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First Report of Downy Mildew of Opium Poppy Caused by Peronospora arborescens in Spain

Opium poppy (Papaver somniferum) is an economically important pharmaceutical crop in Spain with approximately 7,400 ha cultivated annually. In the spring of 2004, severe attacks by a new foliar disease were observed approximately 500 km apart in commercial opium poppy fields in the Castilla-La Mancha and Andalusia regions of central and southern Spain, respectively. The incidence of affected fields ranged from 40 to 50%, and incidence of diseased plants ranged from 20 to 30%. Initial disease symptoms included irregularly shaped, chlorotic-to-light yellow leaf lesions (ranging in size from 0.5 to 4 cm). Affected tissues curled, thickened, and became deformed and necrotic as disease developed. Lesions expanded in size and often coalesced, eventually giving rise to large necrotic areas in leaves or death of entire leaves. In wet weather or conditions of high relative humidity, a dense felt of sporangiophores with sporangia was produced on the abaxial leaf surface and occasionally on the adaxial surface. Microscopic observations revealed sporangiophores branching dichotomically at least four to six times, ending with sterigmata bearing single sporangia. Sporangia were hyaline, elliptical to spherical in shape, and measured 18 to 24 × 14 to 18 μm (average 19 ± 1.2 × 15 ± 1.6 μm). Occasionally, oospores formed in necrotic leaf tissues. Oospores were dark brown (the surface was irregularly ridged) and measured 36 to 46 μm in diameter (average 39 ± 4.4 μm). The oospore wall was 3 to 11 μm thick. On the basis of the observed morphological features of six symptomatic plant samples from fields at Castilla-La Mancha and Andalusia regions, we identified the pathogen as Peronospora arborescens (1). Pathogenicity was confirmed by inoculating 4- to 6-week-old opium poppy plants (cv. nigrum) with an isolate collected from a field in Ecija, Andalusia. Seed of test plants was surface disinfested and germinated under sterile conditions. Plants were sprayed with a suspension of 1 to 5 × 10⁵ sporangia per ml in sterile distilled water. Plants sprayed with sterile water served as controls. There were five replicate plants per treatment. Plants were enclosed in sealed plastic bags and kept in the dark for 24 h. This was followed by incubation in a growth chamber at 21°C, 60 to 90% relative humidity, and a 12-h photoperiod (fluorescent light: 360 μE·m^-2·s^-1). After 5 to 7 days, typical downy mildew symptoms developed in inoculated plants. All control plants remained symptomless. Sporulation by the pathogen on symptomatic leaves occurred when affected plants were sprayed with water, enclosed in sealed plastic bags, and incubated at 21°C in the dark for 24 h. To our knowledge, this is the first report of P. arborescens infecting opium poppy in Spain. Infestations of poppy weeds (Papaver rhoeas) and wild Papaver somniferum were also observed in affected opium poppy fields, which may bear importance in the epidemiology of the disease as alternative hosts for inoculum increase and survival of P. arborescens under field conditions.

References: (1) S. M. Francis. No. 686 in: Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, Surrey, UK, 1981.