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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. 27

Before the rain comes later in the week, take advantage of sunny, calm days:

* This may be your last chance this season for the first application of copper fungicide spray to peach and nectarine trees. Leaf curl, which shows up in the spring, is caused by a fungus that winters as spores on the limbs and around the tree in fallen leaves. Sprays are most effective now, but they need a few days of dry weather after application to really “stick.” If you haven’t yet, spray now.

* Rake and compost leaves, but dispose of any diseased plant material. For example, if peach and nectarine trees showed signs of leaf curl this year, clean up under trees and dispose of those leaves instead of composting.

* Make sure storm drains are clear of any debris.

* Give your azaleas, gardenias and camellias a boost with chelated iron.

* Trim chrysanthemums to 6 to 8 inches above the ground after they’re done blooming. Keep potted mums in their containers until next spring. Then, they can be planted in the ground, if desired, or repotted.

* Prune non-flowering trees and shrubs while dormant.

* Plant bulbs for spring bloom. Don’t forget the tulips chilling in the refrigerator. Other suggestions: daffodils, crocuses, hyacinths, anemones and scillas.

* Seed wildflowers including California poppies.

* Also from seed, plant sweet pea, sweet alyssum, bachelor buttons and other spring flowers.

* Plant most trees and shrubs. This gives them plenty of time for root development before spring growth. They also benefit from winter rains.

* Set out cool-weather annuals such as pansies and snapdragons.

* Lettuce, cabbage, broccoli and cool-season greens can be planted now.

* Plant garlic and onions.

* If you decide to use a living Christmas tree this year, keep it outside in a sunny location until Christmas week. This reduces stress on the young tree.

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