Gray mold attacks autumn roses

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

Foggy conditions prompted recent outbreak

By 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