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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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GENERAL GARDENING

An article about gardening.

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 April 21

This week there’s plenty to keep gardeners busy. With no rain in the immediate forecast, remember to irrigate any new transplants.

* Weed, weed, weed! Get them before they flower and go to seed.

* April is the last chance to plant citrus trees such as dwarf orange, lemon and kumquat. These trees also look good in landscaping and provide fresh fruit in winter.

* Smell orange blossoms? Feed citrus trees with a low dose of balanced fertilizer (such as 10-10-10) during bloom to help set fruit. Keep an eye out for ants.

* Apply slow-release fertilizer to the lawn.

* Thoroughly clean debris from the bottom of outdoor ponds or fountains.

* Spring brings a flush of rapid growth, and that means your garden is really hungry. Feed shrubs and trees with a slow-release fertilizer. Or mulch with a 1-inch layer of compost.

* Azaleas and camellias looking a little yellow? If leaves are turning yellow between the veins, give them a boost with chelated iron.

* Trim dead flowers but not leaves from spring-flowering bulbs such as daffodils and tulips. Those leaves gather energy to create next year's flowers. Also, give the bulbs a fertilizer boost after bloom.

* Pinch chrysanthemums back to 12 inches for fall flowers. Cut old stems to the ground.

* Mulch around plants to conserve moisture and control weeds.

* From seed, plant beans, beets, cantaloupes, carrots, corn, cucumbers, melons, radishes and squash.

* Plant onion sets.

* In the flower garden, plant seeds for asters, cosmos, celosia, marigolds, salvia, sunflowers and zinnias.

* Transplant petunias, zinnias, geraniums and other summer bloomers.

* Plant perennials and dahlia tubers for summer bloom.

* Mid to late April is about the last chance to plant summer bulbs, such as gladiolus and tuberous begonias.

* Transplant lettuce seedlings. Choose varieties that mature quickly such as loose leaf.

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