Basil Dying? Turning Yellow? It May Be Downy Mildew.

Is your basil dying?  Turing yellow and brown?  Join the crowd.  Last year was the first year that my sweet basil didn’t flourish. Sadly, they turned yellow, then brown and subsequently died.  I learned during my master gardener classes that my basil dying was caused by downy mildew.  So if you want basil this year, learn the facts and how to keep downy mildew at bay in your garden.

Why are Basil Dying?

This pathogen, Peronospora belbahrii, which causes downy mildew is deadlier than the Terminator.  It wipes out your sweet basil  (Ocimum basilicum) patch in the blink of an eye.

All of your sweet basil.

In fact, if you live in Florida or Texas, like the Terminator, this pathogen says, “I’ll be back” since it is a year round problem.

Thank goodness for New Jersey harsh winters since it wipes out the pathogen.  But it will be back too.

Where Did this Pathogen Come From?

According to Cornell University, the original outbreak occurred in Uganda in 1933.  It was observed again in Switzerland in 2001, Italy in 2003, France and Belgium in 2004, and subsequently spread to Israel, New Zealand, Canada, the United States, Iran, and several African countries.

In the United States, the pathogen has spread to basil plants in North Carolina, Pennsylvania, New Jersey, Minnesota, Massachusetts, Kansas, and Florida.  Cornell speculates that the pathogen may be in other areas of the United States; however, growers may think that the basil died from other causes.

Twenty percent of basil is imported to the United States.  Leaves could contain the pathogen.  Between contaminated seeds and plants, the pathogen took hold around the world.

How Do You Know if You have Downy Mildew?

According to the University of Minnesota,

• The infection starts at the lower leaves and moves up the plant;
• At first, the plants turn yellow which could be interpreted as a nutrient deficiency.
•  Then, irregular black spots appear.
•  In addition, fluffy gray spores grow on the underside of the leaf.

Eventually the effected leaves turn brown and the plant dies.

The pathogen spreads via contaminated seeds and wind dispersed spores. Once one plant gets sick, the others follow suit.

High humidity and/or dampness like wet springs make this pathogen one happy fella.

The weather conditions of the east coast are prime for the spread of this pathogen.  In New Jersey, we generally have wet springs and very humid summers.

The Research to Come Up with a Fix:

Unfortunately, there aren’t any sweet basil varieties that are not susceptible to downy mildew.

Since 2011, a team of researchers from Rutgers University , University of Massachusetts, Cornell University and University of Florida have assembled to develop a strategy on managing downy mildew in basil.

(If you are interested in getting  more detailed information as to what has been discovered by the team, watch for the videos from the 2016 Basil Downy Mildew Workshop Sessions.)

In addition, the Rutgers team is in charge of  developing new sweet basil varieties that are resistant to downy mildew. The team has one of the largest basil breeding grounds in the United States.

Is All Basil Susceptible?

Not all types of basil are as susceptible as sweet basil.  Less susceptible varieties include:

Moderately susceptible:

• Red Rubin and Red Leaf (O. basilicum purpurescens)
• Thai Basil
• Lemon Std., Mrs. Burns Lemon,  and Lime,

Least Susceptible:

• Blue Spice, Cinnamon, Spice & Blue F1

Interestingly, Eleonora is the first sweet basil to be moderately resistant; however, its level of resistance is not sufficient without using fungicides.  It can be purchased HERE or HERE.

Tips to Grow Sweet Basil:

If you insist on planting sweet basil, here are some tips to keep the pathogen at bay:

• Plant as soon as you can and realize you might only get a spring harvest.
• Make sure that your basil is planted in full sunlight.
• Space your plants to allow for good air circulation.
• Use drip irrigation so the leaves don’t get wet.
• In a greenhouse, keep the lights on at night since this pathogen needs 7 hours of darkness to create spores.
•  In addition, in a greenhouse, keep the humidity below 85% in the canopy of the plants if possible.
• Buy pathogen free seed. (Meaning don’t get seeds from your neighbor’s last year plants.)

What About Fungicides?

Commercial growers can use fungicides as a preventative.  Learn about them HERE and HERE. Unfortunately, organic control for homeowners are insufficient to control the disease.

According to the University of North Carolina,

“Products containing the active ingredients copper or chlorothalonil (the trade name of one product with chlorothalonil is known as ‘Daconil’) are the best and only effective products available to home gardeners. In addition, home gardeners should grow varieties with tolerance if they are worried about basil downy mildew in future years because most chemicals available to the home gardener are not sufficient to control basil downy mildew once it appears.”

Other non-organic approved fungicides reduced the disease by 70-90%.

Once a plant is inflected, get rid of it.  Don’t compost it.

Who Wants Dying Basil.  What’s My Options?

I  felt the same way.  Either harvest early or plant the other varieties of basil listed above until Rutgers creates a hybrid that is resistant to this pathogen.

 Join the Conversation

Did your basil die last year from downy mildew?

Photos by Scot Nelson   here, here, and here.

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