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Effect of Inoculum Density of Sclerotium cepivorum on the Ability of Trichoderma koningii to Suppress White Rot of Onion

Amendment of soil with Trichoderma koningii strain Tr5 grown on autoclaved white millet grain provided between 63 and 79% control of white rot of onion when added to soil containing 10, 25, 50, or 100 sclerotia of Sclerotium cepivorum per kilogram of soil at the time of onion seed sowing. There was no significant difference in the proportion of S. cepivorum infections suppressed among the different sclerotial density treatments. Rhizosphere colonization by T. koningii Tr5 was assessed by incubating onion roots sampled from plants growing in soil with the appropriate density of sclerotia, on a Trichoderma selective medium (Rose bengall-Allisan-streptomycin-Previcur agar) developed for the purpose of the study. Trichoderma spp. isolated were typed by comparison of culture morphology as well as polygalacturonase (PG) (EC 3.2.1.15) and pectinesterase (PE) (EC 3.1.1.11) isozyme profiles to the series of one PG and two PE isozymes known to be produced by T. koningii Tr5. The method was used successfully to assess rhizosphere colonization. Three rates of a millet grain formulation colonized by T. koningii Tr5 were added to soil (1,590, 3,180, and 4,770 kg/ha). At the lowest of these rates, 97% of roots were found to be colonized by isolates which could not be distinguished from T. koningii Tr5, whereas 8% of the roots from nontreated controls were colonized by such isolates. An objective of the study was to determine whether the ability of T. koningii Tr5 to suppress S. cepivorum infections was influenced by increased concentrations of both S. cepivorum sclerotia and T. koningii Tr5-colonized millet grain, and it was found that no further improvements in the percentage of disease suppression were recorded as a result of adding T. koningii Tr5-colonized millet to the soil at more than 1,590 kg/ha at any of the sclerotium concentrations tested.