Molecular detection of Peronospora sparsa in sources of primary inoculum and components of resistance in wild blackberry species

Abstract

Downy mildew caused by Peronospora sparsa is a major disease of blackberry (Rubus sp.) worldwide. The objectives of this research were a) to detect potential sources of primary inoculum of downy mildew of cultivated blackberry and b) to determine some components of resistance to P. sparsa in 4 wild blackberry species from Michoacan Mexico. A region-wide sampling of symptomatic and asymptomatic blackberry plants from nurseries, commercial plots, wild blackberries (R. adenotrichos) and rose plants (Rosa sp.), was conducted. Nested-PCR assays with PS3/PS1 primers in the first reaction followed by a second amplification with PR3/PR4 primers, amplifying a final fragment of 477 bp of the ITS region were used, and then sequenced in both directions. Detached leaflets of 4 common wild blackberry species were inoculated with P. sparsa sporangia under controlled conditions. Sporangiophores (SP) were recorded over a period of 18 days and oospores (OO) were counted 21 days after inoculation; the latent period (LP) and disease severity (DS) were also estimated. Phylogenetic reconstruction with Bayesian inference confirmed the presence of P. sparsa in all collected samples. Rubus adenotrichos, R. pringlei and R. sapidus were high inoculum producers, whereas R. cymosus allowed neither SP nor OO production and had the longest LP and lowest DS. Together, these results indicate that the primary source of inoculum for commercial plantings is mainly the blackberry plants from nurseries, and potentially 3 species of wild blackberries. Rubus cymosus has potential to be used in blackberry breeding programs as a source of resistance to P. sparsa.