Fungicidal activity of Thai medicinal plant extracts against <Emphasis Type="Italic">Alternaria brassicicola</Emphasis> causing black spot of Chinese kale

Crude ethanol extracts and six organic solvent fractions of 10 Thai medicinal plants were evaluated for their antifungal activity against Alternaria brassicicola in laboratory and under greenhouse conditions. The results showed that the ethanol extracts of Coscinium fenestratum, Piper betle, Syzygium aromaticus and Zingiber cassumunar displayed complete mycelial growth inhibition of A. brassicicola at a concentration of 0.1%. Meanwhile, the crude ethanol extract and methanol fraction obtained from the stems of C. fenestratum revealed the greatest inhibition against A. brassicicola at 10%, forming inhibition zones 2.55–2.58 cm in diameter. In the greenhouse experiments, crude ethanol extracts of C. fenestratum and P. betle at 1% significantly (P?<?0.05) reduced the disease incidence at up to 67%, indicating promising preventive and curative activities against A. brassicicola. This activity is similar to that of iprodione, a widely used commercial fungicide. Interestingly, Illicium verum extract showed a greater curative effect (58% disease reduction) than protective effect (47% disease reduction). Because the C. fenestratum extract showed the highest activity against the black spot pathogen both in vitro and under greenhouse conditions, its methanol fraction was further analyzed by spectroscopic techniques. We found that berberine is a key active substance inhibiting mycelial growth of A. brassicicola. The results of this study showed the potential of Thai medicinal plants as alternatives to the use of synthetic fungicides for controlling black spot in Chinese kale caused by A. brassicicola.     

Genetic variation of avirulence genes (AVR-Pi9, AVR-Pik,AVR-Pita1) and genetic diversity of rice blast fungus,Pyricularia oryzae,in Thailand

Genetic variation of the rice blast (Pyricularia oryzae) population in Thailand was investigated based on the nucleotide sequence of three avirulence genes, AVR-Pi9, AVR-Pik, and AVR-Pita1. Sixty rice blast isolates were collected from rice-growing areas around Thailand. Gene-specific primers were used to amplify these AVR genes and AVR-Pi9, AVR-Pik, and AVR-Pita1 were detected in 60, 57, and 23 isolates, respectively. Based on the AVR-Pi9 sequences, we identified one rice blast isolate containing an amino acid change from glutamic acid to aspartic acid. Moreover, two rice blast isolates had identical sequences to the rice blast strain originating in Japan, indicating a potential movement of this isolate from Japan to Thailand. Three AVR-Pik alleles were found, including AVR-PikA (3.51%), AVR-PikD (71.93%), and isolates with two copies of AVR-PikD and AVR-PikF (24.56%). AVR-PikA and AVR-PikF are virulent to Thai rice variety Jao Hom Nin. Six haplotypes of AVR-Pita1 were identified with one deletion and 12 amino acid substitutions. This study revealed that different AVR genes in Thai rice blast populations have different levels of genetic variation: AVR-Pi9 and AVR-Pik genes have a relatively low genetic diversity, while the AVR-Pita1 gene has high genetic diversity. We found AVR-Pi9 was not under selection pressure, while AVR-Pita1 was under purifying selection pressure. In addition, geographic location has influenced the distribution of genetic variation of AVR-Pita1. The information obtained from this study is valuable for the future development of breeding strategies for rice blast resistance in Thailand.