Nobel Rot on your Roses?

With the first rains of the season comes the potential for Botrytis – the disease that can turn lush blooms into a brown, soggy mush.  This common disease is also known as Botrytis blight, bud rot, gray mold, wooly mold and when growing on grapes, the “noble rot” where it can either ruin a crop of grapes, or produce excellent dessert type wine. In our mild Mediterranean climate, we don’t see this disease too much during the dry summer months, but once the rains begin, the wooly gray fungus returns with a vengeance.

With a wide range of host plants, from azaleas to cacti, coast redwood to fuchsias, hydrangeas, roses, and dahlias, this nasty rot affects tender plant parts, weakened, or injured, old or dead non-woody tissue.  Once the fungus is established it can invade healthy green tissue.

Caused by several strains of the fungus Botrytis cinerea, the disease produces a range of effects.  Flower petals become spotted or discolored, flower buds may fail to open, or the buds may rot, leaves and shoots discolor, wilt, decay, and drop, and twigs may die back.  

The disease thrives in warm, humid conditions. While its optimal temperature range is 70 – 77°F, it is active over a wide temperature range.  It requires water for spores to germinate – six or more consecutive hours in contact with free water from rain, fog, splashing from irrigation condensation, or relative humidity greater than 90%.  It survives in decaying plant material or on the soil surface and can overwinter even in cold climates.  It can remain dormant until the weather conditions are right, then spring to life. 

Since it is such a prevalent fungus, prevention is the best approach.  When possible, choose plants that are not susceptible to the disease.  Provide your roses with good cultural care and maintain your garden sanitation – clean up and dispose of fallen leaves and debris to minimize the reservoir for future infection.  Prune out dead or dying tissue and thin the plant canopy to improve air circulation.  Most importantly, avoid overhead watering, or if it is necessary, water in the morning so that plant surfaces have adequate time to dry.

Check out this video to learn more – Four Steps to Cure Botrytis Blight on Roses: Identify the Disease and Save Your Flowers.

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