CONTROL OF TMV IN TOMATOES
Our control experiments were initiated in 1954 at my former working place, in the Agricultural Institute at Kecskemét, on a virus-epidemiological basis. During them it was found that one of the primary sources of infection is formed by seeds issueing from infected plants. The virus living on the seed coat was found to be infective also after 1600 days, counting from the seed-collecting, when seeds were stored at room temperature. That the virus can be found only on the surface of seeds was confirmed by us with a model experiment. In a part of the experiment the seed surface was disinfected by a 2 per cent NaOH solution for 10 minutes and after washing down the disinfectant in every case negative inoculations were obtained from the crushed seeds on the test plants. The efficiency of seed dressing with NaOH was further confirmed in a large number of experiments, so that later on we have worked out this method and suggested its introduction into the practice.
By means of seed dressing however only one of the primary sources of infection is eliminated. We have therefore experimentally tested further on the practical applicability of the hygienical raising of seedlings. The greenhouse of the Institute has been disinfected with a 2 per cent formaldehyde spraying, the soil by steaming, the propagative boxes by flaming, seeds and implements by a 2 per cent NaOH solution. By using this method a temporary success was attained as the earlier 60 to 80 per cent infections were reduced to 6 per cent.
As well known the full realization of the hygiene is however very difficult and expensive as well, therefore in the following the possibilities of therapy were sought after. Thiouracil tested by Holmes (1955) and by others seemed to be a suitable chemical for this purpose. Thiouracil in 0.01 – 0.001 per cent concentration was tested also by us in a large number of experiments (seed dressing, spraying of plants, soil irrigation) and proved to be an outstanding inhibitor of virus multiplication. On account of its inhibiting action also on plant development, its applicability in practice could not be taken in consideration. Thus subsequently after succesful trials by Newell (1954) we have also changed over to examine the virus-inhibiting effect of milk. In our experiments it could be established that among the various milk fractions full milk, skimmed milk and whey display practically the same, nearly 100 per cent, protective effect. Spraying the plants with milk is an infection inhibiting factor. The virus inhibiting substance of the milk namely is not taken up by the plants either through the leaf of the root. Consequently a therapeutic effect of a treatment applied after the infection can be hardly expected. The infection inhibitive action is in an inverse ration to the dilution and the time elapsed after the treatment. For this reason an also practically satisfactory protecting effect can be expected only at the most until a 1 : 5 milk-water dilution for about 10 days following the treatment. On the plant surface a protective film is formed by the milk which