First Report of Damping-Off Caused by Pythium arrhenomanes on Rice in China

Characterisation of Pythium aristosporum Oomycete—A Novel Pathogen Causing Rice Seedling Blight in China

Rice seedling blight is a globally occurring seedling disease caused by multiple pathogens. It is currently the most common disease affecting rice production in northeast China; hence, determining the causal agents, including its biological characteristics, host range, and fungicide efficacy is vital for its effective management. The present study obtained 45 pathogenic isolates from diseased rice seedlings in Suihua, Heilongjiang Province, China. Of these, five pathogens were identified based on their morphology and molecular identification, while 10 oomycete isolates were identified as Pythium aristosporum—the first to be reported in rice seedling blight. Its optimum growth conditions include a temperature of 25 °C, pH 6, and photoperiod of 24 h. Except for soybean (Glycine max (L.) Merr.), black soybean (Glycine max var.), and cucumber (Cucumis sativus L.), P. aristosporum can potentially infect and cause seedling blight on other hosts, such as wheat (Triticum aestivum L.), maize (Zea mays L.), sorghum (Sorghum bicolor (L.) Moench), alfalfa (Medicago sativa L.), oats (Avena sativa L.), and white clover (Trifolium repens L.). Its isolates were found to be highly sensitive to metalaxyl + propamocarb (EC₅₀ = 0.0138 μg/mL) with 84.1% efficacy at 313 μg/mL. These results can serve as the basis for controlling P. aristosporum.

Antimicrobial Activity of Chrysoeriol 7 and Chochlioquinone 9, White-Backed Planthopper-Resistant Compounds, Against Rice Pathogenic Strains

As environmental damage caused by chemical pesticides appears worldwide, eco-friendly agriculture is increasing, and finding eco-friendly pesticide materials has become very important. Chrysoeriol and cochlioquinone, two flavonoids, act as an antibacterial and antioxidant, and increase the resistance of rice to the white-backed planthopper (WBPH). In this experiment, chrysoeriol 7 (C7) and cochlioquinone 9 (C9) were extracted from rice inoculated with the WBPH using MeOH, and cultivars with high extraction efficiency were selected. In addition, the antimicrobial activity of C7 and C9 against various pathogens causing disease in rice was tested. The results show that C7 has antifungal activity against Fusarium graminearum and Pythium graminicola, and C9 show antifungal activity against Cladosporium herbarum, Cladosporium cladosporioides, Gibberella zeae, Fusarium graminearum and Pythium graminicola. When both substances were treated at a concentration of 1000 ppm, they showed high inhibition rates of 62.3% and 36.2% against P.graminicola, respectively. After that, a phylogenetic tree was created to clarify the relationship between the microorganisms whose growth was inhibited and divided into three groups. This result can contribute to the study of biopesticide materials that can control pests and pathogens.

First Report of Pythium sylvaticum Causing Corn Root Rot in Northeastern China

As one of the most planted crops worldwide, corn has continuously increased in importance in China over the last decade. But in recent years, poor stands of corn seedlings have occurred frequently in northeastern China, causing significant economic loss. Mature plants were stunted, the roots were necrotic, and some plants collapsed. We collected soil samples from 5 fields with a history of poor stands of corn seedlings in the Heilongjiang province of China in October 2017. After being planted in the collected soil for 12 days, corn seedlings were uprooted. The pathogen was then isolated as described by Tang et al. (2019). Briefly, the rotted roots were washed in 0.5% NaOCl for 2 min, rinsed in sterile water, and then cut into 1-2 mm segments and placed on cornmeal agar amended with pimaricin (5 μg/ml), ampicillin (250 μg/ml), rifampicin (10 μg/ml), pentachloronitrobenzene (50 μg/ml), and benomyl (10 μg/ml) (PARP+B), which is selective for oomycetes (Jeffers and Martin 1986). After 3 days of incubation in the dark at 25℃, colonies were transferred to 10% V8 juice agar or potato dextrose agar (PDA) and grown for 7 days at 25℃. Based on morphological characteristics, one putative isolate (COPS) was identified as P. sylvaticum (Campbell and Hendrix 1967). On PDA, the culture (COPS) produced creamy white and floccus mycelium. P. sylvaticum (COPS) produced hyphal swellings, but no oogonia or zoospore. Hyphal swellings were globose, terminal, or intercalary, ranging from 12.22-18.55 μm diam. Sequence analysis was performed with the cytochrome c oxidase subunit Ⅱ (COⅡ) gene amplified with primers FM35/FM52 (Martin 2000) and the rDNA ITS amplified with primers DC6/ITS4 (Cooke et al. 2000). For COⅡ gene, BLAST analyses of the 773 bp segments showed 97.93% identity with P. sylvaticum isolate (GenBank Accession No. GU222164.1). For the ITS, BLAST analyses of the 880 bp segments showed 99.89% identity with P. sylvaticum isolate (GenBank Accession No. KY084736.1). Both sequences were submitted to GenBank with accession numbers MK648400 and MK606071 for COⅡ and ITS, respectively. For pathogenicity tests, similar to that described by Ling et al. (2018), four 9-cm petri plates containing 20 mL of 10% V8 juice agar were inoculated with an agar plug (5 mm diam) obtained from a 7-day-old P. sylvaticum culture (COPS) grown on 10% V8 juice agar and then incubated at 25℃ in the dark for 7 days. Nine corn seeds were placed on each plate, after which the plates were filled with 50 g sterilized organic peat substrate. For the controls, seeds were placed on non-inoculated plates of 10% V8 juice agar and filled with 50 g sterilized organic peat substrate. Four replications were inoculated. Plates were maintained in a greenhouse at 23℃. After 14 days, similar symptoms as to those observed in the field were present in the greenhouse, whereas control plants remained symptomless. P. sylvaticum (COPS) was re-isolated from diseased roots as described above, thus confirming Koch's postulates. To our knowledge, this is the first report of P. sylvaticum on corn in China. This pathogen may pose a risk to corn production. The identification of the pathogen will help to develop effective strategies to control the disease.