Erysiphe spp.
Powdery mildew is far from the most severe disease that can infect your beets, but I can’t help but hate it more than just about anything else.
Maybe it’s because this disease is so incredibly common. If a year goes by without some plant coming down with powdery mildew, it’s a miracle!
We link to vendors to help you find relevant products. If you buy from one of our links, we may earn a commission.
While it might be distressingly common, the good news is that powdery mildew also easy to deal with.
There are lots of effective control methods and most of them don’t require harsh chemicals that might harm you or your garden.
The trick, of course, is to identify the problem right away and start fighting back as soon as you can. If the disease isn’t dealt with, it can make your beets grow pathetically small.
Since time is of the essence here, let’s dive into the following topics:
What You’ll Learn
Now, we’re talking about table beets (B. vulgaris subsp. vulgaris) here, but anything in the Beta genus can be susceptible to this disease.
That’s because the pathogen that causes the disease uses all species in the genus as a host.
What Causes Powdery Mildew?
Powdery mildew in beets is caused by species of fungi in the Erysiphe genus, notably E. polygoni, formerly E. betae, syn. Microsphaera betae.
These are obligate parasites, which means they need a living host to reproduce.
That’s good news because it means the fungi don’t need to kill their host to reproduce and that gives us a fighting chance of getting rid of the disease before it causes serious problems.
It can cause some nasty symptoms that can reduce your harvest. Let’s go over that, next.
Symptoms
The symptoms of powdery mildew in beets vary depending on the stage. At first, it produces small powdery spots on the underside of the plant’s leaves.
Usually, it’s the older leaves that are symptomatic to start with. Then the disease spreads to the younger leaves.
As the disease progresses, the entire leaf surface becomes symptomatic. It will be covered in a powdery coating.
This powder is actually the asexual spores of the pathogen, and are known as conidia.
In extreme cases, the older leaves might turn yellow, followed by brown as they dry out and die off. New shoots might be deformed. While beets can withstand a freeze, infected leaves will die in frost.
The powdery coating reduces the plant’s ability to photosynthesize, which can result in stunted growth, and small roots. Those big, beautiful beets you were expecting? They might be sad, pathetic little golf balls.
Obviously, if you like to eat your beet greens or you’re growing Swiss chard, a bunch of sad, powdery leaves aren’t exactly an appealing meal option either.
Life Cycle
The fungus that causes powdery mildew needs warm, dry weather to spread. Ideally, the temperatures will be between 60 and 86°F and the humidity below 60 percent.
It reproduces most readily when the temperatures fluctuate throughout the day. Ideally, the days will be long and dry, with highs in the low 80s and lows in the mid 50s.
The spores can overwinter in warmer areas on any species in the Beta genus. That means beets, chard, and wild beets (B. vulgaris subsp. maritima) can host the pathogen.
In areas with long, deep freezes, the pathogen won’t have a suitable host. These areas generally have much less of an issue with powdery mildew on beets.
That doesn’t mean the pathogens aren’t hiding underground on bits of root left behind in the soil. Even North Dakota has regular powdery mildew outbreaks and we know the winter there isn’t exactly mild.
The same goes for areas with temperatures above 90°F for long periods.
The pathogens can’t tolerate heat and places like southern Arizona will typically have less of a regular issue with powdery mildew than, say, central California.
The pathogen can also live on stored seeds.
The bad news is that once the fungi have found a good host, they start spreading rapidly.
So from the moment those spores appear on the undersides of the leaves to the point where the entire plant is covered and the leaves are dying, it can be a matter of weeks.
Spread
The conidia can be carried on the wind and they can travel a long distance.
The reproductive spores form in the morning and then spread during the afternoon, with germination happening in the evening.
In a few days, these new spores mature and the cycle begins anew.
The spores can also be spread on splashing water, in humid environments through water droplets in the air, or you could even accidentally carry the spores on dirty tools.
Prevention
You can grow resistant cultivars. If you deal with powdery mildew every year, it’s definitely worth seeking them out.
A few common resistant varieties include ‘Kestrel,’ ‘Green Top Bunching,’ ‘Lutz Green Leaf,’ ‘Pacemaker III,’ ‘Red Atlas,’ ‘Red Velvet,’ and ‘Solo,’ but check the seed packet or plant label when you’re shopping.
They will usually note powdery mildew resistance or have “PM” on there somewhere.
Make sure to grow your beets in full sun in a spot with excellent air circulation. Don’t overwater as it raises the humidity in the area.
Avoid overfertilization as well. Too much nitrogen causes excessive leaf growth, which results in crowded conditions that encourage mildew growth.
If you suspect the conditions are just right for infection, you can apply fungicide as a preventative.
Neem or horticultural oil works well, as does a milk spray made from one part milk and five parts water.
Control
Experts and experienced gardeners have come up with lots of ways to deal with powdery mildew.
Here are a few control methods – you can read more about dealing with powdery mildew naturally here.
Hydrogen peroxide and citric acid are powerful tools in the war against powdery mildew. There are many products out there that utilize these common ingredients.
Streptomyces lydicus WYEC 108 is a beneficial bacteria that parasitizes fungi on plants.
You can find it in products like Actinovate SP, available at Arbico Organics.
Bacillus subtilis is another beneficial bacteria that can drive out powdery mildew.
You can also look for products that contain natural ingredients like thymol, which is derived from thyme plants.
For the past few years, I’ve been using products that contain potassium bicarbonate and I’ve had excellent results.
Products like Milstop utilize potassium bicarbonate to kill fungi simply by creating an unwelcoming environment for the pathogens to grow.
If you want to give this product a try yourself, head to Arbico Organics where you will find Milstop in a variety of packet sizes.
Sulfur can’t be used preventatively but it’s quick acting if you use it as soon as you see symptoms. Copper works as well, but it’s less effective than a lot of the other options on this list.
Erysiphe pathogens are adaptable and they can develop resistance to most treatment options.
That’s why I highly recommend that you alternate treatments. Maybe use Milstop one time and then for the next treatment use sulfur.
It might take a few treatments to get the disease under control, but it’s absolutely doable.
Beat Powdery Mildew on Beets
Don’t start thinking all doom and gloom if you see powdery spots on your beet plants.
Sure, it’s a problem, but it’s one you can address quite easily. You can still be digging up a healthy harvest right on schedule if you tackle the problem early.
Maybe you’re struggling to figure out exactly what it is you’re facing. If that’s the case, go ahead and post a few pictures and a description of what you’re seeing in the comments and we’ll do our best to help you figure it all out.
And for more information about growing beets in your garden, check out these guides next:
