First report of powdery mildew on Astragalus sinicus (Chinese milk vetch) caused by Erysiphe trifoliorum in China

Astragalus sinicus L., (Chinese milk vetch) is a traditional leguminous green manure that plays a significant role in maintaining paddy soil fertility to enhance yield and the quality of rice in China. It is also found in gardens, roadsides, farms, fields, riverbanks, open wastelands, and is often used as livestock feed. From February 2019 to 2021, severe powdery mildew infections were observed on hundreds of A. sinicus grown in gardens and at roadsides of Fuzhou city, China. The disease incidence was up to 100% on leaves and stems of A. sinicus. White superficial fungal colonies (circular to irregular patches) were present on both sides of the leaves. Hyphae were flexuous to straight, branched, 4 to 8 µm in width, and septate. Hyphal appressoria were lobulate and solitary or in opposite pairs. Conidiophores were erect and straight, hyaline, and 60 to 120 × 8 to 10 µm (n=30). Foot cell was cylindrical, straight to slightly curved, 22 to 38 × 8 to 10 µm, followed by two to three shorter cells. Conidia were cylindrical-oval to doliiform, 30 to 48 × 13.5 to 24 μm with a length/width ratio of 1.6 to 2.4 (n = 30), formed singly, and without fibrosin bodies. Conidial germ tubes were produced subterminal position. No chasmothecia were found in the collected samples. The morphological characteristics of asexual structures were consistent with the descriptions of E. trifoliorum (Wallr.) U. Braun in Braun and Cook (2012). To verify the identification of the pathogen, the ITS and the part of large subunit (LSU) rDNA gene of the isolates were amplified using ITS1/ITS4 and LSU1/ LSU2 primers (Scholin et al. 1994 and White et al. 1990, respectively) and sequences were deposited in GenBank (ITS: MZ021332, MZ021333; LSU: MZ021334, MZ021335). In BLASTn searches, the ITS and LSU sequences were 99 to 100% identical with those of E. trifoliorum parasitic on Lathyrus magellanicus (LC010015), Medicago littoralis (LC270860), Melilotus officinalis (LC009924) and Trifolium spp., (MN216308, KY660821), as well as E. baeumleri (Bradshaw et al. 2021) on Vicia nigricans (LC010014). Pathogenicity test was performed by gently pressing a diseased leaf onto 10 young leaves of three healthy potted plants, while three non-inoculated plants were used as controls. All plants were maintained in a greenhouse at 20 to 25°C, without humidity control, and natural light. Symptoms developed 7 days after inoculation, whereas the control leaves remained symptomless. The morphology of the fungus on the inoculated leaves was identical to that observed on the originally diseased leaves. Powdery mildew on A. sinicus has been reported as E. pisi and E. polygoni from Korea and China (Shin, 2000; Tai 1979), respectively. Amano (1986) listed E. pisi and Microsphaera astragali (now E. astragali) on A. sinicus from China and Japan. To our knowledge, this is the first report of powdery mildew caused by E. trifoliorum on A. sinicus in China and in general. E. astragali is the most common and widespread powdery mildew species on Astragalus spp. (Braun and Cook 2012) and would be expected on A. sinicus, but this species is genetically clearly different from E. trifoliorum (Bradshaw et al. 2021). The E. trifoliorum complex (clade) is composed of several morphologically well-distinguishable species, besides E. trifoliorum also including E. baeumleri (on Vicia spp.), E. hyperici (on Hypericum spp.), and E. euonymi (on Euonymus spp.), but based on a combination of sequence plus host identity, the collection on A. sinicus can be assigned to E. trifoliorum (Bradshaw et al. 2021). The information in this study extended the host range of E. trifoliorum as well as future studies on A. sinicus in relation to powdery mildew outbreaks in China. References: Amano (Hirata), K. 1986. Host Range and Geographical Distribution of the Powdery Mildew Fungi. Japan Scientific Societies Press, Tokyo, 741 pp. Bradshaw, M., et al. 2021. Mycologia. (In press) Braun, U., Cook, R. T. A. 2012. Taxonomic Manual of the Erysiphales (Powdery Mildews), CBS Biodiversity Series No. 11. CBS, Utrecht, the Netherlands. Scholin, C. A., et al. 1994. J. Phycol. 30:999. Shin, H.D. 2000. Erysiphaceae of Korea. National Institute of Agricultural Science and Technology, Suwon, Korea, 320 pp. Tai, F.L. 1979. Sylloge Fungorum Sinicorum. Sci. Press, Acad. Sin., Peking, 1527 pp. White, T. J., et al. 1990. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA.

Flower and pod development, grain-setting characteristics and grain yield in Chinese milk vetch (Astragalus sinicus L.) in response to pre-anthesis foliar application of paclobutrazol

The number of grains per unit land area is the most important grain yield component in Chinese milk vetch. Flower and pod survival seem to be critical determinants of grain number, which is related to the number of fertile flowers and pods during the anthesis period. Flower and pod growth are frequently considered the key determinants to establish grain number. The objective of this study was to explore the influences of paclobutrazol on flower and pod development, grain-setting characteristics and grain yield in Chinese milk vetch under different concentrations of foliar spray and try to explore the physiological regulatory mechanisms. Field experiments were carried out during the 2017–2018 and 2018–2019 growing seasons at the Dayuzhuang experimental field. The experiment involved the Chinese milk vetch cultivar “Xinzi No. 1” and six levels of foliar application of paclobutrazol, 0, 200, 300, 400, 500, and 600 mg L-1, in treatments CK, T1, T2, T3, T4, and T5, respectively. Foliar spray was applied once, at the squaring stage. In comparison with the CK treatment, all of the paclobutrazol treatments yielded, to various degrees, increased values of the number of inflorescences per unit area, number of pods per unit area, grain-setting rate of pods, and number of grains per pod in all six inflorescence layers, with the largest increases observed in the T3 treatment. In the T3 treatment compared with the CK treatment, from the first to sixth inflorescence layers, the number of inflorescences per unit area was increased by 34.07–58.97%, the number of pods per unit area was increased by 39.69–68.35%, the grain number per pod was increased by 44.31–53.69%, and the grain-setting rate of pods was increased by 1.84–4.89%. An analysis of yield composition revealed that the paclobutrazol spray treatment had little impact on the grain weight of Chinese milk vetch. The correlations between the concentration of paclobutrazol spray and the grain yield of Chinese milk vetch reached a significant level. Grain yield was highest at the paclobutrazol concentration of 373.10 mg/L. The inflorescence contents of gibberellic acid 3 (GA3), indole-3-acetic acid (IAA), and abscisic acid (ABA) were reduced, whereas that of cytokinin (CTK) was increased, by foliar application of paclobutrazol (400 mg L-1, T3 treatment) relative to CK treatment during the stages of flowers and pods developing into grains.

Do Fallow Season Cover Crops Increase N2O or CH4 Emission from Paddy Soils in the Mono-Rice Cropping System?

Cover crop management during the fallow season may play a relevant role in improving crop productivity and soil quality, by increasing nitrogen (N) and soil organic carbon (SOC) accumulation, but has the possibility of increasing greenhouse gas (GHG) emissions from the soil. A year-long consistency experiment was conducted to examine the effects of various winter covering crops on annual nitrous oxide (N2O) together with methane (CH4) emissions in the mono-rice planting system, including direct emissions in the cover crop period and the effects of incorporating these crops on gaseous emissions during the forthcoming rice (Oryza Sativa L.) growing period, to improve the development of winter fallow paddy field with covering crops and to assess rice cultivation patterns. The experiment included three treatments: Chinese milk vetch-rice (Astragalus sinicus L.) with cover crop residue returned (T1), ryegrass (Lolium multiflorum L.)-rice with cover crop residue returned (T2), and rice with winter fallow (CK). Compared with CK, the two winter cover crop treatments significantly increased rice yield, soil organic carbon (SOC) and total nitrogen (TN) by 6.9–14.5%, 0.8–2.1% and 3.4–5.4%, respectively. In all cases, the fluxes of CH4 and N2O could increase with the incorporation of N fertilizer application and cover crop residues. Short-term peaks of these two gas fluxes were monitored after all crop residues were incorporated in the soil preparation period, the early vegetative growth period and the midseason drainage period. The winter cover crop residue application greatly enhanced CH4 and N2O cumulative emissions compared with CK (by 193.6–226.5% and 37.5–43.7%, respectively) during rice growing season and intercropping period. Meanwhile, the mean values of global warming potentials (GWPs) from paddy fields with different cropping crops were T2 > T1 > CK. Considering the advantages of crop productivity together with environmental safety and soil quality, Chinese milk vetch-rice with cover crop residue returned would be the most practicable and sustainable cultivation pattern for the mono-rice cropping systems.