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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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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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