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.