|dc.description.abstract||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
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
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.||en_ZA