A Rainy Season Blight for your Roses

Along with the first rains of the season arrive, comes the potential for botrytis – a disease that can turn lush rose blooms into a brown, soggy mush.  This wide-spread 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.

This blight has a wide range of ornamental host plants – from azaleas to cacti, coast redwood to fuchsias, hydrangeas, roses, and dahlias, and food crops, notably grapes and berries.  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, it 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; its optimal temperature range is 70 – 77°F, though it is active over a wide temperature range.  The fungal spores can germinate and result in new infections after six or more consecutive hours in contact with free water – from splashing, condensation, or high relative humidity.  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 good cultural care to your plants and maintain your garden sanitation – clean up and dispose of garden 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.

Click here to learn more about managing botrytis blight.

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