GENETIC-CHARACTERISTICS OF FUSARIUM VERTICILLIOIDES ISOLATES FROM MAIZE IN COSTA-RICA

Thirty-nine isolates of Fusarium verticillioides from maize seeds from three regions of Costa Rica were classified on fertility, fumonisin p roduction, vegetative compatibility and pathogenicity. The identity of the isolates was verified by sexual crosses with standard tester stra ins and by isozyme analysis. Twenty-three isolates (59%) were mating t ype A(-) and 16 (41%) were A(+); 29 (74%) were female fertile. The iso lates produced high amounts of fumonisin B-1 when grown on sterilized maize grits, 32 isolates producing more than 1000 mu g g(-1), as deter mined by TLC, and 7 less than 1000 mu g g(-1) Vegetative compatibility tests by pairing nit mutants identified 34 vegetative compatibility g roups (VCGs), of which 29 had one member and 5 had two members. Isolat es belonging to the same VCG were obtained from the same seed sample. Two pathogenicity tests with different inoculation methods were perfor med: on toothpick inoculation of 7-week-old maize stalks, 71% of the i solates were pathogenic according to the length of the necrosis formed in the stalk, and on sand inoculation of maize seedlings all the isol ates were pathogenic, according to shoot length and dry weight product ion. Differences in aggressiveness between some of the isolates were r ecorded. It is concluded that natural populations of F. verticillioide s in Costa Rica consist of genetically diverse, highly fertile and pat hogenic isolates that represent a potential risk for disease developme nt and fumonisin accumulation in maize crops.