Search

Choanephora rot of Chenopodium plants (CRC) was observed at the flowering stages in seven plantations of Shanxi Province, China. CRC had caused leaf, stem, and panicle neck rot of C. quinoa, panicle neck and stem rot of C. formosanum, and stem rot of C. album. Typical symptoms included water-soaked, rapid soft rotting, and abundant sporulation on the whole panicle necks, stems, and leaves. Based on morphological characteristics, phylogenetic analyses, and pathogenicity tests, the pathogens were identified as Choanephoraceae cucurbitarum. Sporangiola and sporangiospore of C. cucurbitarum germinated at 30 °C and were able to germinate by two h post-inoculation (hpi). The germination rates of sporangiola and sporangiospore significantly increased at 3 to 4 hpi, and the germination rates ranged from 91.53 to 97.67%. The temperature had a significant effect on the pathogenicity of C. cucurbitarum the optimum pathogenic temperatures for stems of C. quinoa, C. formosanum and C. album were 30 °C after one day post-inoculation. Choanephoraceae cucurbitarum could infect white and red quinoa panicle necks between 20 and 30 °C, and the average lesion lengths were 0.21 to 3.62 cm. Among the five tested fungicides (boscalid, dimethomorph, isopyrazam, propiconazole, and tebuconazole), isopyrazam showed higher sensitivity to sporangiola germination of C. cucurbitarum, with an EC50 value of 0.6550 μg/mL. Isopyrazam and tebuconazole strongly inhibited the sporangiospore germination of C. cucurbitarum, which showed EC50 values of 0.4406 and 0.3857 μg/mL. To our knowledge, the present study found for the first time that C. cucurbitarum is a pathogen causing panicle neck of C. formosanum and stem rot of C. formosanum and C. album, while CRC first appeared in the quinoa panicle necks, and gradually expanded to stems and leaves. [ABSTRACT FROM AUTHOR]

Abstract: Cucumber downy mildew, caused by the fungal pathogen Pseudoperonospora cubensis, is a devastating disease in Shanxi Province, China. Fungicides dimethomorph, metalaxyl and fosetyl-aluminium have been used in the disease management. In this study, 143 isolates of P. cubensis were collected from different geographic locations of Shanxi, and examined for their corresponding sensitivities to these fungicides using the leaf disc bioassay method. The EC50 values of the isolates to dimethomorph, metalaxyl and fosetyl-aluminium ranged from 0.0006 to 5.85 μg/ml (mean = 1.01 μg/ml, SD = 1.27), 0.0869 to >500.00 μg/ml (mean = 85.97 μg/ml, SD = 95.61 for the range of 0.0869–458.10 μg/ml) and 0.0066–114.50 μg/ml (mean = 7.89 μg/ml, SD = 15.33), respectively. Over 80% isolates showed the EC50 values to dimethomorph <2.0 μg/ml. This study is the first to report high levels of resistance to metalaxyl in P. cubensis populations in Shanxi Province, and seven isolates were found to be extremely high resistant with EC50 values >500 μg/ml. The most P. cubensis isolates showed a normal distribution of EC50 range between 0 and 18 μg/ml to fosetyl-aluminium, and a few high resistant isolates were found. No cross-resistance was found between any fungicides used in this study. The concept of resistance index (RI) was used to determine the resistance level to each fungicide for sub-population of the pathogen in each district of Shanxi Province. [Copyright &y& Elsevier]