Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The soilborne plant pathogen Verticillium dahliae Kleb. produces numerous multicellular, melanized resting structures called microsclerotia (MS) inside senescing host tissues. Melanin has been implicated in the long-term survival of fungal resting structures in soil. A method for the laboratory production, isolation, harvest and storage of stable populations of MS was developed and a procedure for plating individual MS was used to quantitatively measure both lethal and sub-lethal treatment effects. MS of 75-106 {dollar}\mu{dollar}m size exhibited faster and more synchronous germination, produced larger colonies and survived long term storage at 24 C better than MS of 53-75 {dollar}\mu{dollar}m or {dollar}{dollar}35 weeks at 24 C. UV irradiation of albino MS for 2 h severely inhibited germination (a lethal effect) and reduced colony size (a sublethal effect). Melanized MS survived better than melanin deficient MS when buried in soil. TCZ treated MS buried in various soils often gave rise to colonies which were not V. dahliae, implicating soil microorganisms in the reduction of their survival. Inoculation of soil with Talaromyces flavus or Trichoderma aureo viride reduced the survival of melanized and TCZ treated MS to 51% and {dollar}<{dollar}10% or 79% and {dollar}<{dollar}14% respectively after 5 weeks burial. A possible mycoparasite isolated from MS (Penicillium sp.) reduced the survival of melanized and TCZ treated MS to 73% and {dollar}<{dollar}20% respectively. The addition of bloodmeal (BM) to soil at 1% w/w eradicated melanized MS and reduced the infection of eggplants with V. dahliae from 90% to 20%. Within 15 days of adding BM the soil pH rose from 5.5 to 8.2 and returned to original levels after 25 days. Since MS suspended above BM amended soil were killed within 6 days, a volatile fungitoxic factor (possibly ammonia) was implicated. Burial of MS in soil amended with 1% w/w urea or 0.5% ammonium acetate completely inhibited germination but 1% ammonium sulphate had no effect.



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