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ISHS Acta Horticulturae 4: Symposium on Vegetable Growing under Glass

EXPERIMENTS WITH SOIL FUNGICIDES FOR CONTROL OF BROWN ROOT ROT OF TOMATO

Author:   G. Crüger
Abstract:
During the last few years brown root rot of tomatoes in glasshouses and outside has become increasingly important in the Federal Republic of Germany. The decrease in yield caused by this disease was less than in other growing areas, since up until now the greater part of tomato growing in the Federal Republic of Germany (F.R.G.) has been done in unheated glasshouses. There the growing period normally only lasts 5 months and only 7 clusters are harvested, As we know the harmful effect of brown root rot does not become really apparent until the 5th cluster is formed. According to the investigations done by us in 1963 in the whole Federal Republic the disease occurs particularly in nurseries which have been tomato growing for many years. The disease is found on the most differing soils and infects all tomato varieties more or less equally. Under the growing conditions in the F.R.G. the decrease in yield may be up to 50%. In connection with this is to be noted that the decrease in yield varies from year to year. Losses are particularly great in years with low temperatures during the summer months.

The rising importance of brown root for tomato growing in the F.R.G. was the reason for more intense research in the Biologische Bundesanstalt. The result of the extensive research done by Termohlen (1962) were used as a basis. As we know Termohlen (1957, 1962) has become convinced that the cause of brown root rot is algrey fungus which forms no spores and which he calls "kurkwortelschimmel". Gerlach & Schneide (1964) examined a large number of tomato roots with this disease, from all tomato growing areas of the F.R.G. In all cases they found the "kurkwortelschimmel" described by Termohlen (1957, 1962). With several isolates they succeeded in making them form pycnidiums. After this the "kurkwortelschimmel" belongs to the genus Pyrenochaeta. In any case it is a fact that brown root rot is caused by a fungus and it should be possible to control it with the use of fungicides.

The present day methods used against brown root rot of tomato are: Soil sterilization by steam or by chemicals or grafting on resistant root stocks. These methods have certain disadvantages. In the case of grafting the harvest almost always begins 14 days later. Besides there is a great danger of transferring viruses by this method. There are also certain disadvantageous effects on the shape and size of the fruits. Grafting is not suitable for all varieties. Certain local knowledge is necessary, for soil fertility plus the combination of root stock and variety have their effect on the quality of the fruit. Soil sterilization by steam requires a considerable amount of apparatus and labour, and the whole process is slow. The known methods of soil sterilization by chemicals require a relatively long waiting time necessary for the escape of the phytotoxic gases. For economically necessary reasons the vegetable growers are eager to use their glasshouses the whole year round and they cannot afford several weeks waiting time which would mean extra money for the chemical soil treatment. For a long time now there has been the wish to have soil fungicides which can be applied either to the growing crop or just before planting time and which have no harmful effect on the crop. Up to now in vegetable crops the use of fungicides for the control of soil inhabiting fungi has been restricted to a few cases as the use of captan, thiram etc. for seed-treatment and calomel and chlornitrobenzenes for control of Plasmodiophora brassicae. Generally speaking the problem of soil fungicides seems still to be in a more or less theoretical stage. There are numerous "invitro-tests" and also many "in vivo" (cold-tests), but there are only very few soil fungicide experiment

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