Powdery Mildew in autumn strawberries

We’ve had high powdery mildew pressure in our strawberry variety trial now for almost 6 weeks, and we’ve been seeing spores on the fruit for about a month now.

Powdery Mildew is unusual in the fungus world in that it does NOT require leaf wetness to infect.  While it thrives in high humidity (75-90%), actual rain (or overhead irrigation) reduces infection.  It also likes warm temperatures, especially 18-27°C.

This propensity to thrive in a warm, humid environment means that powdery mildew pressure is highest in covered crops.

Here’s the disease risk model, presented through Tau Research’s dashboard, that is running in our strawberry tunnel:

We notice in our trial that Monterey, popular variety though it is, is more susceptible to powdery mildew than any of the other day neutral varieties in the trial (we’re growing only University of California varieties this year).  We’re seeing powdery mildew on flowers and fruit, but none on the leaves.

Side note: powdery mildew species are quite specific to their respective host plants.  While lots of other plants get powdery mildews (grapes, apples, broccoli, etc), these mildew species do not infect strawberry.  Strawberry shares a mildew with cinquefoil, ladies’ mantle, eucalyptus and rubus (raspberries, blackberries).

Powdery Mildew Control

In reading about fungicide trials in California, even the best fungicides were only reducing powdery mildew infections to 35% of the untreated control, not totally eliminating them.  This highlights the fundamental importance of reducing humidity and temperature to manage powdery mildew.  Fungicides alone will not cut it.  You must reduce humidity and temperature by venting more.

None of the single-site fungicides can be used off-label during the fruiting season, but that’s no drama really because the California trial that evaluated the PM risk model also evaluated a conventional and an organic program, and they were about the same in terms of efficacy.

Perusing the SGNZ fungicide list, in-season fungicide options are carbenazim (note toxicity to mite predators), Esteem, potassium bicarbonate, and sulfur.  It’s a shame that sulfur vapour is hard on tunnel plastic, because my experience in using “sulfur pots” (the heated pots puff out vaporized sulfur at night) in ornamental production under glass was that we only ever saw powdery mildew if the sulfur pot was broken.  Thankfully, Esteem (polyoxin D zinc) has good efficacy and a 1 day WHP.

See the SGNZ fact sheet for more info about powdery mildew.