Journal of Plant Pathology Archive

Abstract:

In Mexico, the state of Mexico is the main producer of greenhouse-grown roses (Rosa spp.) with ca. 650 ha in production. During the seasons of high relative humidity and mild temperatures, yellow to red or purplish rectan- gular spots delimited by veins were observed on the leaves, whereas on stems and sepals the spots were rounded and reddish to purple. Incidence ranged from 27 to 74% and, in severe cases, plants were defoliated. The pathogen associ- ated with the disease was identified as Peronospora sparsa Berk. based on: the symptomatology shown by leaves and stems of six rose cultivars (Conga, Grande Classe, Opera, Polo, Titanic and Vendela), that were collected during July 2012 in Villa Guerrero (18.97 N, 99.65 W), state of Mexico; sequencing products of the ITS region of rDNA amplified by nested PCR (Lindqvist et al., 1998) using primers PS3 and PS1 (Aegerter et al., 2002) and morphological characters (Horst and Cloyd, 2007)., Hyaline sporangiophores emerging from stomata were observed on the underside of the leaves, which were 300-625 μm in size, branched 3-4 times, with bifurcated tips 5-22 μm in size. Sporangia were hyaline, sub- globose to ellipsoid possessed occasionally a tiny stalk and measured 14-22 × 13-20 μm. No structures were found on stems. The sequence of ITS-rDNA region (GenBank acces- sion No. KF541660) showed 99.1% of similarity index with P. sparsa on Rosa multiflora (AY608610). In the region of study, disease management consists mainly of fungicide ap- plications every 5-7 days, failing which the pathogen will af- fect flower bud aperture and size, thus reducing commercial value of the crop. To our knowledge, this is the first report in Mexico of downy mildew of roses induced by P. sparsa.