Gardening Tips: Treating Basil Plants for Downy Mildew or Basil Blight
A potentially fatal fungal disease called downy mildew is attacking basil plants and has begun spreading in New York, New Jersey, Ohio and Florida. Many gardeners and growers alike are concerned that the disease may spread further throughout the summer and turn the green basil leaves to shades of brown, yellow or gray.
This is a relatively new disease so many gardeners are not familiar with its symptoms and treatment. This basil blight is similar to late blight in tomatoes or downy mildew in cucumbers, squash and melons. These diseases are considered "community diseases" because they move easily from garden to garden.
Basil downy mildew is a new disease in North America as well as Europe. It was first reported in south Florida in October 2007. In 2008 downy mildew was confirmed in both field and greenhouse-grown basil in many states including: North Carolina, Pennsylvania, New Jersey, New York, Massachusetts, Kansas, and Missouri. This disease was also reported in Canada in 2008 and in greenhouse-grown basil in Argentina in February 2008. In 2009 basil downy mildew was found in California. Reports of basil downy mildew in Europe occurred a few years earlier: greenhouses in Switzerland in 2001, Italy in 2003, and Belgium in 2004 as well as France, Israel and New Zealand in 2004. Basil downy mildew has also been detected recently for the first time in Iran and several African countries: Cameroon in 2007, South Africa in 2005, Benin and Tanzania.
If you have it in your garden, you can affect other gardeners and farmers because the pathogen can jump onto their crops and plants. Cornell University’s horticulture department indicated that basil blight was reported in Uganda in 1933 and wasn't reported again until 2001 in Switzerland.
The sign of the blight is the top of the leaf will start yellowing. But as with tomato blight, the fungus grows on the underside of the leaves. Gardeners need to flip the leaves over and look on the undersides of the basil leaves to determine if their plants have basil blight.
There will be a gray to purple dusty growth on the underside, sometimes turning to almost black as the fungus gets ready to release its thousands of spores. Wind will carry the spores off the plant and infect other plants downwind.
Basil Blight Photos
Basil blight was observed recently on ornamental plants related to basil, in particular coleus and salvia. These plants all belong to the Lamiaceae family, which includes basils (Ocimum spp.), mints (Menta spp.), sages (Salvia spp.) and other aromatics. Fortunately, the coleus and basil downy mildew pathogens have now been demonstrated to be genetically different; therefore, these ornamental plants are no longer considered potential alternative hosts. However, there are many ornamental basil varieties that are also hosts to the pathogen affecting basil grown for use as an herb. Contaminated seed is most likely the way that the basil downy mildew pathogen has been able to move between geographically-separated areas, such as Europe and the USA.
If you see the basil blight, remove and use all of the healthy leaves from the plant. It is best to use scissors to cut the healthy leaves off of the plant so as not to disturb any of the spores on the infected leaves. Once removed, cover the plant with a large plastic bag, tie it with a wire or twist-tie at the base, and cut the stems off at the soil level. Discard the plant and do not compost. As with tomato blight, the spores will survive over the winter and infect new plants the following year. Plant your basil in another suitable location.
There are two phosphorous acid fungicides, ProPhyt and K-Phite, that have downy mildew under herbs on the current label. These fungicides were effective in experiments with applications started before or after initial symptoms were found. Greenhouse use is not prohibited. Amistar and Quadris are labeled for use on basil but not specifically for downy mildew; they have the same active ingredient, which has been shown to be effective for this downy mildew.
You can freeze the healthy leaves in plastic freezer bags for future use, or make pesto out of the leaves and freeze this as well.
Basil Pesto Recipe
2 cups fresh basil leaves, packed
4 or 5 sprigs of Italian flat –leafed parsley, stems removed
1/2 cup freshly grated Parmesan cheese
1/2 cup extra virgin olive oil
1/3 cup pine nuts
2 medium sized garlic cloves, minced
Salt and freshly ground black pepper to taste
1 Combine the basil and parsley in with the pine nuts, pulse a few times in a food processor. Add the garlic, pulse a few times more.
2 Slowly add the olive oil in a constant stream while the food processor is on. Turn off the food processor and scrape down the with a rubber spatula. Add the grated cheese and pulse again until blended. Add a pinch of salt and freshly ground black pepper to taste.
Makes 1 cup.
You can add 3 cloves of garlic if you like it stronger. You can substitute chopped walnuts for pine nuts.
See my related article about Tomato Blight at: https://knoji.com/gardening-tips-tomato-blight-causes-and-treatment/