If you’ve got any Sweet Basil plants that look like this, you’re not dealing with a bug. You’re dealing with a new blight on basil called Downy Mildew. And, unfortunately, we’ve got it.
Downy Mildew isn’t knew in gardens; and it’s been affecting cucumbers, squash and other vegetables for years. That Downy Mildew puts a white powdery cover over the leaves of the infected plant.
This Downy Mildew is a mildew of another ilk. It doesn’t turn the Basil leaves white. It first turns them yellow, then brown and then they are dead; and, unfortunately, it’s a rather new scourge in the U.S. having shown up in Florida in 2007 for the first time. What’s its origin? Uganda, no less where it was first reported in 1933. So it looks like it took 81 years to become endemic around the world. The good news is it did take that long to become a worldwide problem.
“Basil growers may recall with frustration a similar situation years ago with another new disease: Fusarium wilt. In just a few years, as the cause of this wilt was being identified, the pathogen became endemic most likely as the result of marketing of contaminated seed. It also is possible that spread of the basil downy mildew pathogen occurred through marketing of infected, basil leaves that were asymptomatic during shipment. Basil in the US has become the leading culinary herb and is available year round.”(Quote taken from Cornell University’s Vegetable MD Online at: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html)
And it grows like weeds in aquaponics and hydroponics systems in both horizontal raft growing troughs and vertical growing systems, so identifying Downy Mildew early is important because it can take out an entire crop.
“Basil downy mildew is caused by the pathogen Peronospora belbahrii. This pathogen thrives in warm, humid conditions. It can move into the garden in infected seed or transplants or as airborne spores.”(Quote taken from Cornell University’s Vegetable MD Online at: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html)
“Plants infected with basil downy mildew first display yellowing of lower leaves. Upon close examination, gardeners will notice that the yellowing appears to occur in sections restricted by major veins. This causes a blocky or angular yellow sections on the leaf.
If the lower surface of the leaf is examined, dark colored spores can be seen as a dirty looking fuzz that grows directly below the yellow sections of the leaf. A small magnifying glass may be useful in viewing these spores. As the disease progresses, infected sections of the leaf turn dark brown to black and leaves may fall off. “(Quote taken from Cornell University’s Vegetable MD Online at: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html)
We just tore an entire Deep Media Bed out of our Greenhouse that had both Sweet Basil and Cinnamon Basil growing in them; and both plants were infected. Sweet Basil is much more susceptible to Downy Mildew but the Spice varieties will also get it.
“The first step in preparing for basil downy mildew is learning the symptoms. Observing spores on the underside of leaves is key to diagnosis. There are other causes of leaf yellowing in basil. Spores are produced during the dark night period, therefore 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 wet paper towel in a closed plastic bag in dark for a day to encourage the pathogen if present to produce spores.”(Quote taken from Cornell University’s Vegetable MD Online at: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html)
“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. Minimizing leaf wetness and reducing humidity to obtain conditions unfavorable for disease development may suppress downy mildew, especially in greenhouses.” (Quote taken from Cornell University’s Vegetable MD Online at: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html)
Our mistake (again) was getting in a hurry to put seedlings into our Grow Beds and purchasing them from the Home Depot Garden Dept. instead of planting the seeds ourselves. One other time we did this, we ended up with a massive infestation of aphids in our Greenhouse. Hopefully, this time we learned our lesson.
Farmers need to regularly inspect their crops for signs of this scourge as once it’s gotten a foothold, it’s impossible to eradicate.
Thanks for following our Blog. We still consider ourselves gardening beginners around here and are sharing what we’re learning as we go. Sorry to have to share bad news about a relatively new plant disease like Basil Downy Mildew.
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