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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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Garden Checklist for week of July 7

Take care of garden chores early in the morning, concentrating on watering. We’re still in survival mode until this heat wave breaks.

* Keep your vegetable garden watered, mulched and weeded. Water before 8 a.m. to conserve moisture.

* Prevent sunburn; provide temporary shade for tomatoes, peppers, eggplant, melons, squash and other crops with “sensitive” skin.

* Hold off on feeding plants until temperatures cool back down to “normal” range. That means daytime highs in the low to mid 90s.

* Don’t let tomatoes wilt or dry out completely. Give tomatoes a deep watering two to three times a week. Harvest vegetables promptly to encourage plants to produce more.

* Squash especially tends to grow rapidly in hot weather. Keep an eye on zucchini.

* Some weeds thrive in hot weather. Whack them before they go to seed.

* Pinch back chrysanthemums for bushy plants and more flowers in September.

* Harvest tomatoes, squash, peppers and eggplant. Prompt picking will help keep plants producing.

* Remove spent flowers from roses, daylilies and other bloomers as they finish flowering.

* Pinch off blooms from basil so the plant will grow more leaves.

* Cut back lavender after flowering to promote a second bloom.

* One good thing about hot days: Most lawns stop growing when temperatures top 95 degrees. Keep mower blades set on high.

* Once the weather cools down a little, it’s not too late to add a splash of color. Plant petunias, snapdragons, zinnias and marigolds.

* After the heat wave, plant corn, pumpkins, radishes, winter squash and sunflowers. Make sure the seeds stay hydrated.

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