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Rose trouble: Attack of the gray mold

Botrytis turns promising buds into mushy mess

Sacramento Digs Gardening
BY Debbie Arrington PUBLISHED NOV 30, 2020

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.

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.

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: http://ipm.ucanr.edu/PMG/r280100511.html

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Keywords:

Sacramento roses cool weather fog wet conditions overwintering mold gardening fungus botrytis garden pests perennials flowers

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Garden Checklist for week of May 19

Temperatures will be a bit higher than normal in the afternoons this week. Take care of chores early in the day – then enjoy the afternoon. It’s time to smell the roses.

* Plant, plant, plant! It’s prime planting season in the Sacramento area. If you haven’t already, it’s time to set out those tomato transplants along with peppers and eggplants. Pinch off any flowers on new transplants to make them concentrate on establishing roots instead of setting premature fruit.

* Direct-seed melons, cucumbers, summer squash, corn, radishes, pumpkins and annual herbs such as basil.

* Harvest cabbage, lettuce, peas and green onions.

* In the flower garden, direct-seed sunflowers, cosmos, salvia, zinnias, marigolds, celosia and asters.

* Plant dahlia tubers. Other perennials to set out include verbena, coreopsis, coneflower and astilbe.

* Transplant petunias, marigolds and perennial flowers such as astilbe, columbine, coneflowers, coreopsis, dahlias, rudbeckia and verbena.

* Keep an eye out for slugs, snails, earwigs and aphids that want to dine on tender new growth.

* Feed summer bloomers with a balanced fertilizer.

* For continued bloom, cut off spent flowers on roses as well as other flowering plants.

* Don’t forget to water. Seedlings need moisture. Deep watering will help build strong roots and healthy plants.

* Add mulch to the garden to help keep that precious water from evaporating. Mulch also cuts down on weeds. But don’t let it mound around the stems or trunks of trees or shrubs. Leave about a 6-inch to 1-foot circle to avoid crown rot or other problems.

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