segovis, Syngenta Crop Protection
  • oxathiapiprolin, FRAC Group 49
  • Greenhouse use only; transplants for retail sale
• Presidio, Valent USA
  • fluopicolide, FRAC Group 43
  • Field use only; 1-day PHI;
  • Adorn labeled for use in the greenhouse
• Reason 500SC, Gowan Company and Bayer CropScience LP
  • fenamidone, FRAC Group 11
  • Field and greenhouse use; 2-day PHI

Organic Materials Review Institute (OMRI Listed) federally registered fungicide products for basil downy mildew control include:

• Actinovate AG (Streptomyces lydicus, Novozymes BioAg Inc.)
• Double Nickel 55 and LC (Bacillus amyloliquefaciens strain D747 Certis U.S.A.)
• Aviv (Bacillus subtilis strain IAB/BS03, STK Bio-Ag Technologies)
• Regalia (extract of Reynoutria sachalinensis, Marrone Bio Innovations)
• Trilogy (neem oil, Certis U.S.A.)
• Milstop, Carb-O-Nator (potassium bicarbonate, BioWorks Inc., Certis USA LLC)
• Oxidate (hydrogen dioxide, BioSafe Systems LLC)
• Oxidate 2.0 (hydrogen dioxide; peroxyacetic acid, BioSafe Systems LLC).
• Cueva Fungicide Concentrate (copper octanoate, Certis USA, LLC)
• Romeo (cell walls of Saccharomyces cerevisiae strain LAS117, Lesaffre Yeast Corporation)

Biopesticide products federally registered for basil downy mildew control that are not OMRI listed include:

• mono- and di-potassium salts of phosphorous acid (K-Phite, Plant Food Systems)
• phosphorous acid, mono- and dipotassium salts (Confine Extra, Winfield Solutions LLC)
• phosphorous acid, mono- and dibasic sodium, potassium, and ammonium salts (Alude and Phostrol, Nufarm Agricultural Products)
• potassium phosphite (Fosphite, JH Biotech, Inc.; Fungi-Phite, Plant Protectants, LLC; Prophyt, Helena Chemical Company; Rampart, Loveland Products, Inc.)
• potassium bicarbonate (Armicarb 100, Helena Chemical Company)
• a combination of potassium phosphate and potassium phosphite (Phorcephite, Loveland Products, Inc.)
• sodium tetraborohydrate decahydrate (Prev-Am Ultra ORO Agri, Inc.)
• hydrogen peroxide, peroxyacetic acid (Rendition, Certis USA LLC)
• hydrogen peroxide; phosphorous acid; mono- and dipotassium salts (Oxiphos, BioSafe Systems LLC)
• citric acid (Procidic, Greenspire Global Inc.)
• hydrogen peroxide; peroxyacetic acid (Sanidate 12.0, BioSafe Systems, LLC)
• Sodium tetraborohydrate decahydrate (Prev-Am Ultra, ORO Agri, Inc.)
• Laminarin (Vacciplant, UPL NA Inc.)

Some important points to consider:

1. Some of the conventional fungicides listed above are sold under different product names, depending on whether the product can be used in the field or greenhouse or for greenhouse transplant use. Other products have both a field and greenhouse use on the same product label.
2. Although a product is listed as a biopesticide, it does not mean it has an OMRI-approved label. All growers should follow labels accordingly. Remember, the label is the law.

Proper control of BDM depends on a number of factors including the environment, disease pressure, and the timing of fungicide applications. Prolonged periods of wet weather and high relative humidity during the production season will make BDM control more difficult regardless of the products used to control it. The amount of disease pressure present in your field will also have an impact on your ability to control BDM. This is especially important in organic production systems where organic products often have better chance of working if disease pressure remains low. This is why growing a basil downy mildew resistant sweet basil is so important; as many organic products as reported by growers have not shown to be as effective as needed.

Research has shown that fungicide applications (e.g., conventional, bio-, or organic) initiated after the start of disease development most often leads to poor control and crop loss. Therefore, it is important to anticipate the arrival of BDM and initiate a fungicide program prior to the onset of disease development. This is also why monitoring the progress of the pathogen in the US is so important. In some areas, the disease might arrive on infected basil transplants from southern states. If this happens, the basil downy mildew will be in present long before the anticipated arrival of the pathogen due to weather patterns.

How products work against basil downy mildew

Conventional fungicides often work by inhibiting spore germination or spore production. Thus, the importance of having them applied prior to the arrival of the pathogen. Some of these products, such as mefenoxam or oxathiapiprolin, move within the plant, giving them an advantage when applied as drip applications. Biopesticides, such as the phosphites, are truly systemic and move up and down within the plants vascular system; however, research has shown that phosphites are more effective as foliar applications than when applied as drip applications. Some biopesticides, such as Oxidate and hydrogen peroxide, act as disinfestants killing spores they come into direct contact with. Because BDM sporulates on the underside of the leaf, these products (and most other fungicides) must reach the undersides of leaves during application in order to be effective. The same holds true for copper products. Copper is a protectant fungicide inhibiting spore germination. Therefore, it must reach the undersides of leaves. Organic products, such as those containing Bacillus and Streptomyces, act as an antigonist against BDM on the leaf surface and must be remain present in high enough populations on the leaf surface to provide control. This is often difficult to do because it requires multiple applications per week with short retreatment intervals. Often, these products are ineffective due to unfavorable environmental conditions. For growers trying to reduce conventional fungicide use, these products as well as disinfectant products will also kill off any biological control agents, so beware.

For information on Rutgers DMR sweet basils, where to purchase seed, as well as control strategies, and ongoing research efforts please follow the Rutgers basil downy mildew breeding program on Instagram at #Rutgersbasil.

Additional Resources:

Tracking basil downy mildew in the US

Managing basil downy mildew

Fungicides for the control of BDM

Controlling basil downy mildew in the greenhouse

By: Andy Wyenandt, Kathryn Homa (IR-4 Project), and Jim Simon, Department of Plant Biology, NJAES, Rutgers University

Basil downy mildew (BDM) can cause significant losses in the greenhouse. Once introduced into the greenhouse it can be very difficult to manage and eliminate. In the past few years, a vast amount of research has been done on understanding BDM biology and controlling it in the greenhouse using different cultural practices. Before we get to control strategies, let’s review what we know about the pathogen.

First, basil downy mildew is an obligate parasite – meaning it needs a living host to survive. As long as basil is in production in the greenhouse there will be a potential source of inoculum. Sources of inoculum can include fresh intact leaves, but also leaves discarded and fallen on the floor or in an open garbage container. This is important for greenhouse growers who produce basil year round or growers who are looking to extend basil production to later into the fall or earlier in the spring. The simplest method to break the disease cycle would be to stop growing basil for a short period of time and keeping your greenhouse as clean as possible. This would help break the disease cycle by removing the host. Sporangia produced by BDM are short-lived. Without a host their survival is only a few hours to a few days depending on the temperature and environmental conditions. The latent period (the time between infection and symptom development) can range from 5 to 10 days depending on the temperature and environmental conditions. This informs us that plants which appear uninfected may actually be infected without symptom development. Therefore, it is critically important to remove all plants from the operation before restarting production (especially if BDM is already present). A good time to stop greenhouse production (i.e., in the mid-Atlantic region or more northern regions) would be after the first hard freeze in the fall – after the freeze kills all potential sources of inoculum that could come from sources outside the greenhouse.

Control strategies using cultural practices in the greenhouse

Reducing relative humidity in the greenhouse

Basil downy mildew requires high relative humidity (>95%) for 7.5 hrs and at least 4 hrs of leaf wetness for sporulation. Sporulation has been shown to be significantly reduced, or not capable when relative humidity is less than 85%. Thus, maintaining relative humidity below 85% in the greenhouse can significantly help reduce spore production. If this is not possible interrupting the dew cycle (i.e., leaf wetness) with 10 minute periods of drying via fanning/venting every 2 to 4 hours can help reduce spore production.

Control using light

Research has shown that infected plants kept under 24 hours of continual light are unable to sporulate, this was also shown to be temperature-dependent. The type of lighting is also important. Incandescent light was fully inhibitory at 15 to 25^oC, but not 10^oC. Narrow band LED illumination with red light has been shown to be more inhibitory than blue light. Thus, lighting basil during the night every few hours at short periods of 10 minutes can help reduce sporulation.

Control using fanning and ventilation

Continuous fanning during the night has been shown to dramatically reduce BDM development by reducing leaf wetness (i.e., dew) and reducing relative humidity (keeping it below 95%). Recommendations from Israel are to initiate fanning when relative humidity reaches 70% in the greenhouse and to stop it when it is below 60%.

The key to controlling and mitigating BDM development in the greenhouse is controlling relative humidity and periods of leaf wetness to help reduce potential sporulation. Using a combination of cultural practices mentioned above can help reduce BDM development, but it will come at a cost to you in the form of additional hardware, temperature and relative humidity monitoring equipment and the cost of electricity. The first step in this process involves understanding where the initial source of inoculum may be coming from. Evidence for BDM being seed-borne is mixed. During the spring-summer-fall, greenhouse basil production will always be at-risk from infections coming from an outside source, including diseased seedlings you may be purchasing. Successfully breaking the BDM disease cycle (without the use of chemical inputs) in greenhouse operations has limited opportunities (e.g., in northern regions where freezing weather occurs). This can only occur in the fall, after freezing weather which can kill all outside sources of inoculum and by not carrying over infected plant material into the winter season, thus the need for a basil-free period during the production cycle. This is especially important in small greenhouse operations that produce basil organically or without the use of chemical input.

These management practices should significantly reduce your BDM problems though will require more of your attention and potentially additional equipment and supplies. Coupling best management practices with new downy mildew resistant basil varieties will further provide protection to you. Try the new basil downy mildew resistant varieties including Rutgers Obsession DMR, Rutgers Devotion DMR, Rutgers Passion DMR, and Rutgers Thunderstruck DMR or other DMR resistant sweet basils such as Prospera, and see which ones work best for you.

For information on Rutgers DMR sweet basils, where to purchase seed, as well as control strategies, and ongoing research efforts please follow the Rutgers basil downy mildew breeding program on Instagram at #Rutgersbasil.

Resources:

Tracking basil downy mildew in the US

Managing basil downy mildew

Fungicides for the control of BDM

Controlling basil downy mildew in the greenhouse

Authors: Andy Wyenandt and Jim Simon, Department of Plant Biology, Rutgers University