| Summary: | The pathogenicity of two isolates of the fungus <i>Verticillium albo-atrum</i> obtained either from lucerne (V1) or tomato(V2) towards four cultivars of lucerne (<i>Medicago sativa</i>.L), namely Maris Kabul, Europe, Vela, and Euver was investigated. The results confirmed that only the V1 isolate is capable of causing wilt disease in these four cultivars. A degree of resistance was observed, however, and of all cultivars Maris Kabul showed the highest degree of resistance. Following pathogenicity testing, production of the phytoalexin medicarpin, synthesis of which is preceeded by an increase in activity of the enzyme phenylalanine ammonia lyase (PAL) was also investigated. The results showed that both the pathogenic isolate (V1) and the non-pathogenic isolate (V2), can induce an increase in PAL activity but that the non-pathogenic isolate induced a greater accumulation of medicarpin in tissues of cv. Maris Kabul. The differential activity between V1 and V2 isolates was further studied using a elicitor produced from mycelial culture filtrate from each isolate of the fungus. It appeared that elicitor from the V1 isolate contains a 'suppressor' molecule that acts 'down stream' of the step in which the increase in PAL activity was induced. The elicitors were partially purified using a combination of Con-A Sepharose affinity and DEAE anion exchange chromatography, and characterized by SDS-gel electrophoresis and treatment with trypsin and periodate. The results indicated a degree of heterogeneity in the elicitors which appeared to be rich in glucose and/or mannose and protein. Treatment with trypsin and periodate confirmed that both elicitotrs are glycoprotein in nature and depend for their activity on carbohydrate moiety. Characterization with SDS-gel electrophoresis revealed that the molecular weights of both V1 and V2 elicitors from the culture filtrates is ca. 29,000. There is also present one minor component (M wt. ca. 45,000) in the V1 elicitor preparation, which may be responsible for the differential activity of the V1 and V2 elicitors.
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