First Report of Blue Mold or Downy Mildew of Pepper from Nurseries in Southeastern Spain
Authors: Díaz M. M. Guerrero , Lacasa A. , Santos M. , Prieto R. Blanco , Marquina J. C. Tello 
Published: May 2007

During October and November 2001, four nurseries reported severe losses in production of pepper seedlings (Capsicum annuum). Plants were affected with the following symptoms: chlorotic spots on upper leaf surfaces along with a dark brown felt and violet reflections on the undersurface of leaves. Spots became necrotic and expanded to include almost the entire blade prompting defoliation that made the plants worthless. These disease symptoms had not been observed in Spain previously. At least four pepper seedbeds were affected and 1.6 million plants (>40% total production) suffered severe defoliation. California type cultivars that produce yellow fruit (Capino, Vélea, and Fiesta) exhibited more severe symptoms compared with cultivars that produce red fruit (Orlando, Haban, Barbadillo, Ribera, and Requena). Lamuyo type cultivars were not severely diseased. Identification of the parasitic fungus from leaves revealed that Peronospora tabacina was the causal agent of downy mildew in pepper, the same pathogen known as the causal agent of tobacco blue mold. Sexual reproductive structures were not found on pepper leaves. Sporangia and sporangiophores corresponded with those described for P. tabacina (synonym P. hyoscyami f. sp. tabacina) (3). The shape of sporangia was spherical in the youngest sporangia and oval to elliptical in mature sporangia (23 × 16 μm). Sporangia were borne on dendritic, dichotomously branched sporangiophores that branched four to eight times and terminated in curved, acute apices. Sporangiophores occurred singly or in small groups. Pathogenicity tests were conducted on California and Lamuyo type pepper cultivars. An inoculum suspension prepared by washing leaves with distilled water was sprayed on seedlings with four true leaves. Inoculated seedlings were maintained at temperatures of 15 to 25°C (night/day). P. tabacina exhibiting the same morphological features as those described above was observed 15 days later on pepper leaves. This disease on pepper was described first in the United States (1,2) and subsequently reported in Greece and Australia (2). The fungus caused disease in nurseries producing pepper seedlings following production of tobacco seedlings or close to other tobacco plants (1). In Murcia, this downy mildew in pepper appeared in pepper nurseries with supplemental heating and did not appear in those without heating. However, the disease spread when diseased pepper seedlings were moved to nonheated nurseries greenhouses. The inoculum may originate from tobacco plants introduced in the greenhouses for the purpose of propagating parasites of whitefly (Bemisia tabaci). Otherwise, tobacco is not cultivated in the Murcia region.

References: (1) G. M. Armstrong and W. B. Albert. Plant Dis. Rep. 17:37, 1933. (2) D. F. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory, On-line publication. http://nt.ars-grin.gov/fungaldatabases/databaseframe.cfm. August 2, 2002. (3) G. Hall. Peronospora hyoscyami f. sp. tabacina. No. 975 in: Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, Surrey, UK, 1989.