Tag Archive | how humidity affects basil

Basil Downy Mildew: You Can Prevent It!

Yes, You Can Prevent Basil Downy Mildew!

Pots of young lettuce-leaf basil.

What could be more refreshing than a garden salad harvested from your own back yard? We love our tomatoes, cucumbers, green onions, ripe sweet peppers, and the ‘Tango’ celery added to mixed greens. And don’t forget the basil!

But you’ve noticed how basil has struggled the past few years despite all the suggestions offered by the garden center. It just doesn’t look quite right, which prompts the question,

**“What Is Wrong With My Basil?”**

Gardeners throughout the world are having issues with basil. From Anchorage to Adelaide, readers at The Farm In My Yard clearly have problems with this herb. And it used to be so easy to grow! Now their leaves develop black spots every summer, and they’re yellow between the veins. This is not appetizing at all.

Does that sound familiar?

Well, join the club! For over a decade, we’ve been dealing with spotty ugly basil, infected by that awful disease known as basil downy mildew. This water mold has been seen in more than 44 of these United States, and in just about every country where this herb is grown. Once the disease has reached your plants, you might think it’s all over.

But it isn’t! By managing it differently than you normally do, you can have beautiful basil once again. I’ve successfully grown it in pots every year since the disease was first identified in the United States. The plants grown in the garden, however, almost always have succumbed to BDM. But there’s more to it than just growing them in pots, as you’ll read later.

You need not worry that Peronospora belbahrii will infect other plants in your garden. BDM targets only basil.

Yellow areas between the veins.

How To Recognize Basil Downy Mildew

As you can see in the photo, above, formerly healthy green leaves developed yellow patches bordered by the main veins. After a stretch of high humidity or rain, the symptoms became readily apparent. The plant looked fine one day, and the next morning it looked like this!

The condition worsened, as it infected more leaves and more stems. The leaves became yellow overall, and black spots showed up. Within a week, the plant was worthless. It had been growing in a large pot with a tomato, so I cut the basil at soil level and threw it in the trash.

We’ve had a very rainy spring and summer so far (2020), and this plant started showing signs of distress by mid June. I picked the good green tips, and kept them indoors in a glass of water until they went into tomato-basil salads.

The local grocery store had full sweet basil in small pots, so I bought one. The first thing to do is to discard the clear plastic wrap around the plant. Why? Because humidity causes the problem in the first place!

This is a pot of healthy sweet basil.

How BDM Starts

Microscopic BDM spores on the leaves need relative humidity above 80% or 85% for only a few hours in order to germinate. Local weather reports will indicate the relative humidity, and you might be surprised by how high that number is at dawn, even during “nice” weather.

Although you might not have had rain or high humidity during the daytime, your basil can still become infected. Here’s how:

As the temperature decreases through the night, the relative humidity rises. It’s usually around dawn, the coolest hours, that the relative humidity reaches its highest reading. With BDM spores on a susceptible variety, 2 or 3 hours of high humidity will get the disease growing. So, even during comfortable daytime weather, the infection gets its start quietly, hours before the alarm clock goes off.

Most years, we can grow basil successfully for a few months before the humidity increases. But over the past few years, BDM has struck earlier in the growing season.

If the summer is hot—in the mid 90’s and above—and the nights don’t cool off very much, the relative humidity won’t rise to the critical 80-85% level required for germination. Unless there’s rain.

Gardeners living where the humidity is always low probably won’t have problems with BDM. But one brief rainstorm could be enough to initiate the disease, if spores are present, even in a normally dry region. If the weather dries sufficiently, though, the disease will subside. Pick off and discard infected leaves, and normal growth should resume.

Wind carries the spores from infected southern-grown plants to northern zones (northern hemisphere) each year. Plants that are shipped to garden centers around the country, infected seeds, and overwintered infected plants are potential sources of basil downy mildew. But the spores do not survive cold winters, and the disease needs a living host.

At Dawn, The First Day

Early symptom–fuzzy gray layer on reverse.

Once the spores germinate, they grow into a gray fuzzy film on the leaf’s reverse. You have to be an early riser to catch this stage of the disease. You’ll notice a change in the leaf’s appearance, showing light green to yellow areas between the main veins.

Later, Or The Next Day

The mold progresses to the next stage, when tiny black dots become visible on the back of the leaf. Those dots are the sporangia that have burst open, releasing spores to infect the next basil that comes along. The leaf becomes more discolored, wrinkled, and spotted. The sporangia can be so thick that most of the leaf reverse is covered in black.

Black sporangia on reverse.

And Then

Yellowing between the veins continues, and leaves develop black edges or spots. Although the infected leaves won’t harm you if eaten (unless there’s a true allergy to the mold), they will not have that rich summer basil flavor and aroma you longed for. It’s time to look for new, healthy young plants. Or start some seeds!

Any basil without good color, either light or dark green, or purple, tastes awful. Whether it’s caused by disease, poor growing conditions, or lack of nitrogen, pale leaves will disappoint your taste buds. Some varieties, notably the lettuce-leaf basils, normally have light green leaves (photo at top).

: Pick all the good leaves.

: Advanced BDM.

Three Suggestions To Prevent Basil Downy Mildew

At the end of this post, I’ll include links to other articles at The Farm In My Yard with information on basil downy mildew. They have all the details you’ll need to grow beautiful basil. And there are methods for growing this herb outdoors in the garden in ways which might decrease the likelihood of BDM.

For disease-free basil, consider these 3 tips:

1. Grow Basil Under Artificial Lights

The Structure

Many gardeners don’t have sunny windows or any garden space to grow basil. You can grow a decent crop, however, under artificial lights, such as fluorescent tubes or LED’s. The more expensive LED’s last longer and consume less energy.

You can buy light cart kits to assemble at home, but they will cost you dearly. Instead, gather a few materials from the hardware store, including the light fixtures, and construct your own light table. Another option would make use of an empty bookshelf. Perhaps the basement or a spare bedroom could become indoor growing space for houseplants and edibles. I recommend a small fan to circulate the air.

A 4′ long plug-in fixture with 2 tubes and a reflector on top will suffice. Smaller units emit much less light and might not provide enough light for herbs. So, for plants that normally require lots of sunlight, try to find space for the larger fixture.

Two or three fixtures lined up about 2′ apart significantly increase the light levels, so you can grow other edibles or houseplants with the basil. Maybe you’d like to add parsley and cilantro to your indoor garden. Lots of possibilities!

For 35 years, I used plant tables originally built by our contractor friend, Wally. I broke them down and reassembled them each time we moved. Constructed from 2″ x 4″ framing, carriage bolts, screws, hooks, small chains, 4′ light fixtures, and 4′ x 8′ sheets of exterior grade plywood, each table was a marvel of functional simplicity. Matte white surfaces reflect light back to the plants, so consider painting the structure, including the plywood, and the room white or off-white.

Be careful using electricity and water in close proximity. If you have any doubts, ask a licensed electrical contractor for advice. But it’s not difficult to set up a simple table with lights. Double the fun and add fixtures under the table, where it’s a bit cooler. Lettuce and mesclun should do well there.

Growing Basil

Keep the plants very close to the tubes—within a couple of inches—and leave the lights on for 14 to 16 hours each day. Yes, this is more hours of light than they’d receive outdoors, but with lower photosynthetic photon flux density (“weaker” light) indoors, plants use this light more efficiently over a longer period of time, resulting in increased growth. Your indoor basil probably will look better than outdoor basil.

Basil likes temperatures from the high 60’s and into the 80’s F, but prefers to be above 70°. You might be able to grow it indoors all year. Keep the soil slightly moist, but not wet.

Pinch back the stems so the basil won’t grow too tall. This prevents lower leaves from being shaded, which will cause yellowing. Cutting back the plants also forces branching, supplying more tender growth for the kitchen. Remember to fertilize every 2 to 3 weeks to keep the foliage a rich green color; pale foliage tastes terrible.

Start seeds or buy young transplants every few months. Basil is an annual, and once in flower mode, it often starts deteriorating. Cutting off the flower buds and a few nodes (pairs of leaves) under them, however, will delay its demise. Place potted basil plants on inverted pots, if needed, to raise young plants closer to the lights.

Microgreens, primarily broccoli.

Growing microgreens in shallow trays is another worthwhile project for your light table. Tender basil seedlings, among many other edible plants grown for microgreens (photo, above), can be harvested 2 to 4 weeks after germination.

Advantages:

Basil is within reach of your kitchen.
You can grow it all year long.
No insects, with careful monitoring. No pesticides.
Pick only what you need, without any of it deteriorating in one of those plastic clamshell containers.
Basil growing in conditioned air (heat or AC) indoors has low relative humidity, so downy mildew will never be a concern. Don’t mist the plants.
Growing plants indoors keeps you connected to nature, which has proven psychological benefits.

Tomato-basil salad.

2. Move Potted Basil Indoors In the Evening

Now that you know how BDM becomes established in your plants, keeping basil in pots is a good solution. Sure, you can also grow it in the summer garden. But having at least one potted basil is a safety net if BDM ravages your garden.

Once rainy weather settles into your neighborhood, or if you’ve noticed BDM in gardens around town, it’s time to pull the pots indoors for the night. Humidity indoors will never reach the levels that exist in the garden outside. Basil I’ve kept indoors has never gotten BDM.

Don’t forget to place the pots back outside in the morning, if it’s warm enough. Basil should receive at least 6 hours of direct sun outdoors. If rainy weather is in the forecast, keep potted basil plants in a bright spot or under lights indoors for the day. Avoid watering them indoors unless it’s absolutely necessary. Plants in 7″ or 8″ pots are easier to move around than basil in huge, heavy ceramic pots. So, consider ahead of time which pots you’re going to use.

A collection of several pots will be easier to wheel back and forth on a cart. How convenient is that?! Treat yourself to one of those carts with a recessed surface, which doubles as a saucer. But don’t let water collect beneath the pots. This will cut off air circulation in the root ball, and could rot the roots. Out to the deck for sunshine, indoors at night for disease-free basil!

3. Use Varieties That Are Resistant to Basil Downy Mildew

Recent breakthroughs in basil breeding show promise for disease-free and pesticide-free growing. Rutgers University developed several new BDM resistant varieties of sweet basil. Look for ‘Rutgers Obsession’, ‘Rutgers Devotion’, ‘Rutgers Thunderstruck’, and ‘Rutgers Passion’. When I ordered seeds this spring, my regular supplier had already sold out.

In Israel, Genesis Seeds and Bar-Ilan University bred the variety called ‘Prospera’. It is available through the commercial arm of the partnership, BIRAD Research and Development Company, Ltd. Your favorite seed house might stock it.

Proven Winners is offering ‘Amazel’. This is a vegetatively propagated variety, and is sterile. It will be sold only as plants since it doesn’t set seed.

In a Maryland trial testing all of these BDM resistant varieties, ‘Prospera’ and ‘Amazel’ did not show any symptoms. Some of the Rutgers plants showed very mild symptoms in autumn, probably because other stressors made them more vulnerable at that time.

I look forward to trying some of these new varieties next year. Check with these suppliers: Johnny’s Seeds, High Mowing Seeds, Harris Seeds, and Proven Winners. As far as I know, only sweet basil is available as disease resistant varieties. I’m not aware of disease resistant lemon and Thai basils, two more of my favorites.

No, none of these new varieties are genetically modified organisms (GMO’s).

(***Update***: I’m pleased to report that the ‘Rutgers Obsession’ basil growing in front of a tomato plant in a 20″ pot outdoors has not shown one speck of basil downy mildew. We’ve had the weather to put it to the test, though, with high humidity and long periods of rainy weather. 9/2/2021)

Concluding

Male American goldfinch.

At the end of the season, as any surviving basil goes to seed, consider leaving it in the garden. The cheery little American goldfinches will appreciate this windfall.

I hope you’re encouraged by these options for growing BDM-free sweet basil. It takes a little effort, yes, but I can’t imagine a season without this fresh herb. I strongly recommend growing the new BDM-resistant varieties.

Good luck, and happy gardening!

Here are two other posts about basil (Ocimum basilicum), with detailed growing instructions:

Growing Basil In Pots: How and Why

Basil Downy Mildew: Symptoms and Solutions

Symptoms of basil downy mildew.

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Growing Basil In Pots: How and Why

May2

2019

updated 3/5/2024

Why Should We Grow Basil in Pots?

‘Valentino’, a lettuce leaf basil, normally is light green in color.

This year, I’ll grow basil in pots. It’s too early to grow basil outdoors, but I am starting seeds now for early sales (transplants and cut herbs) at the farmers’ markets.

Although summer thunderstorms are most welcome after a dry patch, that’s when Basil Downy Mildew becomes a real problem. In this article, I’ll describe the disease and offer tips for growing perfect basil once again.

Basil Downy Mildew was first described in Uganda in the 1930’s. It has been a serious problem in the United States since 2007, when it first appeared in Florida. The disease now has appeared in most of the U.S. and elsewhere around the globe. I lived in Maryland at the time of the first outbreak and was selling plants at farmers’ markets and garden shows.

How To Recognize Basil Downy Mildew

The leaves of affected plants become speckled in pale green or yellowish blotches, mostly bordered by the main veins. Eventually, entire leaves turn pale green.

: Yellow areas between the veins.

: Center leaf beginning to grow pale.

A coating of grayish felt, visible at dawn, covers the bottom of the leaf. Soon afterward, small black dots appear. Those black dots (sporangia) released their spores, which will infect other basil plants. The plant looks sickly, as if it needs fertilizer…or something…and never again has that healthy basil look.

: Black sporangia on reverse.

: Advanced BDM.

Do you recognize some of these symptoms? These characteristics are diagnostic for Basil Downy Mildew. (***Update***: Photograph above, left, was taken at a Charlotte garden center on May 20, 2019. BDM arrived early that year.)

Spores float on air currents, usually migrating north from infected southern-grown plants. They also can be carried on seeds or spread from overwintering local greenhouse plants.

Humidity that rises higher than 85% for a few hours is sufficient to initiate this infection on susceptible varieties. Other water molds affect coleus and seed-grown impatiens, but only basil (Ocimum basilicum) can catch this one. And its name is Peronospora belbahrii.

Can we do something for our beloved basil?…

Easy Solution: Basil In Pots

…Yes we can! Growing basil in pots is the answer! Take the pots indoors in rainy weather to prevent spores from germinating.

But that’s not the only time basil is susceptible to this disease. Nighttime humidity levels can rise quite high even on a day that was perfectly clear and comfortable. When the temperature drops through the night, the lowest temperature usually occurs right around dawn. When the temperature drops to its lowest measurement, the relative humidity increases to the highest level.

Check your local weather report early in the morning. Look for the relative humidity measurement, which can be 85% or 95% at dawn. Rain doesn’t even have to be in the forecast. With spores in the air and basil exposed to high relative humidity, it will become infected. The spores need a few hours of these conditions to germinate. Fortunately, normal indoor conditions never come close to those levels of humidity.

Every year that I moved them indoors at night, the potted basil plants remained BDM free. And almost every year I grew basil outside, BDM was inevitable.

None of the relatively non-toxic sprays I tested worked well enough. But I prefer to not spray herbs at all. Some cultivars introduced over the past several years are recommended as having “some tolerance to Basil Downy Mildew”…but they still can develop the disease in humid areas.

New Varieties

Fortunately, new varieties promise “high tolerance to BDM”. Rutgers University and Van Drunen Farms developed resistant varieties that are available to growers, and, more recently, to retail customers. A huge demand for production of basil downy mildew resistant varieties is driving the research.

(***Update***: Basil ‘Obsession’, from Rutgers, has grown extremely well this summer, and with no BDM. I’ll take some cuttings for winter pots. Several seed suppliers stock these new resistant varieties, but demand might exceed supply, so order early. 9/2/2021)

Long-Time Favorite Herb

Since it was my customers’ favorite herb, I had to figure out a way to grow basil without using chemicals. I couldn’t afford to lose my best-seller. At that time, I started many varieties of basil from seed under fluorescent lights in the basement. They grew larger outside, after transplanting (2½”, 3½”, 4½” pots, and some that were larger), until they were sold at the markets.

Tomato-basil salad.

Knowing that high humidity provided the right conditions for this infection, I moved all the basil flats into the garage at night. Every day! Two small table fans kept the air moving around the plants, so the basil never caught BDM when handled this way.

My business infrastructure was very simple and did not include greenhouse space at that time. During the height of the market season, I grew several dozen flats of basil in various stages of growth at any one time.

These tips were passed along to my customers, who were glad to know how to stay ahead of the problem.

So, I always grow basil in pots, and take them indoors when the humidity is expected to rise high enough to cause trouble. It is a commitment, but the reward is freshly picked luscious basil for summer salads and lasagna. The disease often appears by mid- to late summer, but it can show up earlier.

‘Miniature Purple’ basil for salads or trained as a tiny topiary standard.

Varieties Included:

sweet basil: ‘Genovese’, ‘Genovese Compact’, ‘Fino Verde’, ‘Italian Large Leaf’, ‘Aroma 2’, ‘Nufar’, ‘Marseillais Dwarf’, ‘Amethyst’, ‘Miniature Purple’, and others
lettuce-leaf sweet basil: ‘Napoletano’, ‘Tuscany’, ‘Valentino’
Greek basil: ‘Yevani’, ‘Minette’, and 2 ‘Columnar’ varieties
lemon basil: ‘Mrs. Burns Lemon’
Thai basil: ‘Siam Queen’, ‘Sweet Thai’
‘Magical Michael’
lime (good with cucumbers)
holy: ‘Kapoor tulsi’ (O. americanum v. pilosum)
‘Mexican Spice’
‘African Blue’ (an ornamental basil and an excellent pollinator magnet)
‘Cardinal’ (a cinnamon type; this one and the previous 3 basils have some or good resistance to BDM)

The few that showed any resistance to the disease, however, were not the varieties we prefer in the kitchen.

It wasn’t long before basil started disappearing from other growers’ booths at the markets. So, it was worth the effort to keep basil alive and growing.

Basil In Pots: Potting Up

Start with clean materials. If you’re reusing a 12″ wide pot, clean it out very well, including all the old soil. Don’t discard it, though; throw it under the shrubs or dig it into the flower garden. Even used potting soil has value in the garden. A pot this size will accommodate several stems of basil for the entire growing season.

Wipe down the inside and the outside of the pot with a 10% bleach solution. Allow it to sit for a few minutes before rinsing thoroughly.

Basil is susceptible to several air-, water-, and soil-borne pathogens. That’s why it’s important to begin with clean materials.

Shopping For Basil Transplants

Look for rich green uniform coloring in the foliage. Familiarize yourself with the normal coloring for the varieties you’re interested in growing. For example, ‘Siam Queen’ Thai basil has dark green foliage, while lettuce-leaf basils (top of page) have light green puckered leaves.

Avoid wilted plants and those with brown edges, yellow lower leaves, pale green foliage, splotchy surfaces of leaves, and black spotting on the stems or leaves. Turn the leaves over and look for the tiny black dots of BDM. Inspect purple-leaved varieties carefully. Avoid buying basil that has started flowering.

Underpotted (tightly potbound) and underfertilized basil is stressed and will not last all season. Leaves that are off-color taste awful!

Look for small pots (2½-4½”) with 2 or 3 seedlings in the pot. The plants should have foliage down to the soil or close to it.

Growing Basil From Seed

If you can’t find the varieties you’d like to grow, consider starting them from seed. This gives you an opportunity to try new flavors—maybe lemon or lime, or anise-flavored Thai basil.

Seeds that are kept cool and dry remain viable for a few years, so you won’t have to germinate the entire packet…unless you want to.

Lemon basil sown in 4″ pots.

Pots and Seedling Mix

Have market packs or 4″ pots cleaned and ready. Use the same bleach solution recommended for larger pots, described above.

New peat pots are an option, and can be planted pot and all, without disturbance. Always pinch the bottom open in a few places before transplanting peat pots. This allows roots to quickly grow into the potting soil. Peel off the rim of the pot protruding above the soil level to prevent water from wicking away to the atmosphere. Roots will soon grow through the peat pot in moist soil.

Use pasteurized seedling mix. Most pathogens have been destroyed and its finer particles make better contact with the seeds. Fill containers with the seedling mix to within 1/2″ of the rim, tamping lightly. Fresh, fine-grained, high quality potting soil also works well.

Water gently, using lukewarm (around 85° F) water. Label the pots with the varieties of basil you’re growing and the date—for example, “Sweet basil ‘Genovese Compact’, 4/20/19”. Labels are easily fashioned from a clean, repurposed plastic milk jug.

Now, make a small depression in the center of the pot, about 1/4″ deep. Drop in a few seeds. Or make a few separate small depressions in the pot and drop a seed in each one. The latter is my preferred method; the separation gives each stem a little breathing room.

Cover with 1/4″ of soil. An exception is Thai Basil, which prefers to be sown on the surface of the soil and not covered.

Water

When moistened, basil seeds develop a grayish-white mucilaginous coating. This is normal. The gel holds water next to the seeds and it might attract insects, which help distribute seeds in the wild. And the stickiness helps seeds adhere to the soil.

Place the pots in a flat or a tray and give them warm sun indoors. Basil germinates within a week in damp soil at temperatures in the 70’s and 80’s F. If it’s too chilly, you can place the pots of basil on a heat mat. Seeds won’t germinate and young seedlings will fail in cold soil.

Keep the soil damp to moist but not wet. Never let water collect in the tray. Use lukewarm water, and water gently so the seeds won’t become dislodged. You could water from the bottom but pour off the excess once the soil surface has moistened.

Now That They’re Germinating

As soon as the seeds germinate, give them at least 6 hours of direct sunlight. This ensures shorter sturdy stems that are less susceptible to damping off disease. Use artificial light (see “Maintenance”, page 2) if you don’t have sunny windows.

Seedlings (not basil) stretching due to insufficient light.

With favorable outdoor conditions, young seedlings can be placed outside for at least part of the day. Don’t expose them to temperatures below 70°, or to wind or full hot sun if they have been indoors for a while. Check the weather forecast.

Basil needs warmer temperatures than most herbs and vegetables. Even if it’s frost-free and the garden is ready, don’t be tempted to plant basil until the soil has warmed up. Basil will either sit still or deteriorate in cool or wet soil.

Damping off is a fungal disease that appears as a narrowed, light brown section of the stem that causes the stem to bend over, killing the seedling. Avoid overly wet and humid conditions, stagnant air, low light, and cold temperatures. Young seedlings are more susceptible to damping off than older plants.

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Headings

Page 1: Why Should We Grow Basil In Pots?, How To Recognize Basil Downy Mildew, Easy Solution: Basil In Pots (New Varieties), Long-Time Favorite Herb (Varieties Included), and Basil In Pots: Potting Up (Shopping for Basil Transplants, Growing Basil from Seed)

Page 2: Repotting Basil In Pots (The Soil, Score the Root Ball, Add Plants, A Small Pot of Dwarf Basil), Maintenance For Basil In Pots (Light For Basil In Pots, Temperature, Watering Basil In Pots, Dealing With Slugs and Snails (Caterpillars), Fertilizer For Basil In Pots, Harvesting and Hormones), and About That Basil Downy Mildew (Growing Basil Outdoors)

Pages: 1 2

Basil Downy Mildew: Symptoms and Solutions

May10

2018

Healthy basil.

Basil Downy Mildew In the U.S.

Basil downy mildew is a devastating disease caused by the pathogen Peronospora belbahrii. This disease has been active in the United States since 2007, and for several years before that in Europe. BDM is thought to have originated nearly 90 years ago in Africa.

This water mold targets a very specific host—basil. Just basil. There are related species that attack other members of the Lamiaceae family, such as coleus, but it’s basil I’ll concentrate on today.

I’m writing about it now because some of you might have given up growing basil. If you’ve struggled with it the last few years, I’m going to offer some tips so you can once again enjoy your home-grown basil. And then I’ll share my favorite Tomato-Basil Salad recipe, a summertime staple around here.

Symptoms Of Basil Downy Mildew

Yellowing between main veins.

Let me begin by describing the symptoms of BDM. The plants might start the season looking great, all green or purple and… oh, that fragrance!… You probably harvested several clippings, using them in salads, on pizza, in tomato sauce. What’s summer without fresh basil?!

And suddenly it went downhill from there. The leaves developed blotchy yellow or pale green sections between the main veins. The discoloration then spread over most of the leaf surface. The leaf reverse (the lower surface) was covered in a fuzzy gray film early the next morning, followed by little black dots.

Those tiny black specks are the fruiting structures (sporangia) of basil downy mildew. They puffed out microscopic spores that were carried on the breeze to infect other basils. The plants became spotty overall, and never regained their strength.

In only a few days, the affected leaves turned yellow. Some of the tips of the stems remained green, but, eventually, they, too, showed symptoms. And most of the leaves dropped off.

You fertilized, you spoke kind, encouraging words. And you commiserated with friends. Yet nothing helped. How disappointing!

: Center leaf beginning to grow pale.

: Black sporangia on reverse.

Spores and Relative Humidity

All is not lost. With some insight into how this disease works, you might be able to stop the progression of BDM and get more mileage out of your plants. Another option is to start with healthy new transplants and take precautions so it doesn’t happen again.

You might have noticed that basil downy mildew started showing up and spreading quickly during wet or humid weather. Good observation! In fact, spores require around 85% relative humidity for a few hours in order to germinate.

Even if the weather is quite dry or only slightly humid, the basil still can become infected. How? As the nighttime temperature drops, the relative humidity climbs. After a period of high humidity, the spores begin to germinate on susceptible varieties of basil.

Let’s say the daytime temperatures are in the 80’s F, with comfortably low humidity around 40%. With no rain in the forecast, you might think you’re in the clear. The problem is that the period of time just before dawn is when the relative humidity is at its highest reading of the day. It could easily reach 90%! The lower the temperature drops during the night, the higher the relative humidity rises.

So…limiting humidity around basil can keep the plant just outside the reach of the disease. If you live in an area that gets extremely hot and dry in the summer, your basil might survive unscathed. That’s happened only 2 or 3 times in my gardens since BDM started showing up in the mid-Atlantic and southern states.

Reducing Humidity

Potted sweet basil.

How do you limit the humidity? Simple! That’s why I’m writing this now, before everyone gets their basil plants into the garden. Here are some suggestions for you:

Basil grown in the garden should be situated in an area with excellent air circulation. Don’t plant it in the middle of a bed, with neighboring plants all around, or next to a fence.

Planting basil near a paved surface, or surrounded by paved surfaces, has the benefit of having less square footage that’s able to absorb moisture. The goal is to reduce as many sources of moisture as possible. And that includes overhead watering! If you’ve read the posts about tomatoes, you’ve learned the importance of keeping foliage dry.

BDM can show up in one neighborhood while entirely missing the next one. It might not appear at all if the summer is very hot and dry and if the nights don’t cool down very much. Or it might not show up until rain returns after a long, dry summer.

Certain weather conditions, such as frequent rainstorms and persistently high humidity, foster the widespread and enduring presence of basil downy mildew. Once the spores are in the air, growing basil is a lost cause… Or is it?

The Ultimate Solution: Potted Basil

This pot will come inside at night, after releasing the toad.

Basil grows well in containers. An airy exposure up on the deck could be a good spot for it. Or sitting on a brick patio, or under the umbrella when it’s especially hot. Check your plants every day for those tell-tale light-colored blotches, and remove those leaves immediately.

If it looks like a losing battle, move the pot indoors for the night. That’s right! Get used to it if you want to—no, must—have fresh basil.

As long as you’re not living in the middle of a bog, the humidity indoors will never reach the levels required to grow BDM. And then place the basil back outside the next morning. On rainy days, keep the potted basil indoors, in a bright spot. Yes, it’s work, but worth the effort.

A few varieties of basil don’t get the disease, and plant breeders are working feverishly to bring more to market. Naturally, all of my favorites, including ‘Genovese’, ‘Yevani’, ‘Mrs. Burns’ Lemon’, ‘Tuscany’, and ‘Siam Queen’ DO get basil downy mildew. And, yes, I DO bring in the potted plants at night once the disease has reached the area or before rainy weather comes near.

Those that, in my experience, seem to be resistant are ‘Cardinal’, ‘Kapoor tulsi’ (holy basil), and ‘African Blue’ basil, but they’re not my favorites in the kitchen. ‘Eleonora’ is another that has been advertised as having “intermediate” resistance, but I have seen advanced cases of BDM on this variety.

(***Update***: New cultivars developed by Rutgers University will be worth trying. Look for ‘Rutgers Devotion DMR’, ‘Rutgers Obsession DMR’, ‘Rutgers Passion DMR’, and ‘Rutgers Thunderstruck DMR’. I grew ‘Rutgers’ Obsession DMR’ outdoors all summer, in 2021, and still have a cutting growing in the kitchen window. This variety did not develop any symptoms of BDM. Success! 1/22/2022)

Always remember: when you think about basil, think about how relative humidity can affect its health.

Tomato-basil salad.

Tomato-Basil Salad

Here’s a recipe for Tomato-Basil Salad, like Caprese salad, but with extra bits:

Ripe tomatoes, cut into chunks, juice included
Yellow or white onion, thinly sliced
Cucumber, alternately-peeled (no need to peel the thin-skinned ‘Diva’), halved and sliced
Mozzarella cheese, medium chunks
Sweet basil, usually ‘Genovese’ for us
Extra virgin olive oil
Red wine vinegar, or white wine vinegar if you prefer
Fresh Italian oregano, chopped; dried will work
Salt and pepper to taste

Measurements aren’t that important. Tomatoes are the main ingredient, and you can use all reds or mix the colors. Not too much onion. Certainly enough basil, and don’t use so much oregano that it overpowers the rest. Mother likes hers more vinegary. Oh, and get a nice loaf of bread for dunking. Enjoy!

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