Spring rains bring more roses, more rose problems

By Joan Morris 5/28/10

The unusual and persistent spring rains that have lingered through May have made shedding winter’s cloak a more onerous task. But there are some advantages to all that free water from the heavens: The roses are spectacular this year.

But like the roses, the rain also has come with thorns. While the unseasonal precipitation has jump-started our rose gardens, it also has brought an increased risk of disease.

“There have been an abundance of blooms on roses and other flowering shrubs,” says Jolene Adams, vice president of the American Rose Society and a nationally recognized judge and expert on roses, “but there’s probably not a garden around that isn’t having some trouble with those dread diseases.”

While rose lovers always do battle against the opportunistic fungi that attack the buds and wither leaves, this spring has produced more than its fair share of problems with blackspot, powdery mildew and rust.

Adams, who has 165 roses growing on an 80-by-40-foot lot in Hayward, says gardeners have options when it comes to treating rose diseases. “Depending on the gardener’s philosophy, you can treat it or wait it out. But either way, you need to practice good sanitation.”

That means removing diseased leaves from under and around the roses and disposing of them promptly in your municipal green waste bins. Don’t add to a compost pile, Adams says, because you could spread diseases that way.

In California, none of the fungal diseases is deadly. These cool, moist spring days will soon give way to heat and the dry summer. But the fungus can turn America’s favorite flower into an eyesore — something that sends most rosarians into despair — and some can weaken the bush.

Blackspot covers the leaves and stems with unsightly dark blemishes, while powdery mildew dusts stems, leaves and buds in a velvety white powder. Rust also discolors leaves. Blackspot and rust cause leaves to drop, which weakens a plant’s ability to perform photosynthesis and feed itself.

One way to avoid the diseases could be to start with roses that are proven to be disease-resistant. The San Jose Municipal Rose Garden is one of 15 in the United States where new varieties of roses are tested before being marketed. And this year, the garden is filled with blooms and fragrance.

“Water is one of the best fertilizers,” says Terry Reilly, co-founder of Friends of the San Jose Rose Garden. “We’ve got some roses now that are really popping.”

The garden is filled with All-America award-winning roses, Reilly says, judged high not only for their beauty, color, scent and aging qualities, but also on their resistance to fungus and other common maladies.

“We are not seeing any particular damage,” Reilly says, “and I think that’s because we’ve got proven winners.”

The garden, planted in 1937 and now with about 2,000 rose bushes, is in the running for America’s Best Rose Garden. The top five online vote-getters will be visited by judges who will make the final designation. (Go to www.rose.org to cast a ballot.)

If you’re thinking about adding roses to your garden, you still have plenty of time to plant. Although the bare-root season is done, nurseries carry roses in five-gallon pots that are already budded out and ready to start blooming.

With the weather forecast still unsettled, there is one thing we can be certain of. For those who want to stop and smell the roses, they won’t have to look hard to find them.
Just remember an umbrella.

Remove diseased leaves from under and around the roses and dispose of them in your municipal green waste bins. You don”t want to put the clippings and debris in your own compost pile, rose expert Jolene Adams says, because that will spread the fungus through the compost.

Rust and blackspot can be treated with fungicidal sprays, although Adams suggests reading labels carefully and finding the least toxic option available. Neither can be eradicated, only controlled.

Powdery mildew can be easily washed off. Adams suggests hosing off the roses in the mornings, before 11 a.m., so that the sun and wind will dry the leaves before nighttime temperatures drop. Adams also recommends using a dry mulch. Horticultural oils with sulfur or jojoba oil also can be effective in treating powdery mildew.

When pruning roses this winter, Adams recommends drenching the bare canes with a dormant spray. Repeat the application in March and April.