by Linda Carloni, Alameda Backyard Growers
Alameda Sun, Thursday, June 14, 2018
Powdery mildew may well win the race for “most common problem in Alameda food gardens.” Here’s what happens.
It’s June or July, and the vegetable plants in the garden are getting big. They look lush and green and are starting to produce. But then the leaves of a zucchini plant start to get funny, white, powdery spots on them. The spots are blotchy and won’t brush off. At first, the white stuff doesn’t seem to be bothering the veggies. But within a few days the white blotches have spread to another summer squash and the cucumber vine next to it. The leaves have started to distort and maybe a few little vegetables also have brown spots. Eventually, the powdery mildew will start to kill the leaves and can reduce vegetable production significantly.
Powdery mildew is composed of a group of fungi that attack many vegetables. In Alameda, zucchini, other summer squash, cucumbers and peas seem to be most commonly affected, but it can also attack tomatoes (yellow blotchy spots), lettuces, beans and other plants. Different fungal species affect different vegetables. Powdery mildew is particularly widespread in Alameda because it thrives in moderate temperatures (60 to 80 degrees F) and the shady conditions so prevalent in our small backyards. Powdery mildew does not require moisture to germinate and can survive well in our dry summers.
The good news is that powdery mildew generally doesn’t attack until after the plant has begun producing, so frequently the plant will continue to produce food even if it is infected.
The bad news is that powdery mildew is difficult to manage once it starts. If mildew is found early, the problem can be mitigated by cutting off the affected leaves or washing off the blotches. For best results, get as much of the mildew off as possible. Using a least-toxic oil, like a plant-based oil, or a biological product like Serenade at the beginning of an outbreak may also help.
Trying to avoid the problem in the first place is most effective. If powdery mildew has appeared in the garden previously, try to plant vegetables that are not as susceptible or look for mildew-resistant varieties. Plant the most susceptible vegetables in the sunniest place possible. Make sure lots of air circulates around the plants, and don’t over-fertilize. Check the plants often to spot any mildew problems early and take action to mitigate it.
Learn even more about powdery mildew at http://ipm.ucanr.edu/PMG/PESTNOTES/pn7406.html.