Studies on the survival of Verticillium dahliae in soil

[摘要] Various aspects concerning the survival of Verticillium dahliaeKleb. in soil, in the field, green-house, and laboratory werestudied. 'With biological control in mind, attention was paidto factors affecting antagonists of V. dahliae and to colonization of microsclerotia (MS) in soil. Possible activationof antagonists during the period of absence of host plantswas considered.Regression analysis of survival data indicated that individualMS are capable of surviving up to 43 months in soil in theabsence of host plants. Soil moisture and temperature couldnot 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 afterthe plants had been killed. Antagonistic fungi did not appreciably increase in the rhizosphere soil after the release ofpathogen propagules.Bacteria and actinomycetes antagonistic to V. dahliae couldbe stimulated to increase in high pH soil by the additionof MS and fertilizers containing phosphate. In low pHsoil, which favoured fungal antagonists, these tendencieswere less obvious. The largest numbers of bacterial andactinomycetous antagonists occurred in high pH soil in whichV. dahliae survived best.The attrition rate of MS was fastest in low pH (c. pH 4,5)soil. However, it was established that fungal antagonistswere not mainly responsible for the attrition.Active microbial invasion of MS in soil was establishedby electron microscopic studies. Apparently lysed cellwalls 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 antagonistsare capable of actively destroying MS in soil, but thisprobably is a much slower process than that which was observedin acidified soil.Various techniques were used to establish the fact thatattrition was much faster in acidified than in alkalinesoil. The use of several techniques demonstrated thatthe effect of ·low pH on the attrition of the pathogen wasreal and not a reflection of the inadequacy of a singletechnique. It was also evident that the effect of lowpH was fungitoxic and not fungistatic to the pathogen.Experiments to test the validity of the claim that the Al-ionis toxic to V. dahliae at very low concentrations indicatedthat the attrition of V. dahliae was as fast in low pH soildevoid of aluminium salts as in aluminium-amended soil.Soil acidification may be considered as a control measure.However, practical and economic considerations will prohibitits implementation. Apart from the cost factor, most plantsdo not tolerate such a low soil pH. Liming of the soil wouldbe necessary, with the result that favourable conditions areagain created for renewed increase in pathogen numbers.In a green-house study it was found that varying the moisturecontent of the soil and incorporating N and P, were ineffectiveas measures to reduce Verticillium populations. However, inflooded and air-dried soils, significant decreases occurred.The addition of urea at 0,25% or higher to the soil, reducedVerticillium populations appreciably. Various organic soilamendments gave diverse results. Maize residues, followed bysoyabean 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 beproduced in rotation with paddy rice, flooding may be of practical use.,Air-drying of the soil will depend on weatherconditions, but it may be possible to devise agronomic practices to speed up the drying-out process and thereby reducepathogen populations. This 'finding should, however, bestudied under field conditions to verify the results obtainedin the laboratory.The reduction of MS in the soil after amendment with plantresidues holds promise as a control measure. Various cropscould effectively be used in rotation with cotton and when theresidues are incorporated into the soil, a significant attrition rate could be expected. However, the large quantity(1%) of residues required to effectively reduce the populationof V. dahliae suggests that attrition would not be as fastunder 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 MSindicated that they are composed of numerous thin- and thickwalledcells. The cell walls varied in thickness and wereimpregnated with melanin which also occurred in the matrixbetween the individual cells. These properties confer resistance to attrition to the MS. In the soil they apparentlygerminate over extended periods and give rise to limited hyphalgrowth which exhausts the reserves of the less resistant cells.However, some of the more resistant cells may remain· dormantand retain the viability of the MS. This may explain thesurvival of some propagules even under the adverse conditionsto which they were subjected in the present study. It alsoexplains why V. dahliae is such a difficult pathogen toeradicate under normal agronomic conditions.

[发布机构] University of the Free State