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


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Garden checklist for week of July 13

Put off big chores and planting until later in the week when the weather is cooler. In the meantime, remember to stay hydrated – advice for both you and your garden.

* Keep your vegetable garden watered, mulched and weeded. Water before 8 a.m. to reduce the chance of fungal infection and to conserve moisture.

* Water, then fertilize vegetables and blooming annuals, perennials and shrubs to give them a boost. Feeding flowering plants every other week will extend their bloom.

* Give vegetable plants bone meal or other fertilizers high in phosphate to stimulate more blooms and fruiting.

* Add some summer color. Plant petunias, snapdragons, zinnias and marigolds.

* From seed, plant corn, pumpkins, radishes, winter squash and sunflowers. Plant Halloween pumpkins now.

* Pinch back chrysanthemums for bushy plants and more flowers in September.

* 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.

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