Ask a Master Gardener: Is there such thing as 'sudden rose death?'

If you’re a gardener in Santa Clara Valley and the Peninsula, you know how lucky we are to live here. Whether you garden in your backyard, in raised beds or in pots, you know our soil is incredibly fertile and our Mediterranean climate lets us grow year-round. As gardeners, we often look for tips on how to garden more successfully. That’s where the UC Master Gardeners of Santa Clara County come in. Our volunteers are part of the University of California’s Cooperative Extension program and we are trained to use research-based information to educate our fellow residents on sustainable gardening and landscaping practices specifically for our area.

Through our monthly "Ask a Master Gardener" column, we will answer your questions and offer tips. Have a question? Email [email protected] or leave a comment at the end of this month's column.

Is there such a thing as 'sudden rose death?'

In the past three years I’ve lost five rose bushes. They were all in full bloom but in less than a week, the leaves turned brown, the blossoms dried up, and they expired. Neighboring plants were unaffected. What could have happened? -- R.C., Menlo Park

Answer: We examined the photos you sent, and we suspect the main problem is Botrytis blight (or gray mold), which is caused by the fungus Botrytis cinerea. We noticed twig die-back and bud rot (buds that did not open). There are larger pruned canes ("trunks" that grow up from the base of the plant) that are dead and some of the live canes have target-like blotches, which also are symptoms of Botrytis.

Since the roses are all in the same location, we suspect they were all infected by the fungus. Botrytis is such a prevalent fungus, prevention is the best approach. Plant roses that are not susceptible to Botrytis blight. Reduce the humidity around plants by providing good air circulation, modifying irrigation and reducing ground cover. Deadhead any infected flowers immediately and dispose of fallen leaves and petals; prune out infected canes, buds and flowers.

Also, the leaves have brown or black purplish spots suggesting black spot or Diplocarpon rosae. The leaves turn yellow around the spots and fall off. This fungus grows when leaves remain wet for more than seven hours, which could happen with overhead watering, spraying to wash off aphids or watering nearby plants. If you wet leaves, make sure to do so early in the day so leaves have a chance to dry before evening. A drip line or soaker hose is the best way to water. Remove all debris from the base of the rose but don’t compost it. Affected leaves can be pruned out, but do this during the dormant season (late fall to winter).

Here's a UC website that has a lot of on rose diseases. Good luck!

What is powdery mildew, and how do I treat it?

Powdery mildew is one of the most frequent topics in our plant clinics this time of year. It looks like talcum powder has been sprinkled on both sides of leaves, and in full-blown cases, the leaves may turn completely white or gray. It affects both vegetables and perennial flowering plants.

There are many different species of powdery mildew, and the good news is that each species only attacks specific plants. That means while powdery mildew on your zucchini plant may infect others also in the cucurbit family (like cucumbers and melons), it will not infect your roses nearby. More good news -- it rarely grows on vegetable fruits, just primarily on the leaves -- so you can still eat the vegetables.

One thing to mention: Several popular varieties of zucchini have natural spots of white pigment, which some people assume is the beginnings of powdery mildew or another disease. If the white spots are uniformly positioned in or between the leaf veins, it’s probably a natural physical trait of that variety.

How to treat powdery mildew

• During the early stages of infection, wash it off with sprays of water to reduce spreading. Spray in the morning to prevent other fungal diseases from taking hold.

• Selectively prune overcrowded plants to increase air circulation.

• For mild to heavy infections, try a horticultural oil spray such as neem oil. You may need to repeat application every seven to 14 days. (Don’t apply oils when temperatures are above 90 degrees).

• Remove infected plant parts -- but assume that since powdery mildew spreads with wind, you can expect reinfection.

• Avoid heavy nitrogen fertilizer late in the summer because succulent tissue is more susceptible to infection.

• Next year, choose a powdery mildew-resistant hybrid variety!

Do you have gardening tips for August?

• Prune your apricot and cherry trees after harvesting fruit and before the end of August. By pruning at least six weeks before the first rainfall, you reduce the chance of a fungal disease called Eutypa dieback. Fungal spores enter fresh wounds through splashing water or rain, so you want your pruned trees to be healed by fall.

• Dry fresh herbs. Now is the time to dry and preserve your garden’s herbs. Many herbs can be tied into small bundles and hung upside down in a dark, warm, and dry place with good air circulation. After removing stems, mint, sage and bay leaves can also be scattered on a tray to dry. Here are drying tips.

• Protect against sunscald. Summer heat can sunburn fruits and vegetables, especially tomatoes, peppers and persimmons. When extreme temperatures are forecast, place shade cloth or row cover material over them -- and make sure the plants are well-watered.

Want to speak with a master gardener about a plant problem? UC Master Gardeners of Santa Clara County offers Plant Clinic Online, a monthly clinic held via Zoom that puts residents in touch with experts who can help diagnose their plant problems. Upcoming clinics are scheduled for 7 p.m. on Aug. 8, Sept. 12 and Oct. 10. You can also submit questions to our Help Desk.

Find more garden events and classes here.