How to Get Rid of Mildew on Plants in San Francisco
Powdery and downy mildew look alike on the leaf and want opposite things from your hose. Here, telling them apart is most of the job.
White dust settles on a zucchini leaf in the middle of a gray August. A pea vine goes yellow-blotched on top and furry underneath. Both get called mildew, and both get the same panicked treatment, which is how one of them gets worse. They are different diseases that want opposite conditions, so how to get rid of mildew on plants in San Francisco depends entirely on which one you have. The tell is easy once you know where to look.
Why mildew thrives here
Downy mildew only makes spores on leaf undersides after rain or very heavy fog, and they vanish once the leaves dry. They form in darkness or subdued light in a near-saturated atmosphere, and free moisture sitting on the leaf is what lets them infect. That is a summer night in the Sunset, written up as a lab condition.
Powdery mildew works the other way and still gets what it wants here. Moderate temperatures of 60°F to 80°F plus shade are its most favorable conditions, and its spores are sensitive to heat above 90°F and to direct sun. The July heat that shuts it down in Sacramento never shows up on the coast. The daily rhythm fits too: powdery mildew is favored by warm, dry days and cool, damp nights, a fair summary of summer in the Mission or on Bernal Hill.
The split runs roughly by microclimate. Foggier blocks hand downy mildew the wet leaf surface it needs; sunnier inland pockets, warm by day and damp by dark, feed powdery mildew. Most gardens here see both across a year.
Powdery or downy?
Powdery mildew vs. downy mildew: powdery mildew is a group of fungi growing as a thin layer on the leaf surface, and the white dust is its spores, which need no free water to germinate. Downy mildew grows inside the leaf and pushes spore stalks out through the underside, and it needs standing moisture on the leaf to infect.
Powdery mildew often starts as yellow spots on the upper surface of older leaves, then develops the characteristic powdery growth on both the upper and lower sides. It enlarges in a circular pattern with no clear boundary at the leaf veins, and it sits there through a dry afternoon. Badly affected leaves twist, yellow, die, and drop, exposing the fruit underneath to sunburn. (On artichoke, eggplant, pepper, tomatillo, and tomato, one powdery mildew species makes yellow patches with almost no powder, which is why blotched tomato leaves get misread.)
Downy mildew shows light green to yellow angular spots on the upper surface, and angular is the key word: the patches are boxed in by the leaf veins rather than spreading in circles. Flip the leaf and you find fluffy growth underneath, white to purple on peas and cucurbits, with nothing powdery on top. Older leaves go first, and the lesions turn brown and dry. On broccoli and cauliflower it can turn systemic, laying black streaks inside the stems and floret branches, and it hits seedlings hardest: a young plant with badly infected cotyledons and first true leaves may die.
Two practical tells. Check on a foggy morning before the leaves dry, since downy mildew's fuzz vanishes once they do while powdery mildew stays all day. And under a hand lens, powdery mildew spores hang in chains on one stalk; downy mildew spores sit on branched stalks that look like little trees.
Which crops get hit
Downy mildew is the cool-season problem, and this city grows cool-season crops nearly year-round. It takes lettuce, spinach, peas, the cole crops including broccoli and cauliflower, and the cucurbits, where cucumber is most often infected. Cool, moist weather in early spring and late fall favors it, and the coastal translation is that the SF version of that season runs far longer. Peas go in on the foggy side from February through November, per the planting calendar, which hands pea downy mildew a ten-month runway.
Powdery mildew hits a wider list: cucurbits worst of all, so zucchini, summer squash, cucumber, pumpkin, and melon, plus peas and beans, lettuce, artichoke, carrot, beet, radish, turnip, and the tomato family. One of the two cucurbit powdery mildews, Erysiphe cichoracearum, is flagged as common in areas with cool springs and summers, which is the coast in one line.
Now the reassuring part, which should change what you do about half the time: on many of these crops it is not worth treating. Powdery mildew on lettuce rarely becomes serious enough to need treating at all, and on cole crops and the tomato family it seldom does either. Squash and cucumbers are the ones that genuinely lose ground.
Getting rid of mildew
Both diseases are canopy problems, so the first fixes are structural.
Space them out and open up the air. Spacing to reduce canopy density and humidity cuts the spread of downy mildew on cucurbits, so the catalog's 24 to 48 inches for zucchini and 12 to 24 for cucumber are not suggestions, and lettuce wants its 4 to 12 inches rather than a seeded mat. Freely circulating air and letting plants dry between irrigations matter more than anything else you can do: prune out excess foliage, strip the lowest squash leaves so the crown is not sitting in its own damp shade, and think hard about a bed jammed against a solid fence in a wind shadow. That bed stays wet all day.
Spend your sun on the susceptible crops. Shade is one of powdery mildew's favorite conditions and coastal light is thin already, so the sunniest spot you have belongs to the cucumbers and squash rather than the lettuce, which will forgive you. Go easy on nitrogen too, since too much pushes lush foliage that shades itself.
Read the packet for resistance. Downy mildew has resistant broccoli varieties and lettuce cultivars; powdery mildew has resistant cucumbers, melons, squash, pumpkins, peas, and cole crops. Where the pressure never lets up, that line on the packet beats a pretty catalog photo.
Deal with the infected material. Prune out and discard diseased tissue as soon as a powdery mildew infection appears, and destroy infected crop debris and susceptible weeds before planting the next crop. Spinach downy mildew attacks only spinach, so a year off lowers its incidence outright. Infected leaves go in compost, green waste, or the trash; a properly maintained pile kills the spores, but if yours never heats up, use the green bin. Never leave stripped leaves lying on the bed.
How to water each
Here is where people go wrong: the right hose technique for one disease is the wrong one for the other.
For downy mildew, keep leaves as dry as you can. Water at the base, avoid overhead sprinklers, let plants dry between irrigations, and use raised beds so the ground drains.
For powdery mildew, the opposite holds. Its spores cannot germinate in free moisture and may be killed by it, so overhead sprinkling can actually reduce the disease. Timing is the whole trick: wash the spores off midmorning so the moisture dries fast. In the fog belt that rule has teeth, because a leaf wetted at four in the afternoon will not dry before the marine layer returns, and a wet leaf overnight is what downy mildew has been waiting for. Treat the overhead wash as a spot treatment on confirmed powdery mildew, on a clear morning, never as an irrigation plan.
Soggy soil and a wet canopy are different failures, though. Mildew lives on leaves, so a badly draining bed is its own diagnosis and the signs of overwatering plants are worth ruling out separately. The dry direction counts too: oil sprays go badly on a drought-stressed plant, so know the signs of underwatering plants before you spray.
When to spray
Start with what isn't on offer. For downy mildew in a home garden there is no recommended fungicide at all; the whole program is cultural. If your diagnosis is downy mildew, the bottle stays on the shelf.
For powdery mildew, several least-toxic materials are available: horticultural oils, neem oil, jojoba oil, sulfur, and the biological fungicide Serenade. They are not interchangeable. Oils work best as eradicants and can reduce or eliminate a mild to moderate infection you can already see. Sulfur is preventive only and must go on no later than the first sign, so buying it after the leaves are white is buying it too late. Never apply an oil within two weeks of a sulfur spray in either direction, never apply either above 90°F or to a drought-stressed plant, and know that some squash and melon varieties are sensitive to sulfur. Whatever you spray, cover the leaf undersides too.
Living with mildew
A fog-belt food garden is going to have mildew in it. The climate that makes this city so good at greens and peas and brassicas keeps both mildews comfortable twelve months a year, and no spray changes the weather. What changes your season is the habit: turn a few leaves over on a foggy morning, note which surface the growth is on and whether the patches are angular or circular, and let that decide whether you reach for the hose, the pruners, or nothing at all.
Frequently Asked Questions
What is the fastest way to get rid of mildew on plants?
There is no fast answer until you know which mildew you have. For confirmed powdery mildew, a midmorning overhead wash knocks spores off immediately, and a horticultural or neem oil can reduce a mild to moderate infection you can already see. For downy mildew, the fastest real progress is pulling the worst leaves, opening up the spacing, and stopping any overhead watering.
Does baking soda get rid of powdery mildew?
The home-garden list of least-toxic fungicides for powdery mildew runs to horticultural and plant-based oils, sulfur, and the biological fungicide Serenade. Baking soda is not on it. The oils are the ones that work on an infection you can already see, and sulfur only helps if it goes on before or at the very first sign.
Can I eat vegetables from a plant with mildew?
These are plant diseases, and the damage they do is to the plant rather than to you. Powdery mildew fungal growth does not usually grow on the vegetable itself, though pea pods can pick up brownish spots and severely infected plants tend to produce fruit with little flavor. Downy mildew can dark-spot pea pods and, in broccoli and cauliflower, streak the inside of stems and floret branches black, so cut away any discolored tissue.
Why does mildew keep coming back in my San Francisco garden?
Powdery mildew fungi need living plant tissue to survive, and a city that grows lettuce and cole crops year-round never gives them a break. Add nights damp enough for downy mildew to sporulate and summer days that rarely get hot enough to hurt powdery mildew, and there is no seasonal reset. Manage the conditions, and the pressure drops even though it never goes to zero.
Sources: UC IPM on downy mildew in cole crops, UC IPM on downy mildew in lettuce, UC IPM on powdery mildew in vegetables, UC IPM on powdery mildew in peas and beans, and UC IPM on powdery mildew in lettuce.