Although it feels like we are being inundated with new downy mildew diseases, downy mildew of basil was actually first found in Uganda in 1933. The current outbreak started in the United States in field-grown basil in south Florida in October 2007. By 2008, basil downy mildew was found in field- and greenhouse-grown crops in many states as well as Canada. Complete crop loss occurred for some growers since they were not prepared for this disease on this crop. By 2009, basil downy mildew was found in the western United States, and early in 2011 it was reported in Hawaii.
Reports of downy mildew began a few years earlier in Europe, where the disease is now considered endemic. It was found in Swiss greenhouses in 2001, Italy in 2003, and in both Belgium and France in 2004. Basil downy mildew has also been detected for the first time in Israel, New Zealand, Iran and several African countries: Cameroon in 2007, South Africa in 2005, plus Benin and Tanzania. It appears safe to say it has been found worldwide in the past 10 years.
For several years, the pathogen (Peronospora belbahrii) was described as the same as that found on salvia, mentha and even coleus since these crops are in the same family as basil — lamiaceae. Sophisticated lab methods have since determined that this downy mildew is a new one and generally specific to basil. Discussions with researchers do indicate that the lines between the downy mildews on these related crops are not always sharp making it possible for more than one downy mildew to be found on a single plant genus.
Ornamentals like salvia/ lamium and coleus are not considered potential alternative hosts of basil downy mildew. However, there are many ornamental types of basil that are also hosts to the pathogen affecting basil grown for use as an herb. It seems more likely to me that these ornamental basils may become infected by spores movement from the basil produced for culinary purposes.
Basil downy mildew can be seed-borne and, depending on the researcher, it is felt to have spread so quickly worldwide based on this feature alone. Some research confirmed that Florida strains were genetically identical to Swiss strains of this pathogen. Others, however suggest that movement of infected basil leaves (for culinary purposes) might have spread the disease. For instance, 20 percent of the fresh basil crop in the United States comes from offshore. When research has been
conducted on basil seed (specifically ‘Genovese’) they found that four of 17 seed lots were infected with the downy mildew pathogen at a rate up to 0.33 percent. This was reported as high enough to result in an epidemic.
Once downy mildew has been found in a crop, the fungus is easily wind-dispersed; and this is felt to be the main way it spread throughout the eastern United States every summer since 2008. A monitoring program was started in 2009 to try to determine whether the fungus moved north from Florida, now that it is established in Florida. I have not found any mention of oospore production by this downy mildew, although in field-grown basil this should be a concern. Downy mildew on sunflower and many other crops is known to carryover from season to season by production of oospores allowing it to overwinter in fallow soil. This is apparently possible with the impatiens downy mildew but I could find no mention of it on basil.
Characteristic yellowing is seen on the top of leaves that often is confined between the leaf veins, making it angular in shape. On the undersides of such yellow areas, the gray to nearly black spores of the downy mildew pathogen develop on the undersides of infected leaves. Unfortunately, there are two other diseases on basil that can be confused with basil downy mildew. The first is foliar nematode, which I have seen on some vegetatively propagated basil. The second is bacterial blight caused by Pseudomonas cichorii.
Using seed not infested with the basil downy mildew pathogen is a critical step to prevent the disease. However, this may not be easy currently since seed testing and “certification” usually lags behind its need. If we are growing the least expensively produced basil seed (open pollinated) it has likely not been certified free of pathogens. You really cannot require the highest seed quality without being willing to pay for it.
The second line of defense would be to choose cultivars of basil that are resistant to downy mildew. Cultivars choice for container production are not often determined by the producer but by the distributor or end user. Variety evaluations have shown that all of the most commonly used culinary sweet basil (Ocimum basilicum) are more susceptible than the some of the exotic and ornamental basils such as Ocimum citriodorum and Ocimum americanum.
Trials in New York and New Jersey showed that the worst downy mildew symptoms were seen on sweet basil varieties ‘Aroma 2’, ‘Genovese’, ‘Martina’, ‘Italian Large Leaf’, ‘Magical Michael’, ‘Nufar’, ‘Opal Purple Variegated’, ‘Poppy Joe’s’, ‘Queenette’, and ‘Superbo’. Fewer symptoms were found on ‘Red Leaf’ and ‘Red Rubin‘ with similar low severity of downy mildew observed in each of the lemon and lime basil varieties (‘Lemon’, ‘Lemon Mrs. Burns’, ‘Sweet Dani Lemon Basil’, and ‘Lime’). No symptoms were found on leaves of ‘Spice’, ‘Blue Spice’, and ‘Blue Spice Fil’.
In an evaluation conducted on Long Island in 2009, ‘Cinnamon’, ‘Queenette’ (Thai basil), and ‘Red Rubin’ were less severely affected than ‘Superbo’. This is good news for ornamental producers of basil but not for anyone producing the crop for culinary uses whether fresh leaves or containers destined for the specialty grocery chain store.
The Greenhouse Environment
Research on conditions that promote basil downy mildew has been conducted. The pathogen develops most quickly at 68° F and not at all if temperatures are lower (54° F) or higher (80° F). It seems odd that the disease is so severe in Florida where daily temperatures may exceed 80° F regularly. It is my experience that if you provide enough water, temperature becomes less important. Thus, even at high temperatures, watering from overhead or daily rainfall (like Florida in the spring, summer and fall) can result in severe downy mildew.
In addition, at least six hours of leaf wetness are needed for infection to occur after exposure to sporangia of the pathogen. This means that in a greenhouse, you can reduce disease severity by keeping the leaves a dry as possible especially when temperatures are around 68° F. Avoid overhead irrigation, spraying fungicides late in the day or irrigating late in the day. It is also possible to reduce downy mildew diseases by spacing plants and using fans to promote leaf drying and reduction of RH around the plants.
There have been many trials performed on field-grown and some on container, greenhouse-grown basil for prevention of downy mildew. Trials on field-grown basil since the mid 2000s have shown that certain products are clearly more effective than others. Unfortunately, the fungicide labels that may be used for field-grown basil are not the same as those that may be used on greenhouse, container-grown basil.
Azoxystrobin is very effective in the field but not broadly labeled for greenhouse use on basil. Syngenta has pursued some 24 (c) labels on Heritage (azoxystrobin) and obtained them in Florida, Alabama and California. Similarly, mefenoxam is very effective but without a 24 (c), Subdue MAXX could not be used by greenhouse basil growers. Syngenta does have labeling for its use in greenhouses in California, Michigan and Alabama. Finally, we have recently received national labeling for Micora (new active ingredient — mandipropamid), which has greenhouse grown basil on its label. Until California registers Micora (4 hour REI), we have a 24 (c) on Revus (also mandipropamid) for basil downy mildew in greenhouse production. Follow the labels for rates and intervals as well as rotations. Each of these products has provided very good to excellent control of basil downy mildew under numerous field trials in multiple states. In some trials, the addition of a wetting agent has dramatically improved control of basil downy mildew even with the most effective fungicides.
Although these fungicides are legal (at least in some states) and effective they are not OMRI listed. If that is your need, a few fungicides are currently labeled for this new disease. These do not include highly effective products however. Cease is a somewhat effective biological control that is OMRI listed and labeled for this use. MilStop is also labeled for organic use and is labeled for this use. It is also somewhat effective. OxiDate is labeled for use outdoors and in greenhouses and has limited residual activity. There are three phosphorous acid fungicides that list control of herb downy mildew: ProPhyt, Fosphite and K-Phite. These products are effective in many cases for prevention of downy mildew but are not OMRI listed. Unfortunately, if you must produce organic basil, your choices are few and less effective than other products for basil downy mildew. You must, therefore rely on environmental changes if at all possible.
Basil downy mildew was found and reported in Europe as early as 2001, but some of us were still surprised by this disease in 2012 when it became a serious problem for greenhouse container growers. There is really no excuse for this in the time of the Internet. We all need to be more proactive in determining what the next “disastrous” disease may be and work towards preventing the event. It has always been and will always be better to prevent a problem than try to contain it after its has escaped. Unfortunately, that is often the first time we notice a problem may exist.
Learn how to diagnose and fight disease on your basil crop.