Gray mold gives rose gardens winter blues

Here I was, with a garden full of December rose buds prompting visions of gift bouquets. Then came the rain – and most of those beautiful buds turned to mush.

It was an attack of gray mold, and another reminder that it’s time to prune.

Gray mold is the descriptive nickname of the fungal disease botrytis. It’s common in late fall and early winter rose gardens, and this season’s outbreak looks particularly yucky.

Recent rain storms 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 the 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 of them 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 and blooms. (This contains other fungal spores, too, for powdery mildew, rust, black spot and other rose diseases.) Dispose of that old mulch and replace it with fresh mulch.

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