Roberts P D, Raid R N, Harmon P F, Jordan S A, Palmateer A J
Southwest Florida Research and Education Center, University of Florida, Immokalee.
Everglades Research and Education Center, University of Florida, Belle Glade.
Plant Dis. 2009 Feb;93(2):199. doi: 10.1094/PDIS-93-2-0199B.
Basil is grown as a specialty crop in greenhouse and field production in Florida and other regions of the United States. Downy mildew on basil (Ocimum basilicum) was detected from four production sites (Collier, Hendry, Miami-Dade, and Palm Beach counties) in south Florida in the fall of 2007, and within months, it was also found in west-central north Florida (Hillsborough County). Incidence reached nearly 100% on some of the affected crops and caused complete yield losses on basil grown both in the field for fresh market and potted herbs market. Symptoms developed during transit on basil that appeared symptomless at harvest. Symptoms initially appeared as yellowing on the lower leaves that was typically delineated by the veins, although in some cases the entire leaf area of the leaf surface was affected. A gray, fuzzy growth was apparent on the abaxial leaf surface. Microscopic observation detected dichotomous branching, hyaline sporangiophores (220 to 750 × 4 to 9 μm) bearing single sporangia. Sporangia were light brown, ovoid to slightly ellipsoid, and measured 14 to 15 × 15 to 18 μm. Oospores were not observed. Leaves of potted basil plants and coleus (Solenostemon scutellarioides) were inoculated with a suspension containing 1 × 10 sporangia/ml and sprayed till runoff (approximately 15 ml per plant) with a hand-held pressurized aerosol canister. Plants were covered with a plastic bag for 24 h and maintained in the greenhouse under ambient conditions. Noninoculated plants served as controls. After 7 days, symptoms typical of downy mildew occurred only on the inoculated basil plants and sporulation was confirmed microscopically. The internal transcribed spacer regions of an isolate collected in Hendry County were sequenced bidirectionally. The consensus sequence was deposited into GenBank (Accession No. FJ346561). Sequence data matched (100% homology) with a Peronospora sp. reported on sweet basil in Switzerland (GenBank Accession No. AY884605) and was similar (99% homology) to an isolate (GenBank Accession No. DQ523586) reported on coleus, although inoculation to coleus failed to confirm pathogenicity on this host. The sequence data also distinguished the isolate from P. lamii (87% homology) previously reported to occur on basil. The pathogen was identified as a Peronospora sp. based on morphological characteristics and sequencing homology (1-3). References: (1) L. Belbahri et al. Mycol. Res. 109:1276, 2005. (2) S. Francis. CMI Descriptions of Pathogenic Fungi and Bacteria. No. 688. CMI, Kew, England, 1981. (3) A. McLeod et al. Plant Dis. 90:1115, 2006.
罗勒在美国佛罗里达州及其他地区作为特色作物在温室和田间种植。2007年秋季，在佛罗里达州南部的四个生产基地（科利尔县、亨德里县、迈阿密-戴德县和棕榈滩县）检测到罗勒（Ocimum basilicum）上的霜霉病，数月内，在佛罗里达州中北部西部（希尔斯伯勒县）也发现了该病。在一些受影响的作物上发病率接近100%，导致用于新鲜市场的田间种植罗勒和盆栽香草市场的罗勒完全绝收。症状在收获时看似无症状的罗勒运输过程中出现。症状最初表现为下部叶片发黄，通常由叶脉界定，不过在某些情况下，叶片整个叶面区域都会受到影响。叶片背面有明显的灰色绒毛状生长物。显微镜观察发现二叉状分支、具单个孢子囊的透明孢子囊梗（220至750×4至9μm）。孢子囊浅褐色，卵形至稍椭圆形，大小为14至15×15至18μm。未观察到卵孢子。用含有1×10个孢子囊/毫升的悬浮液接种盆栽罗勒植株和彩叶草（Solenostemon scutellarioides）的叶片，并用手持式加压气雾剂罐喷雾至径流（每株植物约15毫升）。植株用塑料袋覆盖24小时，并在温室环境条件下养护。未接种的植株作为对照。7天后，典型霜霉病症状仅出现在接种的罗勒植株上，且通过显微镜确认有孢子形成。对在亨德里县采集的一个分离株的内部转录间隔区进行双向测序。一致序列存入GenBank（登录号FJ346561）。序列数据与瑞士报道于甜罗勒上的一种霜霉属真菌（GenBank登录号AY884605）匹配（100%同源），与报道于彩叶草上的一个分离株（GenBank登录号DQ523586）相似（99%同源），不过对接种彩叶草未能确认该寄主上的致病性。序列数据也将该分离株与先前报道于罗勒上的lamii霜霉（87%同源）区分开来。根据形态特征和测序同源性，该病原菌被鉴定为一种霜霉属真菌（1-3）。参考文献：（1）L. Belbahri等人，《真菌研究》109:1276，2005年。（2）S. Francis，《英联邦真菌研究所病原真菌和细菌描述》第688号。英联邦真菌研究所，英国基尤，1981年。（3）A. McLeod等人，《植物病害》90:1115，2006年。
