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Gray mold attacks autumn roses

Foggy conditions prompted recent outbreak

Rose bloom with gray mold
Gray mold, also called botrytis, ruins rose blooms. (Photo: Debbie Arrington)

Gray mold has me feeling blue. I love autumn roses. Hundreds of rosebuds in my garden had me envisioning fresh bouquets this holiday season.

But most of these November roses will never open. Instead, they turn to mush on the stem.

The reason? Gray mold. That’s the descriptive nickname of the fungal disease botrytis. It’s common in November rose gardens, and this season’s outbreak looks particularly yucky.

Rain storms in early November kicked it into full gear. Recent foggy conditions nurtured its rapid advancement.

Gray mold – which actually looks more tan or brown on the rosebud – needs moisture for growth in plant tissues, particularly tender flower petals. To prevent outbreaks, keep flower buds dry. That’s all but impossible on damp, foggy days.

Gray mold starts out looking like pink measles or brownish water spots on light-colored flowers. Those brown spots rapidly grow until the fungus consumes the whole petal. The flower never fully opens.

Gray mold also attacks African violet, asters, begonia, carnation, chrysanthemum, cyclamen, cymbidium, gerbera, geranium, gladiolus, hydrangea, marigolds, orchids, petunia, poinsettia, primrose, ranunculus, snapdragon, zinnia and many other garden favorites.

According to UC Integrated Pest Management program, the best control of gray mold is “good sanitation.” Clip off infected blooms, put them in a plastic bag and dispose in the trash. Do not compost them; that just recycles the spores back into the garden.

Pick up fallen blooms and petals around the bush and dispose of them, too. After pruning when roses are dormant, rake out old mulch and fallen foliage. (This contains other fungal spores, too, for powdery mildew, rust, black spot and other rose diseases.) Then dispose of that old mulch and replace with fresh mulch.

For more information on gray mold, check out these pest notes from UC IPM: http://ipm.ucanr.edu/PMG/r280100511.html


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Dig In: Garden checklist for week of Jan. 29

Bundle up and get work done!

* Prune, prune, prune. Now is the time to cut back most deciduous trees and shrubs. The exceptions are spring-flowering shrubs such as lilacs.

* Now is the time to prune fruit trees, except apricot and cherry trees. Clean up leaves and debris around the trees to prevent the spread of disease.

* Prune roses, even if they’re still trying to bloom or sprouting new growth. Strip off any remaining leaves, so the bush will be able to put out new growth in early spring.

* Prune Christmas camellias (Camellia sasanqua), the early-flowering varieties, after their bloom. They don’t need much, but selective pruning can promote bushiness, upright growth and more bloom next winter. Feed with an acid-type fertilizer. But don’t feed your Japonica camellias until after they finish blooming next month. Feeding while camellias are in bloom may cause them to drop unopened buds.

* Clean up leaves and debris around your newly pruned roses and shrubs. Put down fresh mulch or bark to keep roots cozy.

* Apply horticultural oil to fruit trees to control scale, mites and aphids. Oils need 24 hours of dry weather after application to be effective.

* This is also the time to spray a copper-based oil to peach and nectarine trees to fight leaf curl. Avoid spraying on windy days.

* Divide daylilies, Shasta daisies and other perennials.

* Cut back and divide chrysanthemums.

* Plant bare-root roses, trees and shrubs.

* Transplant pansies, violas, calendulas, English daisies, snapdragons and fairy primroses.

* In the vegetable garden, plant fava beans, head lettuce, mustard, onion sets, radicchio and radishes.

* Plant bare-root asparagus and root divisions of rhubarb.

* In the bulb department, plant callas, anemones, ranunculus and gladiolus for bloom from late spring into summer.

* Plant blooming azaleas, camellias and rhododendrons. If you’re shopping for these beautiful landscape plants, you can now find them in full flower at local nurseries.

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