Impact of organic N on corky root in organically cultivated greenhouse tomatoes

Corky root of tomato caused by Pyrenochaeta lycopersici, is a major soil-borne disease in Swedish organic greenhouse production where tomatoes are grown in soil-based substrate. The corky root attacks the roots of the tomatoes and results in a suberisation of the roots reducing the plants ability to take up water and nutrients. Currently, for most producers, corky root resistant grafted plants is a presumption for the production but apprehensions are that this will affect the taste and may result in a build-up of resistance. Other methods for controlling the disease are urgently needed. A way to decrease the problem is to replace the substrate but this is very expensive. In the present study we tested the hypothesis that a proper N fertilization regime decreases disease severity. A greenhouse study was conducted using ungrafted tomato plants which were grown in a substrate naturally infected with corky root at three levels of additions of organic N, combined with three different pH levels. A high N addition increased the yields whereas a high pH reduced the yields. We conclude that tomatoes can resist corky root better at low pH. The reduced tomato yield at high pH cannot be explained by a calcium deficiency due to calcium immobilization at high pH. For organic tomato growers, a substrate with a low pH and a proper N fertilization is recommended.