Molecular diagnosis of thiophanate-methyl resistant strains of Fusarium fujikuroi in Japan

Fusarium fujikuroi is the pathogen of rice bakanae disease, and is subclassified into gibberellin and fumonisin groups (G- and F-groups). Thiophanate-methyl, a benzimidazole fungicide, has been used extensively to control F. fujikuroi. Previous investigation showed that F-group strains are thiophanate-methyl sensitive (TMS), while most G-group strains are thiophanate-methyl resistant (TMR) in Japan. The minimum inhibitory concentration (MIC) in TMS strains was 1–10 μg mL-1, while that in TMR strains was higher than 100 μg mL-1. E198K and F200Y mutations in β2-tubulin were detected in TMR strains. A loop-mediated isothermal amplification-fluorescent loop primer (LAMP-FLP) method was developed for diagnosis of these mutations, and was applied to 37 TMR strains and 56 TMS strains. The result indicated that 100% of TMR strains were identified as having either the E198K mutation (41%) or the F200Y mutation (59%), while none of the TMS strains tested showed either mutation. We found one remarkable TMR strain in the F-group which had a F200Y mutation. These results suggest that E198K and F200Y mutations in β2-tubulin contribute to thiophanate-methyl resistance in F. fujikuroi.

Characterization of Prochloraz Resistance in Fusarium fujikuroi from the Heilongjiang Province in China

Prochloraz is widely used to control the rice bakanae disease caused by Fusarium fujikuroi. The current study was aimed at monitoring the development of F. fujikuroi resistance to prochloraz in the Heilongjiang province, and at analyzing the fitness of F. fujikuroi strains with different resistance levels. The results indicated that the majority of the 89 F. fujikuroi strains collected from the Heilongjiang province were resistant to prochloraz, with resistance frequency reaching 92.1%. To assess the field resistance risk of prochloraz, 21 F. fujikuroi strains with different resistance levels were selected to investigate their biological characteristics and assess their fitness. Mycelial growth, sporulation, and germination rates were significantly different among the tested strains. However, when grouped into two sub-populations, no significant difference was tested between prochloraz-resistant and prochloraz-sensitive strains. Pathogenicity assays revealed that the disease severity index of prochloraz-resistant strains was higher than that of prochloraz-sensitive strains. Cross-resistance assays showed no cross-resistance between prochloraz and five other fungicides, namely phenamacril, ipconazole, tebuconazole, carbendazim, and fluopyram. Ffcyp51A gene overexpression was observed in the prochloraz-resistant F. fujikuroi strains, following exposure to prochloraz. Collectively, these results indicated that F. fujikuroi resistance against prochloraz was severe. Furthormore, prochloraz-resistant strains were highly fit and could potentially become a dominant population in rice fields, consequently resulting in yield loss.

Estimation of yield loss and relationship of weather parameters on incidence of bakanae disease in rice varieties under shallow low land ecologies of Eastern India

Aim: To assess yield loss in different rice varieties due to rice bakanae disease and to understand the role of weather parameters on disease incidence and losses. Methodology: Roving survey and surveillance was carried out for three consecutive years to assess the status of rice bakanae disease among twenty popular rice varieties growing in Eastern India. Field experiment was conducted for three successive years, i.e., 2016, 2017 and 2018 at Cuttack. Highly susceptible variety Pooja was selected for the experiment. Data on disease incidence was collected daily from 10 days after transplanting to harvesting stage and data were recorded at standard meteorological week. Correlation was performed to find the effect of different weather parameters on the incidence of bakanae disease on rice. Results: Disease being seed borne caused significant reduction in yield and quality. The maximum disease incidence was recorded in Pooja with 18.25, 28.50 and 40.25 per cent in three years. The same variety showed maximum yield loss of 35.26 per cent compared all other varieties. The data on relationship of bakanae disease incidence with weather parameters showed that the rainfall (-0.444) had significant negative correlation during 2018, while RH (0.525 and 0.606) had significant positive correlation during 2017 and 2018. Interpretation: This study gives some information for formulating breeding programs to develop resistant varieties and management measures for preventing Bakanae disease.