After returning from a lovely 10-day vacation in the greener Pacific Northwest, I went to check on the usually prolific Avant Garden share plot and was dismayed and disappointed to see our once-healthy, sturdy zucchini and squash plants covered with the evil powdery mildew! Leaves were paper thin and crumbling, the fruit was stunted, and the vines were white and cracked. It was almost a total loss.
I was hoping to avoid the funky fungus this year (the garden was affected last year, so I know the spores are lurking in the soil) by keeping a close watch on the entire garden including random checks on all 43 raised beds, removing any leaves with signs of the fungus, and spraying the leaves with water and Neem oil. My method seemed to work…until it didn't. Oh well, we'd had a great harvest up until early August, delivering hundreds of pounds of zucchini, yellow squash, and cucumbers to local food assistance organizations. Luckily, powdery mildew (PM) doesn't infect tomatoes, eggplant, peppers, kale, or Swiss chard, our other share plot goodies. However, all cucurbits can succumb to powdery mildew if exposed so we are keeping a keen eye on monitoring the remaining cucumbers and winter squash.
A bit about this pesky pathogen. Powdery mildew is the name bestowed upon a disease caused by two closely related fungi (Sphaerotheca fuliginea and Erysiphe cichoracearum). Yellow spots first appear on stems, petioles, and leaves, enlarging as the mycelium grows and produces spores, thus the powdery appearance. If not removed or mediated, the plant will take on the desperate look described above. Unfortunately, this can happen any time of the year, but crops are especially at risk when the humidity is 50 to 90%. S. fuliginea prefers warmer weather while E. cichoracearum flourishes in cooler weather.
How can we prevent powdery mildew (PM)?
- Find and plant crops resistant to it.
- Control weeds (the pathogen will overwinter in the weeds).
- Space plants appropriately for adequate air circulation.
- Prune overcrowded areas to increase air circulation.
- Soil should have balanced nitrogen amendments.
- Survey your crops! At the first sign of PM, remove the offending plant part (usually the leaves) by throwing them in the green waste bin, not on the ground or in the compost bin, and spraying the leaves with water early in the day. Remember PM is spread by the wind and pests and will remain in your compost.
- Apply organic fungicides such as sulfur, Neem oil, or potassium bicarbonate. Sodium bicarbonate (baking soda) is effective also.
Good luck…I'll keep you posted on our gardening mishaps and adventures in Avant Garden in Benicia!