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 × 104 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 × 106 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.
