Laccase enzymes (benzenediol: oxygen oxidoreductase, EC 1.10.3.2) play a major role in the degradation of phenolic compounds such as lignin. They are common in fungi and have been suggested to participate in host colonization by pathogenic fungi. Putative mycoviruses have recently been isolated from the causal agent of pine pitch canker disease, Fusarium circinatum Nirenberg & O'Donell. In this study, the effects of single and double mycoviral infections on laccase activity, growth rate and pathogenicity were investigated in fourteen F. circinatum strains. Extracellular laccase activity was analyzed by the Bavendamm test, image processing and a spectrophotometric method. Mycelial growth, in vivo pathogenicity and seedling survival probability were also determined in Monterrey pine (Pinus radiata D. Don) seedlings. The findings showed that (i) mycelial growth of isolates from the same fungal population was homogeneous, (ii) the presence of mycovirus appears to increase the virulence of fungal isolates, (iii) co-infection (with two mycoviruses) caused cryptic effects in fungal isolates, and (iv) laccases embody a possible auxiliary tool in fungal infection. The prospects for biocontrol, the adaptive role of F. circinatum mycoviruses and the importance of laccase enzymes in host colonization are discussed.
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