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Rose trouble: Attack of the gray mold

Botrytis turns promising buds into mushy mess

Rose with botrytis
This California Dreamin' rose shows effects of botrytis. The fungus attacks blooms or buds moistened by dew, fog or sprinklers. (Photos: Debbie Arrington)

Our warm autumn weather prompted many roses to push out fresh blooms in November. But instead of opening into full healthy flowers, the buds quickly browned and seemed to disintegrate on the bush.

That’s botrytis in action. Right now, it's by far the most common problem in Sacramento-area rose gardens.

Nicknamed gray mold, botrytis is a common fungus that attacks a broad range of ornamental and edible plants – especially roses. It causes bunch rot in grapes, and can take hold with as little as four hours of moisture.

Spray of roses with botrytis
A spray of Enchanted Evening roses is turned into
brown mush by botrytis.

So, even though weather has been mostly dry, botrytis opportunistically attacks blooms moistened by morning dew, fog or sprinklers.

This is the second wave of botrytis in Sacramento this year. We saw a similar (and unusually early) outbreak in late April and May when spring rains and cool weather coaxed the fungi into action. Botrytis overwinters in mulch around the bush, waiting for just the right combination of conditions: Cool, cloudy days and damp flower petals. In Sacramento, that weather is much more common in November than May.

Moisture is key. Botrytis needs moisture for growth in plant tissues, particularly tender flower petals. To prevent outbreaks, keep flower buds dry. Of course when it starts raining or nights turn foggy, that’s all but impossible.

Botrytis eventually will overwhelm the flower and turn it into soft mush. The earliest stages look like pink measles or brownish water spots on light-colored flowers. Those brown spots quickly grow until they consume the whole petal. The fungus grows so fast that the flower never fully opens.

Besides roses, botrytis 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.

Botrytis bloom
Here's another rose bloom (a Pink Promise hybrid tea rose)
completely ruined by botrytis.

According to UC Integrated Pest Management program, the best control of botrytis 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.

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


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Dig In: Garden Checklist

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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