A plant disease called Basil Downy Mildew has been yellowing basil leaves, stunting basil plants, and interfering with gardeners’ pesto dreams for many years. According to Cornell University, this basil disease was first identified in the USA in Florida in 2007. Since then, the disease has spread pretty much across the entire country, and beyond.
What is basil downy mildew?
Basil downy mildew is a plant disease that causes yellowing and browning of basil leaves. Because the first signs, the changes in color of the leaves, are also signs of other problems, like a lack of soil fertility, identifying downy mildew requires a closer look at the plant.
In the two plants above, the first shows leaves that are less brightly green. The tops of the leaves show some yellowing, and there are brown patches on the leaves, too. The undersides of its leaves are pale.
The other plant is actually the exact same plant, just a few weeks earlier in the summer. Back on July 22, the leaves were brightly green, un-mottled, and the backs of the leaves were also green.
To see if downy mildew is causing the change in the leaves, from brightly green to a mix of paler green with yellow and brown, turn a yellowed leaf over to look underneath.
If downy mildew is causing the problem, there will be purple-gray dots that are the spores of the disease that you can see if you look close. You might need a magnifying lens to see them well. From further away, the underside of an affected leaf looks almost silvered.
The good news for home gardeners is that not all basils are affected by the downy mildew. Finding resistant basils that have the flavor we desire is the next step in renewing our pesto dreams.
My home garden basil varieties trial
This year I planted eight kinds of basil. My goal was to identify hardy, good-flavored varieties for my yard. All but two varieties (‘Amazel’ and ‘Red Rubin’) were grown from seed that I bought from Strictly Medicinal Seeds.
The ‘Red Rubin’ seeds were from Nichol’s Garden Seeds. The ‘Amazel’ basil was a plant sent to me (for free) from Proven Winners.
The information below is what I’ve learned so far for each of the eight basil varieties currently growing in my yard (two are about to come out, because of the downy mildew).
Sweet Genovese basil (Ocimum basilicum)
Genovese basil has the “classic” basil flavor. It is the variety I have relied on for many years to fill my freezer with pesto, to be used in winter.
However, this is one of two varieties in my garden that currently are turning yellow and brown from downy mildew. The downy mildew pictures above are of this variety.
The good news is that I already harvested leaves from my two Genovese plants, early in July, to make pesto. That pesto is in the freezer now, waiting for its moment to shine in appropriate meals.
Sweet lettuce leaf basil (Ocimum basilicum)
Lettuce leaf basil is the other variety that is fading fast in my garden as a result of basil downy mildew. To be honest, this variety looks worse than the Genovese basil.
In the past, this has been a great basil to have in the garden, because the large leaves fit so nicely onto a sandwich.
If I had known what was coming to the plant this summer, I would have brought more of it into the kitchen before now.
Kivumbasi lime basil (Ocimum canum)
This is a new basil variety for me. I was curious about the flavor and wanted to know more. The great news is that the plant, so far, shows no signs of being affected by basil downy mildew.
The plant in my garden has small leaves, and the entire plant is not as large and robust-looking as some varieties. The flavor is the funny part. It really does taste like a mix of lime and basil.
I have not been super-successful in finding ways to use this basil in the kitchen. The description at Strictly Medicinal Seeds, where I purchased the seeds, suggests using the leaves in tea. Before I pull the plant from the garden, to make way for fall vegetables, I will harvest the remaining leaves to dry for herb tea.
The other way I’ve used the leaves is to make lime-basil popsicles. The result was a little odd, but you never really know until you try, right?
Mrihani basil (Ocimum basilicum)
This is another new basil for me, and it also shows no signs of basil downy mildew. The flavor is very good; it leans a bit toward anise. I am saving leaves for tea.
The leaves are large and scallop-edged. The scalloped edges are pretty in the garden, and the large size makes for an easy harvest. The plant is also large, for a basil.
The flower spikes start out green but mature to purple. Bees love them.
Thai basil (Ocimum basilicum)
So far, I see no signs of basil downy mildew on this variety. The plants make tidy, globe-shaped mounds. The flower spikes are purple, which makes an ornamental contrast to the leaves. The basil flavor is spicy and good, but not exactly pesto-like.
If you order pho from a Thai restaurant, this is likely to be the basil that comes in the pile of green things that accompany your soup.
Greek basil (Ocimum minimum)
This small-leaved variety also shows no signs of basil downy mildew. The plant is small all over, with a neat, tree-like shape if given proper spacing at planting time. My garden tended toward crowdedness this year, so my Greek basil did not develop the classic shape.
I have grown this basil before. The small leaf-size slows down harvest a little, but the strong, spicy flavor means a little bit goes a long way. The flavor leans toward cloves.
Red Rubin basil (Ocimum basilicum)
This is the basil variety I grew last year, when I first decided to look into a wider range of basil varieties that might resist the basil downy mildew disease. Of all the non-Genovese varieties in the garden this year, this one has the most Genovese-like flavor.
Like last year, the plant does not show any signs of having basil downy mildew.
The leaves do shade toward purple. Both years that I’ve grown this variety, the plants have not grown to a large size. The leaves, though, are large enough that harvesting is easy.
‘Amazel’ sweet basil from Proven Winners (Ocimum basilicum)
This plant was sent to me by Proven Winners, and it fit nicely into my plan to try more basil varieties this year. The plant shows no sign of basil downy mildew. The leaves are large-ish, brightly green, and the flavor is similar to that of ‘Red Rubin’ — not quite as sweet as the classic Sweet Genovese, but close.
Cornell University reports that the seeds of ‘Amazel’ are sterile, which means that, even if we manage to collect seeds from the plants, they won’t grow next year. However, many gardeners purchase herb plants, instead of seeds, for the garden each year. If they buy an ‘Amazel’, they will get a plant with good flavor that won’t suffer from basil downy mildew.
How to manage basil downy mildew in the organic home garden
Currently, no good treatment options exist for basil downy mildew in home gardens, even in gardens that are not managed organically. Commercial non-organic farms do have some chemical options, but even those are not perfectly effective.
Organic home gardeners should consider other strategies.
Change our expectations
Some of us may choose to branch out, flavor-wise, and bring some differently flavored pestos to the table. Certainly, there are many flavor options to try! Maybe a batch of lime-basil pesto would be less odd than my lime-basil popsicles. Maybe, too, I will eventually love lime-basil popsicles.
Plant resistant varieties
‘Amazel’ is not the only new Genovese-type basil variety that shows resistance to the downy mildew disease. A Basil Downy Mildew article from Wisconsin Cooperative Extension suggests ‘Eleanora’ as a resistant variety to try. Cornell University (linked earlier in this article) lists several additional varieties for home gardeners to try:
Many of the above varieties can be purchased from Johnny’s Selected Seeds.
Change the timing of planting and harvest
Looking back at the first two pictures in the article, you can see that the plant on July 22 looked great. Evidence of disease did not show up until later. If I had planted more Sweet Genovese basil earlier in spring, I could have harvested loads of delicious basil leaves before August, with its heat and humidity that encourage downy mildew diseases.
Then, as the disease became noticeable, I could have removed them from the garden, replanting that space with another crop, knowing that I had plenty of pesto stored for use in later months.
Purchase steam-treated seeds
Some basil seeds are steam-treated to kill the basil downy mildew pathogen. The information might not be easy to find in a seed catalog or even online, but untreated seeds can spread the disease to your garden if they came from infected plants.
The disease can also come into your garden on the wind, which we can’t do anything about, but making sure our tools are clean, that plants we purchase to set into our gardens are healthy, and buying disease-free seeds are ways we can slow the spread of this disease.
Herb tea in my future
Right now, my garden still contains several basil plants. They won’t all be there much longer, though. Soon, I will be harvesting leaves to dry (in my most excellent Excaliber dehydrator) for herb tea. Then, I will remove most of the plants to make room for planting vegetables that I want to grow for fall and winter harvests.
The Thai basil plants will stay in the garden until frost, to support the many pollinators that visit them every day.