I've included some links below to some websites that may help you identify exactly what is going on with your tomatoes. From what you have described, it sounds to me like you have some type of blight problem going on. If that is the case, disinfecting your greenhouse is going to be essential to preventing further spread.
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By McMillan968 (Guest Post)07/07/2008
I lost some last year from the overly wet weather! (FLOODS) I took off ALL the leaves EXCEPT the very top and they lived.
By henry (Guest Post)06/28/2008
You may have blight- caused by a fungus in the air. it likes damp warm condition and was bad last year, it is the same a potato blight and could have caught it from nearby potatoes?
Burn all the dead leaves etc. I changed my soil for this year - hope it goes OK.
Here are questions related to Tomato Plant Leaves Turning Black.
I potted a tomato plant in a white bucket and filled it with soil a little over half full and put it on the porch as we live on the second floor on a busy street. I noticed the plant itself was turning a dark color. I gave it some Miracle Gro and the bottom leaves on the plant went yellow so I took those off. Now the rest of the plant has turned just about all black. There are 2 tomato's growing at the top and they are getting big. What is the plant lacking?
By Gail B
By Cathy Yeater07/20/2011
I would say drainage problem and not enough soil.
My tomatoes leaves are turning black underneath. What causes this? They are blooming at this time.
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Hardiness Zone: 6a
Peggy62 from Chillicothe
It could be a couple of things.
Normally, leaf scorch is a result of stress from drought, or caused from pesticide burn or fertilizer burn. Check these first.
It you rule these out, then what you are describing could be blight. There are two types of blight, early and late. Both are a fungal disease commonly affecting potatoes and tomatoes. Symptoms on leaves start as tiny brown spots, which develop into greenish gray or brown areas that can expand to cover the entire leaf. These spots are sometimes surrounded by a ring of yellow tissue on the upper surface of the leaves and a ring of white fungal growth on the undersides of the leaves. Affected leaves drop early, which exposes fruit to sunscald.
Treatment is straightforward: remove or destroy infected plants and get rid of all debris. In your case, it's best to relocate your plant away from the others until you figure out whether or not it's infected. Pick off all infected leaves, and avoid watering from overhead because the splash may launch the disease spores. Don't over fertilize. If you believe there is a need, get the appropriate garden fungicide for vegetable crops and apply according to directions.
If your plant ends up dying a premature death, make sure you get rid of all of the dead plant debris, then wash and sterilize the pot with a solution of 1 part bleach to 10 parts water. When replanting, look for disease-resistant cultivars. Also, preventative copper-based fungicides may sometimes help reduce the spread of early blight.
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