What makes rose buds turn brown?

This First Prize bud never got to bloom -- it's been infected by botrytis cinerea. (Photos: Debbie Arrington)

Rotten roses follow botrytis fungal outbreak

By Debbie Arrington

My bushes are full of rotten roses. I know I'm not alone.

Blame botrytis. That fungus put a real damper on my December garden. Warm weather in early November pushed out new growth and a late round of blooms, just in time for Christmas.

That got me excited. I could fill the house with holiday bouquets or bring arrangements to December gatherings. After an often-difficult year, my roses were going to be gorgeous.

Instead, Marilyn Monroe turned to mush. First Prize was a total loss. These beautiful big buds became brown and died before they ever opened. Other roses developed odd discolorations. Irresistible, a cute white mini, looks like it has measles. Bruises cover the petals of Pink Promise.

Ouch!

Marilyn Monroe is no beauty in this case.
Fog creates ideal conditions for botrytis.

Rain and fog brought on the outbreak, and this fungal infection quickly spread. The opportunistic spores are everywhere. Botrytis cinerea loves high humidity, according to the UC Cooperative Extension master gardeners. Nicknamed gray mold and blossom blight, this fungus can be a real problem for fruit growers in coastal areas. In cherry orchards, it rots blossoms and ruins the crop. Grape growers often have to deal with botrytis.

In Sacramento, it shows up in late fall to attack roses and other flowering shrubs. It forms masses of gray brown spores that can move around by wind or water. The spores linger on dead buds or fallen leaves, waiting for moisture before attacking the plant. During dry falls or winters, it's not an issue. But a blanket of fog creates ideal conditions.

If left to thrive, botrytis can cause twigs to die back and can damage new growth. The best solution? Trim out the problems.

The master gardeners recommend removing and disposing of fallen leaves and debris around plants. (Don't compost it; put it in the trash.) Also, prune out any dying twigs or blooms.

To help stop more fungal outbreaks, avoid overhead watering of roses. Prune to promote good air circulation around bushes. That's next month's job.

For more on botryits: https://bit.ly/2BaPgYq or http://ipm.ucanr.edu.