Black Spots on Tomato Leaves

Peter,
Could you help me out please. The tomatoes in Jack’s garden have black spots on the leaves. How do I fix?
Thanks,
Teemu

Err, that’s awkward. Black spots on Tomato leaves is a tricky problem. We were plagued by this disease for many years. Actually there are a number of diseases that can produce black spots, but let’s assume that the spots are caused by bacteria.
At Scotsburn, we almost completely eradicated the disease through good nursery hygiene and a ‘balanced’ feeding program. In the bad old days we were continually spraying copper solutions (like Bordeaux mixture) to prevent the bacterium infecting our Tomatoes but we found moving Tomato pots up onto benches much more effective. Moving to benches reduced the amount of water that was splashed around the plants – splashed water can pick up and transmit disease.
We also worked on the theory that our plants needed to be growing vigorously to prevent the disease attacking. This is a good theory but it turns out that the execution was a ‘bit out’. We were feeding our Tomatoes so much that they became lush, soft and very prone to minor physical damage which gave the bacterium access to our plants.
The solution came from Peter Wood who used to run Woodlyn Nursery and he happens to be my mum’s cousin. Peter suggested we cut back the Phosphorus (P out of the NPK ratio) in our fertilizer blend. I have previously related Phosphorus to fatty food. Great if you need it but seriously addictive. Tomatoes love it and it doesn’t take much to get your plants jumping out of their skins. So what did we do? We found a ‘balanced’ liquid fertilizer with equal quantities of Nitrogen and Potassium and almost no Phosphorus. Problem solved.
Can this be fixed at home? I think so. If you are planting in pots, use a quality potting mix. I recommend Debco’s Terra Cotta and Tub mix. If you choose to add additional fertilizer I suggest Phostrogen – Tomato Food (phew their analysis agrees with mine, takes the Potassium levels even higher). We are in the process of trialing Debco’s Organic potting mix but I don’t have any suggestions for organic fertilizers in pots yet.
I haven’t mentioned growing in soil. Soil is great, use organic fertilizer like manure. Well prepared soil protects your Tomatoes from many disease problems.
Image from University of Minnesota. Michelle Grabowski. Click on the image to see the original.