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Attack of the gray mold spoils fall roses

How to prevent the spread of this yucky fungal disease

Gray mold ruined this Gemini hybrid tea rose bloom. The fungus got just enough rain earlier this month to deface fall roses.

Gray mold ruined this Gemini hybrid tea rose bloom. The fungus got just enough rain earlier this month to deface fall roses.

Debbie Arrington

My visions of Thanksgiving bouquets are quickly turning to mush. Gray mold is attacking my roses.

Gray mold is the descriptive nickname of the fungal disease botrytis. It’s common in November rose gardens, and this season’s outbreak came early.

Damp conditions in early November gave gray mold a big boost. Gray mold – which actually looks more tan or brown on the rosebud – needs moisture for growth in plant tissues, particularly tender flower petals. And this month, the fungus got just enough rain to explode among my pretty fall roses.

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 and eventually the whole flower. The bud never fully opens.

Gray mold also attacks many other favorite flowers including African violet, aster, begonia, carnation, chrysanthemum, cyclamen, cymbidium, gerbera, geranium, gladiolus, hydrangea, marigold, orchid, petunia, poinsettia, primrose, ranunculus, snapdragon and zinnia.

Two rose hips
Rather than clipping off the whole bud, pull off
the infected petals and leave the rose hips.

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.) Dispose of that old mulch (again in the trash, not compost) and replace with fresh mulch.

Generally, I snip off buds infected with gray mold before they have a chance to drop. (That’s my game plan for spring botrytis outbreaks.)

But that strategy is problematic in mid-November. The bush needs its hips – the fruit located at the base of the blooms – to mature; that’s the plant’s signal to go into dormancy and shut down for the winter. If the spent flowers (and forming hips) are removed, the bush keeps on pushing out new growth. (That makes rose pruning in December and January a bigger pain.)

Master rosarian Dave Coop shared this tip on how to control botrytis while also allowing the hips to ripen. Instead of snipping off the spent bloom, gently pull off its petals. (Make sure to wear gloves when working with roses.) Discard the brown infected petals. The clean hip can then ripen, turning bright orange or red. And the bush can start shutting down for winter.

This method also helps control future fungal outbreaks. Instead of the botrytis-packed petals laying in wait under rose bushes, the gray mold is bagged up and removed from the garden; that will cut down on infections next spring – and later in the year, too. (Again, that’s “good sanitation.”)

For more information on gray mold, check out these pest notes from UC IPM:


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For week of Nov. 26:

Concentrate on helping your garden stay comfortable during these frosty nights – and clean up all those leaves!

* Irrigate frost-tender plants such as citrus in the late afternoon. That extra soil moisture increases temperatures around the plant a few degrees, just enough to prevent frost damage. The exception are succulents; too much water before frost can cause them to freeze.

* Cover sensitive plants before the sun goes down. Use cloth sheets or frost cloths, not plastic sheeting, to hold in warmth. Make sure to remove covers in the morning.

* Use fall leaves as mulch around shrubs and vegetables. Mulch acts as a blanket and keeps roots warmer.

* Stop dead-heading; let rose hips form on bushes to prompt dormancy.

* Prune non-flowering trees and shrubs.

* Clean and sharpen garden tools before storing for the winter.

* Brighten the holidays with winter bloomers such as poinsettias, amaryllis, calendulas, Iceland poppies, pansies and primroses.

* Keep poinsettias in a sunny, warm location – and definitely indoors overnight. Water thoroughly. After the holidays, feed your plants monthly so they’ll bloom again next December.

* Rake and remove dead leaves and stems from dormant perennials.

* Plant spring bulbs. Don’t forget the tulips chilling in the refrigerator. Daffodils can be planted without pre-chilling.

* This is also a good time to seed wildflowers and plant such spring bloomers as sweet peas, sweet alyssum and bachelor buttons.

* Plant trees and shrubs. They’ll benefit from fall and winter rains while establishing their roots.

* Set out cool-weather annuals such as pansies and snapdragons.

* Lettuce, cabbage and broccoli also can be planted now.

* Plant garlic and onions.

* Bare-root season begins now. Plant bare-root berries, kiwifruit, grapes, artichokes, horseradish and rhubarb.

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