Ask a Master Gardener: What's wrong with my roses?

COURTESY TO THE BELLINGHAM HERALDSeptember 2, 2013 

Question: I love roses, but I'm ready to give up on several of mine that have black spots on their leaves. What could it be, and can I do anything about it?

Answer: When Shakespeare's McBeth uttered the words, "Out, out, damn spot!" he didn't know he'd be joined in his sentiment by thousands of gardeners with black spot on their roses.

What you have is simply called black spot. It is a fungus that can begin in the spring, but is most active in warm summer weather when there is humidity, rainfall or overhead watering that spreads it. Black spot can severely weaken a rose plant. A good remedy is to spray with a combination of two teaspoons of baking soda and two teaspoons fine-grade horticultural oil dissolved in a gallon of water.

When you prune your roses, but sure to clean up the soil of all leaves and debris, and discard them. Don't compost them! Then, before the new growth occurs, spray both the plants and soil with a dormant-season horticultural oil or lime sulfur. You will destroy many disease organisms and insect eggs that could be a problem come spring.

And remember, all you hearty and "I don't give up on my roses no matter what" gardeners: Be sure to stop and smell then on occasion! You deserve a reward for your persistence.

Q: A friend told me to deadhead my flower plants. Sounds drastic, but I didn't want to appear totally ignorant. What does it mean, and why should I do it?

A: All plants have a critical job to complete in their lifetime: They are destined to procreate, sometimes in small numbers, sometimes in numbers hard to believe. A plant produces flowers not to please humans, but to attract pollinators to its flowers, thus ensuring a supply of seed that will allow the plant to produce new plants, thus fulfilling its destiny.

When a flower begins to die, the seeds it contains start to ready themselves for their ultimate job, and energy that comes to every flower is diverted from flowering to producing seeds. So you see the picture. Cut off the dying flowers, stop the seed production, and prolong the flowering of the plant.

I find deadheading to be mindless and relaxing. Beyond that, I like to keep my plants blooming as long as I can. You can, too. Just deadhead.

Q: What can I do about powdery mildew on my squash and potato plants? Will it do serious damage to the fruit?

A: Step outside. Summer days are warm, evenings are cool, and we have a fair amount of humidity going on. Perfect conditions for the fungus among us.

The good news is that powdery mildew is easy to recognize - a white or gray coating on the surface of the leaves. The seriously bad news is that it can attack virtually every plant growing in your garden: vegetables, fruit trees, grasses, flowers and shrubs. It is particularly fond of roses, grapes, apples, potatoes, peas and zinnias.

Complicating this problem is that a whole host of nasty fungi cause powdery mildew on different plants. It not only is unattractive, it can cause serious distortion of the leaves or growing tip. It generally weakens your plants.

Good air circulation is essential. Thin out crowded conditions and you'll lessen the chance of powdery mildew, or any of its cousins. If you've had a problem this year, next year spray a fungicide before the disease shows its ugly head. To be effective, not even one leaf can yet be affected by the mildew when you spray. And rake up and dispose of leaves and debris on the ground.

It's a knotty problem in our little piece of heaven, these mildews. I have no doubt they'll outlive humans. But in the meantime, try to outsmart them with your vigilance.

ABOUT THIS COLUMN

Ask a Master Gardener will appear in The Bellingham Herald weekly through the summer growing season. If you have a gardening question you'd like answered in the column, please email it to newsroom@bellinghamherald.com.

Master Gardener Kathleen Bander is a resident of Bellingham and life-long gardener. For more information on Whatcom County Master Gardeners, go to whatcom.wsu.edu/ch/mg

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