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Fog and cold lead to attack of gray mold

Botrytis outbreak on roses, poinsettias follows chilly, damp weather

This Marilyn Monroe rose is showing early signs of botrytis: The red spots are the symptoms.

This Marilyn Monroe rose is showing early signs of botrytis: The red spots are the symptoms.

Debbie Arrington

Sacramento experienced record cold weather this past weekend and thick damp fog that seemed to just hang around.

How will this combination of cold and damp affect our gardens? A rapid shutdown to dormancy. Any fall foliage clinging to branches is dropping now.

A pale peach rose with grey mold on petal edges
Gray mold turns petals to mush.

This weather also kicks some fungal diseases into high gear. My roses saw a massive attack of botrytis – gray mold – and it’s not pretty.

According to the National Weather Service, downtown Sacramento recorded lows of 32 degrees on Dec. 17 and 18 – records for both dates. With relentless fog, Sunday (Dec. 18) was the coldest day in downtown Sacramento since January 2001; the high topped out at 42.

The fog is linked to all our ground moisture, thanks to recent rains. Lack of wind and sun keeps the fog trapped close to the ground. And that fog creates ideal conditions for gray mold.

Gray mold – which actually looks more tan or brown on the rosebud – needs moisture for growth in plant tissues, particularly tender flower petals. To prevent outbreaks, keep flower buds dry. That’s all but impossible on damp, foggy days.

Thanks to dry and warm fall weather, I had dozens of rose buds in my garden and was hoping to make some Christmas bouquets. But after so much fog and frosty nights, I’ll have few if any roses for my holiday table.

Gray mold starts out looking like pink measles or brownish water spots on light-colored flowers. Those brown spots rapidly grow until the fungus consumes the whole petal. The flower never fully opens.

Another victim to gray mold this season: Poinsettias. If left outside, they turn to mush.

Gray mold also attacks African violet, asters, begonia, carnation, chrysanthemum, cyclamen, cymbidium, gerbera, geranium, gladiolus, hydrangea, marigolds, orchids, petunia, primrose, ranunculus, snapdragon, zinnias and many other garden favorites.

According to UC Integrated Pest Management program, the best control of gray mold is “good sanitation.” Clip off infected blooms, put them in a plastic bag and dispose of them 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.

(Since this is already pruning time, I’ll remove those infected blooms in a pass through my garden before I tackle pruning the canes.)

After pruning, rake out old mulch and fallen foliage. (This contains other fungal spores, too, for powdery mildew, rust, black spot and other rose diseases.) Then, dispose of that old mulch and replace it with fresh mulch.

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


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Dig In: Garden Checklist

For week of March 19:

Spring will start a bit soggy, but there’s still plenty to do between showers:

* Fertilize roses, annual flowers and berries as spring growth begins to appear.

* Watch out for aphids. Wash off plants with strong blast from the hose.

* Pull weeds now! Don’t let them get started. Take a hoe and whack them as soon as they sprout.

* Prepare summer vegetable beds. Spade in compost and other amendments.

* Prune and fertilize spring-flowering shrubs after bloom.

* Feed camellias at the end of their bloom cycle. Pick up browned and fallen flowers to fight blossom blight.

* Feed citrus trees as they start to blossom.

* Cut back and fertilize perennial herbs to encourage new growth.

* Seed and renovate the lawn (if you still have one). Feed cool-season grasses such as bent, blue, rye and fescue with a slow-release fertilizer. Check the irrigation system and perform maintenance. Make sure sprinkler heads are turned toward the lawn, not the sidewalk.

* In the vegetable garden, transplant lettuce and kale.

* Seed chard and beets directly into the ground.

* Plant summer bulbs, including gladiolus, tuberous begonias and callas. Also plant dahlia tubers.

* Shop for perennials. Many varieties are available in local nurseries and at plant events. They can be transplanted now while the weather remains relatively cool.

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