Unfortunately, closed greenhouses retain heat and unwanted humidity which can spell problems if left unchecked, especially during prolonged periods of overcast days and cooler weather.
Remember, high relative humidity equals wet leaves which can favor the development of diseases such as Botrytis or bacterial diseases such as bacterial leaf spot. To avoid potential problems, do your best to vent out as much humidity as possible on a daily basis in the early morning or late afternoon. Turn on circular fans to promote air movement and finish watering earlier enough in the day so leaves have enough time to dry out before nightfall. Any flats with transplants with suspect leaf spots should be pulled out of the production facility and isolated until a proper diagnosis can be made.
As a side note, stressed transplants where water and/or fertility have been withheld are also more prone to disease development. A light fertilizer application may go a long way in improving transplant health before setting in the field.
Please see Table E-14 in the upcoming 2020/2021 Mid-Atlantic Commercial Vegetable Production Recommendations guide for a list of control options for important diseases in greenhouses.
