We are now in the season of misty mornings which are fine for romantic poets but for gardeners it is fungus weather. Given moist warm or cool atmospheric conditions air borne spores abound, ready to settle on any vulnerable foliage or fruit. Fresh air is the answer to minimising the risk as much as possible.
Overnight condensation forms on greenhouse plants, on the glass and the greenhouse structure itself. As summer merges into autumn the daily drying time takes even longer and conditions are ripe for fungal attacks. Some of the heritage varieties of tomatoes are delicious, but. One of the reason they fell out of favour was their lack of resistance to disease.
Tomato leaf mould – Cladosporium fulvum with its yellowing lef lesions and under leaf felty patches loves fungus weather. Then there is potato blight – late blight, which kill exposed plants and makes affected fruit inedible.
Perhaps the most widespread disease of all is Botrytis cinerea – grey mould which can attack a wide range of plants under a wide range of temperature and moisture conditions. It starts as a brown lesion which quickly spreads and matures into a mass of grey spores which given one tap they become air borne releasing thousands of spores which under moist warm conditions can germinate in 4 hours. Our answer to botrytis is vigilance, cleanliness and air.
By the end of September tomato plants an b stripped of almost all their leaves and watering reduced to barely moist. Every bit of plant debris needs collecting up and any signs of botrytis cut out with a sharp knife, or in a very bad attack, the plant removed. Where a plant is still carrying several trusses of fruit cutting out botrytis will leave a vulnerable wound. This is where, immediately after cutting out the lesion, dusting the wound completely with Sulphur powder will prevent reinfection. A little ventilation left open day and night, unless frost is forecast, and plenty of daytime vent will help keep the air moving and the botrytis at bay. Over the last few years we have had a few late frosts and in disease free greenhouses plants have gone on fruiting well into autumn.