In March of 2014, blackberry (Rubus fruticosus) greenhouse seedlings with leaf symptoms that included yellowing on the leaf surface and reddish brown angular lesions with necrotic centers and chlorotic margins were detected in the University of Arkansas Fruit Research Station in Clarksville, AR (35°32.065′ N, 93°24.3564′ W). Symptoms were observed on multiple blackberries in the greenhouse, with an estimated prevalence of 25%. Three symptomatic samples of the affected plants were submitted to the Plant Health Clinic in Fayetteville, AR, for diagnosis. Sporulation was observed on the underside of the symptomatic leaf tissue. Hyaline sporangiophores were observed emerging from stomata on the undersides of leaves, 295 to 620 × 4 to 6 μm with long, straight trunks and were branched 3 to 4 times with bifurcated tips, with a length of 5 to 23 μm. Typically, one branch of each pair curved inward and one reflexed. Sporangiophores ended with sporangia that were round or slightly ovoid, colorless to yellowish-brown, and 14 to 22 × 11 to 18 μm. The causal agent was morphologically identified as Peronospora sparsa Berk (1,2). The identification was confirmed using a molecular method directly from plant tissue. DNA was extracted from two samples (~3 × 3 mm) from each of three symptomatic leaves, followed by PCR amplification using P. sparsa-specific rDNA-ITS region primers P1: 5′-CACGTGAACCGTATCAACC-3′ and P2: 5′-GATAGGGCTTGCCCAGTAG-3′ (GenBank Accession No. Y15816) (4). DNA amplification was successful, resulting in a product of 94 bp, confirming that P. sparsa was present in the symptomatic blackberry tissue. Sporulating leaf tissue was laid on the underside of surface sterilized blackberry leaves from three plants with a similar genetic background and incubated at 17°C with a 12-h photoperiod in a moist chamber. Sporangiophores and sporangia developed on the underside of lesions on previously uninfected leaves 16 days after inoculation. As a control, leaves from the same three plants were surface sterilized and placed in a moist chamber and incubated at 17°C with a 12-h photoperiod and examined 16 days later. No lesions or sporulation developed on the controls. Previously, P. sparsa has been reported on blackberry in California (3) and Mexico (5). To our knowledge, this is the first report of P. sparsa causing downy mildew on blackberry in Arkansas.
References: (1) M. J. Berkeley. Gardeners' Chronicle 14:307, 1861. (2) M. A. Ellis. Page 15 in: Compendium of Raspberry and Blackberry Diseases and Insects. APS Press, St. Paul, MN, 1989. (3) D. Farr et al. Page 486 in: Fungi on Plants and Plant Products in the United States. APS Press, St. Paul, MN, 1989. (4) A. Hukkanen et al. Eur. J. Plant Pathol. 116:225, 2006. (5) A. Rebollar-Alviter et al. Plant Dis. 93:674, 2009.
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