The use of histochemical analysis and stem colonization for distinguishing reactions of alfalfa to verticillium wilt

Abstract

The objectives of this study were to investigate resistant responses in cultivated alfalfa (Medicago sativa L.) to verticillium wilt and to develop a selection procedure to distinguish resistant, tolerant, and susceptible genotypes utilizing phenolic compound production. The level of phenolic compound accumulation in infected seedlings was significantly higher than noninfected seedlings. Seedlings of resistant or tolerant entries accumulated higher levels of phenolic compounds than susceptible entries during the first 7 days following inoculation. The results indicate that the levels of phenolic compound accumulation and distribution in the stem vascular region exhibit quantitative differences associated with resistance to Verticillium albo-atrum infection. Furthermore, this chemical response appears to be genetically inherited. The level of stem colonization in resistant entries was slightly lower than tolerant and susceptible entries during the first 2 weeks following inoculation. Susceptible seedlings died during the 8 weeks postinoculation. Six months following inoculation, stem colonization dropped to near zero in the resistant genotypes, whereas it remained high in the tolerant plants. The absence of symptoms is not a reliable indicator of verticillium wilt resistance. Patterns of phenolic compound accumulation and long-term stem colonization can be used as more accurate criteria for distinguishing resistant genotypes from tolerant and susceptible genotypes. Key words: Verticillium albo-atrum, phenols, resistance.