A biological control approach of armillaria root rot of tea
Onsando, James Micah
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Field, greenhouse and laboratory experiments were conducted between June 1987 and September 1991 to investigate: (i) effects of soils amended with locally available organic agricultural wastes (ii) effects of sub-lethal doses of basamid (iii) effects of commonly used inorganic fertilizers in tea (iv) effects of age and surface area of coffee pulp (v) effects of soil moisture in coffee pulp amended soil on the proliferation of Trichoderma spp. in Kenya tea soils (vi) antibiotic activities of Trichoderma spp. in Kenya tea soils on Armillaria mellea and (vii) the potential of coffee pulp in the management of Armillaria root rot of tea. The organic amendments, saw dust, coffee pulp and husks in a 10:1 ratio, filter press cake, cow manure, sheep manure and goat manure in this study influenced the population dynamics of soil microorganisms differently both in the greenhouse and field. Coffee pulp and husks significantly (p=0.05) enhanced Trichoderma proliferation more than the other organic amendments. The enhanced proliferation of Trichoderma spp. (T. koningii, T. harzianum and T. longibrachiatum by coffee pulp was rate dependant with 16 and 22 grams carbon giving the highest populations without significant (p=0.01) differences between them. The peak Trichoderma proliferation was on the fifth month after treatment application, implying that for sustenance of high Trichoderma populations in the soil for biological control of Armillaria root rot of tea, a second or even third application would be necessary. The coffee pulp had the added advantage of significantly (p=0.05) enhancing high proliferation of non-Trichoderma fungi and bacteria, which in turn could make synergistic antagonism against A. mellea possible. The coffee pulp and husks in a 10:1 ratio was as effective as basamid in the management of Armillaria root rot of tea in the field. Unlike basamid, coffee pulp significantly (p=0.05) enhanced tea yields. The usage of basamid at different rates, some of them sub-lethal to Trichoderma and inorganic fertilizers NPKS 25:5:5:5, NPK 20:10:10, S/A and DAP at 150kg N/ha was significantly inferior to coffee pulp in the enhancement of microbial proliferation including that of Trichoderma spp. Possible contributory factors to the superior enhancement effect of the coffee pulp to microbial proliferation include adequate food reservoir and optimal soil pH for microbial activities. Fresh and one-month-old coffee pulp and husks enhanced significantly (P=0.05) more microbial proliferation including that of Trichoderma spp. more than the older pulps. The population proliferation with age was quadratic and dependant on carbon content. Increase in coffee pulp and husks surface area, improved the material's ability to enhance Trichoderma proliferation and that of other microorganisms only for a short duration. This implies that in order to sustain high Trichoderma populations, more frequent applications of the ground coffee pulp and husks would be required than when the normal coffee pulp is used. Dry soils at 15 to 25 moisture content significantly hampered Trichoderma proliferation. Wet soils at 35 to 50 moisture content significantly (P=0.05) enhanced Trichoderma spp. proliferation. The better survival of Trichoderma spp. in very wet coffee pulp amended soils, indicated their tolerance to anaerobiosis. The results suggested therefore that for maximum biocontrol benefit of coffee pulp due to proliferation of Trichoderma, application should be restricted to very wet months of the year. Metabolites of Trichoderma isolates significantly (P=0.05) inhibited mycelial and rhizomorphal growth of A. mellea. T. koningil (isolate 9), T. longibrachiatum (isolate 3) and T. harzianum (isolate 4) had the highest antibiotic activities against A. mellea. A mixture of metabolites of the 9 isolates did not exhibit synergistic effects.