The saprophytic fungus Botrytis cinerea (gray mold) lives primarily on dead or decaying organic matter, but in favorable conditions such as excessive humidity, this fungus can very quickly become an aggressive parasite on healthy plants. Quickly settling on dead plant tissue, the spores can inhabit drying flowers, damaged leaves, and even scars from sucker removal.  The greatest risk of infection occurs at a temperature of 57℉ – 59℉ (14°C – 15°C) and during periods of high relative humidity (about 97-98%). This happens most often in early spring, late autumn, or when there is not sufficient ventilation in an indoor growing environment.

In the first stage of the disease, grayish-brown spots appear on the stems of tomato plants. These spots can vary in size and are covered with a fluffy fungal bloom. Very often, the plant dies above the site of infection. Small, black sclerotia (compact masses of spores) can sometimes be observed on the surface of the affected tissue. As the disease progresses, the stem under the infection becomes hollow. In early spring crops, undercover, B. cinerea can also infect the lower parts of the tomato stem, located just above the ground. This can happen especially if the pathogen has been transferred through cuttings. In this case, the infection causes the stem to rot at the base.

The pathogen also attacks leaves, flowers, and fruit. Gray-green or yellowish spots appear on infected leaves with less pronounced concentric zoning and gradually increase in size until a fluffy gray bloom appears, then the leaves dry up. From a yield point of view, gray mold can attack flowers and young fruit buds, which causes the buds to drop. When gray mold attacks larger fruits, water-soaked soft spots appear, the color of the stain changes, a gray fungal coating appears, and the entire fruit becomes infected. Wet rot, caused by gray mold, results in a significant loss of crop.

Prevention
Since Botrytis cinerea can survive on plant debris, thorough cleaning of garden spaces, greenhouses, and contaminated tools is crucial in staving off infection for future seasons. It is best to use drip irrigation (to prevent wet foliage), and water early in the morning to keep the plants and soil dry at night. Remove the plant’s oldest leaves and infected leaves to improve airflow, and be sure not to create open wounds, which would be more susceptible to infection.
Please note there are no tomato varieties with resistance to gray mold, and once an infection has begun, there is no way to ‘cure’ it. To control a gray mold outbreak after noticing the first symptoms of the disease, plants can be treated with alternating fungicides so as not to build a fungicidal resistance to one in particular. Daconil, Copper, Neem, and Mancozeb are a few fungicide options.