First Report of Downy Mildew on Basil (Ocimum basilicum) in China

Abstract

HomePlant DiseaseVol. 99, No. 11First Report of Downy Mildew on Basil (Ocimum basilicum) in China PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Downy Mildew on Basil (Ocimum basilicum) in ChinaX. Y. Kong, S. Wang, S. L. Wan, C. L. Xiao, F. Luo, and Y. LiuX. Y. KongSearch for more papers by this author, S. WangSearch for more papers by this author, S. L. WanSearch for more papers by this author, C. L. XiaoSearch for more papers by this author, F. LuoSearch for more papers by this author, and Y. LiuSearch for more papers by this authorAffiliationsAuthors and Affiliations X. Y. Kong S. Wang S. L. Wan C. L. Xiao F. Luo Y. Liu , Sanya Science & Technology Academy for Crop Winter Multiplication, Winter Multiplication Crop and Plant Protection Research Center, Sanya Key Laboratory for Pest Management of Winter Cucurbits and Vegetables, Sanya 57200, Hainan, China. Published Online:26 Aug 2015https://doi.org/10.1094/PDIS-01-15-0077-PDNAboutSectionsSupplemental ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat In December 2014, Sanya City (18°N) of Hainan Province, the southernmost city of China, suffered an onslaught of downy mildew over large basil-growing areas. According to our investigation, almost 40% of the plants in the region (70 km2) were infected by the disease, which spread very rapidly. Given favorable conditions, the disease rate could reach 100% three to five days after the attack, making prevention and control extremely difficult. The seedling and adult plant stages of basil are subject to infection by the disease. In the early stages of the disease, the infected parts of the green leaf blades turn yellow, and then brown spots develop at the leaf margins. In the later stages of the disease, the spots spread along the margins or main veins of the leaves into larger areas, amassing as irregular, brown, necrotic spots that can consequentially wither the blades, and more seriously, cover more than half of the leaves. Meanwhile, the leaf back can be completely covered with a grayish-black layer of mildew. In the final stages of the disease, all the leaves of the plant will be infected, whereupon they will turn brown. When examined under a microscope, large amounts of sporangiophores and sporangia were found in the samples of the mildew layer collected from the infected leaves. Sporangiophores were hyaline, 197.0 to 595.3 μm (average 370.6 μm) in length, branched two to seven times, and ending with two slightly curved branchlets having a single sporangium at the tip of each branchlet. The longer branchlets measured 14.3 to 38.3 μm in length (average 23.2 μm), while the shorter ones were 2.4 to 22.1 μm in length (average 8.1 μm). Sporangia were oval or circular, ranged from brown to dark brown, measured 19.8 to 33 μm in length (average 27.6 μm) and 15.1 to 29.3 μm in width (averaging 23.0 μm) (a length/width ratio of 1.2), and produced germ tubes by direct germination. Based on these morphological characteristics, the causal agent was identified as Peronospora belbahrii Thines (Belbahri et al. 2005; Thines et al. 2009). Furthermore, genomic DNA was extracted from infected plant tissue according to Dellaporta et al. (1983). The complete ITS rDNA region was amplified and sequenced using primers ITS5 and ITS4 (Nagy and Horvath 2011). The consensus sequences were deposited in GenBank (Accession No. KP657570), and a BLAST analysis in the NCBI database revealed 99% similarity to all of the P. belbahrii sequences previously reported in regard to basil (EU863410 and KJ960193). For the preparation of the inoculum, infected leaves were flooded with sterile distilled water to collect the sporangia. The sporangia were washed once with sterile distilled water and the suspension was diluted to 1 × 105 sporangia/ml. The sporangia suspension was sprayed onto 15 healthy leaves grown in the same period for inoculation, and sterilized distilled water was sprayed on 15 healthy leaves serving as controls. The experiment was repeated 10 times. Every replicate was placed in a separate petri dish under 12 h of light at 20°C, and 90 to 95% relative humidity. Five days after inoculation, the leaves inoculated with the suspension showed typical symptoms of the disease while the control leaves showed no symptoms. Infected leaves under the microscope showed large amounts of P. belbahrii fungi. Therefore, all of the results suggest that the downy mildew on basil was caused by the fungus P. belbahrii. To our knowledge, this is first report of downy mildew on basil in China.