Koike S T, Ochoa O E
University of California Cooperative Extension, Salinas, 93901.
Department of Plant Sciences, University of California, Davis, 95616.
Plant Dis. 2007 Mar;91(3):326. doi: 10.1094/PDIS-91-3-0326A.
In 2006, field-planted strawflower (Helichrysum bracteatum (Venten.) Andr.) grown for cutflowers in coastal San Mateo County was severely infected with a downy mildew disease. Initial symptoms consisted of irregularly shaped chlorotic lesions. Lesions were typically vein delimited, and in particular, did not cross the central longitudinal vein of the lanceolate leaves. Lesions were large and could exceed 6 cm long. Profuse white sporulation developed on the abaxial sides of the lesions. As disease progressed, lesions became gray brown and necrotic. Cutflower stems having symptomatic foliage were unmarketable. Hyaline conidiophores emerged from stomata, branched dichotomously, and had branch tips ending in swollen vesicles bearing sterigmata. Conidia were hyaline and ovoid to globose in shape. The pathogen was identified as Bremia lactucae Regel. To establish pathogenicity, strawflower plants were spray inoculated with a conidial suspension of 1 × 10 conidia/ml, incubated for 24 h in a dew chamber (18 to 20°C), and then maintained in a greenhouse (22 to 24°C). After 14 days, symptoms and signs of downy mildew developed on inoculated plants and the pathogen was confirmed to be B. lactucae. Untreated control plants did not develop downy mildew. Because lettuce (Lactuca sativa L.) is grown extensively in this coastal region, we investigated the pathogenicity of B. lactucae from strawflower on lettuce by inoculating a lettuce downy mildew differential series that includes 20 lettuce lines, four wild L. serriola lines, and strawflower. A conidial suspension of 1 × 10 conidia/ml was applied to all plants; plants were then incubated in a growth chamber with irradiation at 15°C. After 14 days, only the strawflower plants showed downy mildew lesions and sporulation. Collected from diseased field-grown strawflower, 100 downy mildew conidia were measured and had dimensions of 24.4 × 17.4 μm. These conidia were significantly longer than the B. lactuca conidia collected from lettuce (21.8 × 18.1 μm) grown in a nearby area (LSD [P = 0.05] = 2.2), though dimensions were within the reported length range for B. lactucae (12 to 31 μm) (2). To our knowledge, this is the first report of downy mildew caused by B. lactucae on strawflower in California. Inoculation studies indicate that these strawflower and lettuce pathogens from California likely belong to distinct formae speciales (2), though a B. lactucae isolate in Italy was able to infect both hosts (1). The pathogen has been reported on strawflower in Florida, Egypt, Italy, and the United Kingdom (1). References: (1) A. Garibaldi et al. Plant Dis. 87:315, 2003. (2) W. M. Morgan. CMI Descriptions of Pathogenic Fungi and Bacteria. No. 682. CAB International, Wallingford, UK, 1981.
2006年，在沿海的圣马特奥县用于切花种植的露地麦杆菊（Helichrysum bracteatum (Venten.) Andr.）受到霜霉病的严重感染。最初症状表现为形状不规则的褪绿病斑。病斑通常由叶脉界定，特别是不会穿过披针形叶片的中央纵脉。病斑较大，长度可能超过6厘米。在病斑的背面产生大量白色霉层。随着病情发展，病斑变为灰棕色并坏死。带有症状叶片的切花茎无法进入市场。透明的分生孢子梗从气孔中伸出，二叉状分枝，分枝顶端有膨大的泡囊，泡囊上着生小梗。分生孢子透明，形状为卵形至球形。该病原菌被鉴定为莴苣盘梗霉（Bremia lactucae Regel）。为确定致病性，用浓度为1×10个分生孢子/毫升的分生孢子悬浮液对麦杆菊植株进行喷雾接种，在露室（18至20°C）中培养24小时，然后置于温室（22至24°C）中。14天后，接种植株上出现霜霉病的症状和病征，病原菌被确认为莴苣盘梗霉。未处理的对照植株未发生霜霉病。由于生菜（Lactuca sativa L.）在该沿海地区广泛种植，我们通过接种一个包括20个生菜品系、4个野生刺莴苣（L. serriola）品系和麦杆菊的生菜霜霉病鉴别系列，研究了来自麦杆菊的莴苣盘梗霉对生菜的致病性。将浓度为1×10个分生孢子/毫升的分生孢子悬浮液施用于所有植株；然后将植株置于温度为15°C、有光照的生长室中。14天后，只有麦杆菊植株出现霜霉病病斑和霉层。从患病的田间种植的麦杆菊上采集了100个霜霉病分生孢子进行测量，其尺寸为24.4×17.4微米。这些分生孢子明显长于从附近地区种植的生菜上采集的莴苣盘梗霉分生孢子（21.8×18.1微米）（最小显著差数法[P = 0.05] = 2.2），尽管其尺寸在报道的莴苣盘梗霉长度范围内（12至31微米）（2）。据我们所知，这是加利福尼亚州麦杆菊上由莴苣盘梗霉引起的霜霉病的首次报道。接种研究表明，来自加利福尼亚州的这些麦杆菊和生菜病原菌可能属于不同的专化型（2），尽管意大利的一个莴苣盘梗霉分离株能够感染这两种寄主（1）。该病原菌已在佛罗里达州、埃及、意大利和英国的麦杆菊上有报道（1）。参考文献：（1）A. Garibaldi等人，《植物病害》87:315，2003年。（2）W. M. Morgan，《英联邦真菌研究所病原真菌和细菌描述》第682号。英国沃灵福德的国际应用生物科学中心，1981年。
