Grey mold and powdery mildew remain as fungal diseases of greenhouse tomato that are very difficult to control. Resistance to these two diseases has not been found, and these fungi can move into the greenhouse easily from the outside. Both pathogens can colonize many species of weeds, as well as horticultural crops, so it is extremely important to maintain a weed-free environment. Botrytis cinerea is the causal agent of grey mold. It can be controlled with low humidity, but that is hard to achieve in a plastic hoop house on a rainy day.
Many growers install hot water lines below the tomato plants to dry the foliage.
It is important to exchange warm, moist air with cool, dry outdoor air at least once a day, preferably about four p.m. so the plants will be drier at night. Roof vents or a small exhaust fan at the peak of the house can reduce the humidity dramatically. The fungicides registered for use on grey mold of greenhouse tomato in New York, Botran and Exotherm Termil, have only preventative action, not curative. Deleafing is a standard practice to remove infected leaves and improve ventilation near the moist soil and floor. Reducing the plant population density in the Northeast can help control grey mold and increase yield, with the optimum spacing about 4 to 4.5 square feet per plant.