Studies on the survival of Verticillium dahliae in soil
Baard, Schalk Willem
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Various aspects concerning the survival of Verticillium dahliae Kleb. in soil, in the field, green-house, and laboratory were studied. 'With biological control in mind, attention was paid to factors affecting antagonists of V. dahliae and to colonization of microsclerotia (MS) in soil. Possible activation of antagonists during the period of absence of host plants was considered. Regression analysis of survival data indicated that individual MS are capable of surviving up to 43 months in soil in the absence of host plants. Soil moisture and temperature could not be related to the attrition of the pathogen. However, microbial colonization of the MS could have had an effect. Pathogen propagules were released into rhizosphere soil after the plants had been killed. Antagonistic fungi did not appreciably increase in the rhizosphere soil after the release of pathogen propagules. Bacteria and actinomycetes antagonistic to V. dahliae could be stimulated to increase in high pH soil by the addition of MS and fertilizers containing phosphate. In low pH soil, which favoured fungal antagonists, these tendencies were less obvious. The largest numbers of bacterial and actinomycetous antagonists occurred in high pH soil in which V. dahliae survived best. The attrition rate of MS was fastest in low pH (c. pH 4,5) soil. However, it was established that fungal antagonists were not mainly responsible for the attrition. Active microbial invasion of MS in soil was established by electron microscopic studies. Apparently lysed cell walls and eroded areas in the immediate vicinity of bacteria indicated that enzymes may be involved in the deterioration of the cell walls. It is concluded that antagonists are capable of actively destroying MS in soil, but this probably is a much slower process than that which was observed in acidified soil. Various techniques were used to establish the fact that attrition was much faster in acidified than in alkaline soil. The use of several techniques demonstrated that the effect of ·low pH on the attrition of the pathogen was real and not a reflection of the inadequacy of a single technique. It was also evident that the effect of low pH was fungitoxic and not fungistatic to the pathogen. Experiments to test the validity of the claim that the Al-ion is toxic to V. dahliae at very low concentrations indicated that the attrition of V. dahliae was as fast in low pH soil devoid of aluminium salts as in aluminium-amended soil. Soil acidification may be considered as a control measure. However, practical and economic considerations will prohibit its implementation. Apart from the cost factor, most plants do not tolerate such a low soil pH. Liming of the soil would be necessary, with the result that favourable conditions are again created for renewed increase in pathogen numbers. In a green-house study it was found that varying the moisture content of the soil and incorporating N and P, were ineffective as measures to reduce Verticillium populations. However, in flooded and air-dried soils, significant decreases occurred. The addition of urea at 0,25% or higher to the soil, reduced Verticillium populations appreciably. Various organic soil amendments gave diverse results. Maize residues, followed by soyabean pods caused the fastest attrition of NS in the soil. The addition of urea to soil at high rates would not be practical as a control measure. In situations where cotton can be produced in rotation with paddy rice, flooding may be of practical use.,Air-drying of the soil will depend on weather conditions, but it may be possible to devise agronomic practices to speed up the drying-out process and thereby reduce pathogen populations. This 'finding should, however, be studied under field conditions to verify the results obtained in the laboratory. The reduction of MS in the soil after amendment with plant residues holds promise as a control measure. Various crops could effectively be used in rotation with cotton and when the residues are incorporated into the soil, a significant attrition rate could be expected. However, the large quantity (1%) of residues required to effectively reduce the population of V. dahliae suggests that attrition would not be as fast under field conditions as it was under laboratory conditions. A long term rotation might thus be necessary. Electron microscopic studies on the fine structure of the MS indicated that they are composed of numerous thin- and thickwalled cells. The cell walls varied in thickness and were impregnated with melanin which also occurred in the matrix between the individual cells. These properties confer resistance to attrition to the MS. In the soil they apparently germinate over extended periods and give rise to limited hyphal growth which exhausts the reserves of the less resistant cells. However, some of the more resistant cells may remain· dormant and retain the viability of the MS. This may explain the survival of some propagules even under the adverse conditions to which they were subjected in the present study. It also explains why V. dahliae is such a difficult pathogen to eradicate under normal agronomic conditions.