I was giving a talk about diagnosing problems on tomato plants when someone asked me what they could do about powdery mildew. I was surprised at this question as powdery mildew usually shows up in August when rain and cooler weather appear. I did not expect to hear about it in May. I had already heard of a case of early blight on tomatoes in Vancouver. There must be a connection. For someone who grows tomato plants, I dread the thought that my crop could be affected. I have also planted cucumbers and pumpkins in the garden. Will they get mildew as well? As of last night, my plants look healthy but I will continue to watch them carefully.
So what does powdery mildew look like? Above is an example of the white fuzzy growth happening on the leaves of pumpkins. This is an old photo taken in late fall.
Here is another photo of powdery mildew starting on the leaves of a zucchini plant in my garden two years ago. Not to be mistaken for the white blotches on some zucchini leaves which is normal, powdery mildew will cover the whole leaf and you can see above how it looks different. In the fall I don’t worry about powdery mildew as the plant still has lots of green leaves needed for photosynthesis. To control powdery mildew, remove affected leaves as you see them. This will also let more light into the plant and provide better air circulation. So how did powdery mildew happen? Powdery mildew appears when conditions are right. It’s usually when we have warm days and cool nights and cloudy days. Yes, I just described the weather for this month, didn’t I? So what can you do now? Try washing the leaves off with water from the hose but make sure your plants go dry into the night. If your plant is totally covered with powdery mildew, remove it and replant. That’s the good thing about spotting this issue now. You still have time to replant. You can plant squash seeds in June and still have a harvest. I think perhaps we may have rushed the season and planted too early. Perhaps by planting later, we could have avoided the powdery mildew. Powdery mildew can also happen when we go through a dry period. Think about it, the last two days are the first decent rainfall we have had for a while. Be sure your plants are planted in full sun. Shady gardens with poor air circulation suffer most from powdery mildew.
Another pest in the garden this month is the cabbage moth. You will probably recognize the cabbage moth as she has two white wings with black dots on them and can be seen flying around gardens from late April to September. She is busy looking for places to lay her eggs. She loves your Brassica crops the best. Crops such as broccoli, cabbage, kale, brussel sprouts, cauliflower, turnip and kohlrabi are loved by the cabbage moth. Once she has laid her eggs, they hatch into larva which eat away at the leaves of your Brassica crops. Above is a photo of a cabbage moth larva. Its busy eating leaves and getting bigger each day.
So how do you control cabbage moth? Hand pick caterpillars daily from vegetables if they are not too numerous. Floating row covers shown above work well when the plants are small. The cover protects plants from egg laying moths early in the season. I find the covers effective when plants are small and susceptible to being eaten to the ground. Once the plants are larger they can tolerate some damage.
Have you seen problems on your tomato plants? If the weather is conducive to powdery mildew, we could also see early blight on tomatoes. Normally the lower mainland doesn’t see early blight. So far the master gardeners have identified one case of early blight in Vancouver. Watch for black lesions on the stems of your tomato plants. This is a killer for tomatoes and if they get early blight they will have to be removed. I spotted these brown spots on my San Marzano tomato plant last week. I quickly removed them. A week later the plant is looking green and healthy so I am not sure if blight has hit my garden or not. Praying it doesn’t happen. Just in case I will be growing a few tomato plants in the greenhouse to be guaranteed a crop.