|Botrytis on a hybrid tea rose
Seasons Greetings from Botrytis cinereaFellow rosarian Paul McCollum sent this photo to me and asked: "What is causing this on this rose - I can only speculate."
It's a form of the very prevalent Botrytis cinerea. This fungal disease is airborne so the spores are everywhere. They germinate in small droplets of water which linger on the petals, aided by high humidity, poor air circulation, and warm temperatures.
The pink spots are most visible on lighter colored rose petals and usually the ones on the outside of the flower. The fungus is most attracted to the softer tissues of flowers and sometimes hips.
As the fungus matures, it feeds off the dead tissues and can produce brown rot if the moisture is sustained or gray mold - the fully developed fungus which issues spores.
With the pink water spots, usually, the petals drop before it develops the gray mold. But if the rotted outer petals dry somewhat and prevent the flower from opening, Botrytis will develop into what Victorian's called "Gray Ghost in the Garden."
There is no cure but you can control it by promptly removing rotting, unopened buds and disposing of them in your green waste container. The disease spreads rapidly, especially if your plants are too close together.
Botrytis cinerea affects many garden plants, fruits, and vegetables, especially in the late season. Pruning all of your rose bushes in January will also obviously put an end to it for the season.