| dc.description.abstract | Pyrethrum production has recently been threatened by a disease characterized by wilting
and subsequent death of pyrethrum plants. Information on etiology and epidemiology of the
disease is scanty and generally not reliable. This study was carried out to identify the pathogen(s)
that cause the disease and to investigate the conducive environmental conditions for the disease
development. ,In addition to the above objectives, some commercially recommended clones and
varieties were also screened for resistance to the wilt disease.
Diseased plant and rhizosphere soil samples were collected from five different pyrethrumgrowing
areas, namely Kisii, Molo, Mwongoris, Sotik and Limuru for isolation and identification
of pathogens.
Identification of fungal isolates from diseased plant materials revealed the presence of
Fusarium oxysporum at all the sampling sites. All the isolates were found to be pathogenic to
clone L175/487 plants, colonizing the vascular (xylem) system of inoculated plants.
Nematodes isolated from all the five sites were identified as Meloidogyne hapla and
Pratylenchus penetrans. However, samples from Sotik had Tylenchus species at very low
population levels.
Use of high levels-of either fungal inoculum alone or nematode-fungus combination led
to increased severity as well as early onset of the wilt disease during pathogenicity tests. These
results indicate that inoculum build up in the soil, where no control measures are practised, can
have disastrous effects on the pyrethrum field stands as well as crop productivity.
Plants inoculated through root dipping method developed symptoms much earlier than
those inoculated through soil drenching, though the symptoms of the disease were basically
similar at comparable inoculum levels. However, unilateral symptom development was observed
in plants inoculated by soil drenching in addition to other symptoms.
Symptoms of the wilt disease consisted of initial chlorosis of lower leaves, beginning with
the leaf margins and progressing towards the leaf petioles. This was followed by necrosis of the
chlorotic regions and eventual drying up of the leaves. At high inoculum levels, symptoms were
more severe, with withering and epinasty of leaves even without initial chlorosis.
Influence of temperature on wilt development was clearly demonstrated by exposing
plants of clone L175/487 to ambient temperatures of 17.7°,21.5°,25.0° and 30.0°C. Plants ,
inoculated and incubated at high temperatures of 25° and 30°C developed wilt symptoms much
earlier and more severely than plants incubated at lower temperatures of 17.7° and 21.5°C.
Different soil moisture levels of 25-30, 45-50, 65-70 and 90-100% displayed significant
differences in their influence on wilt development in pyrethrum. In general, wilt symptoms
developed earlier at low moisture levels than at high moisture levels.
A field experiment on the effect of prevailing temperatures and rainfall (a pre-requisite
for soil moisture levels) was carried out at Kisii, Molo, 01 Joro-Orok and Limuru. These sites
experienced ambient temperatures of 19.9°, 13.1°, 13.7° and 13.0°C respectively and
corresponding rainfall amounted to 164.9, 215.5, 98.4 and 16.9mm, during the first four months
of the experimental period (June to September 1994). Wilt symptoms developed earliest on plants
at the Kisii trial site followed by plants grown at Limuru, 01 Joro-Orok and Molo in that order.
A test carried out to investigate the effect of pH of agar medium on growth of fungal
isolates indicated that high pH levels favour colony growth. Colony diameters on the fourth day
of incubation increased from 2.3cm to 5.3cm as pH increased from 4 to 8.
Screening of different clones and varieties for resistance to wilt disease gave no promising
result since all the eight clones and three varieties screened succumbed to the disease. The longest
time taken for wilt to develop under greenhouse conditions was 51h months (for clone L/75/487)
and 6 months under field conditions at Mwongoris (for clone KrI74/122). | en |
