Aggressiveness and genetic structure of seedborne pathogen, Gibberella fujikuroi causing Bakanae disease in rice [in Laguna and Nueva Ecija, Philippines]

Forty isolates of Gibberella fujikuroi were collected from Nueva Ecija and Laguna and characterized by morphology, mating type (MAT) and mating population by genetic crosses and MAT-PCR. The identification of the isolates was confirmed by Elongation Factor (EF)-1 infinity gene sequences and diversity was assessed using Universally Primed-PCR (UP-PCR). Aggressiveness of the 40 isolates was tested in two rice varieties (PSBRc54 and PSBRc18) under the laboratory while aggressiveness of the 20 isolates were tested in the greenhouse and microplot conditions and their in vitro fumonisin production were analyzed using Enzyme Linked Immunosorbent Assay (ELISA) and High Performance Liquid Chromatography (HPLC) with the three types of fumonisins (FB1, FB2 and FB3) detected by the latter. Since all isolates that have been crossed with mating population C tester strains were infertile, MAT-PCR was utilized. The ratio of MAT1:MAT2 is 10:25, which deviated from the Mendelian ratio of 1:1. The effective population number for mating type, Ne sub(mt), is 28.57% of the count (total population). Twenty three isolates were confirmed as G. fujikuroi based on sequence analysis of EF-1 infinity gene. Under laboratory condition, elongation and stunting of seedlings as measures of aggressiveness varied widely. Correlation (r=0.76) of aggressiveness of 40 isolates of G. fujikuroi between the two varieties was high, but correlation between two trials were moderate (r=0.44-0.50). Significant genotypic variation for aggressiveness of isolates (P=0.05) were observed for both varieties. Repeatabilities of aggressiveness in both trials in PSBRc54 (84.5-89.1) and PSBRc18 (86.5-88.4) were high. Isolate and isolate-environment interaction on the two varieties were also significant (P=0.05) across two trials. Heritability was medium (0.54-0.55) on PSBRc54 and PSBRc18 due to significant isolate-environment interaction and error. In the greenhouse and microplots, moderate to high correlation (r=0.60-0.89) of isolate aggressiveness was obtained between the two rice varieties. Repeatability estimates for PSBRc54 and PSBRc18 were 43.5 and 19.5, respectively across ratings. Significant genotypic variation of aggressiveness (P=0.05) was found in PSBRc54. Aggressiveness of 20 G.fujikuroi on two varieties widely varied under microplot and greenhouse conditions. In the microplot, mean plot height was higher than the greenhouse. Repeatability estimates on both varieties were lower in the microplot due to error. No correlation of aggressiveness of isolates was found on both rice varieties between greenhouse and microplot. High degree of genetic variation was observed among 20 isolates as analyzed by UP-PCR. No correlation between molecular data and aggressiveness was found, suggesting primers were not specific to the loci in the genome that are associated with aggressiveness. Of the 23 G. fujikuroi isolates collected, three isolates are fumonism producers as detected by ELISA and two FB1 producers by HPLC.