Characterization of <i>Alternaria</i> Species Associated with Leaf Spot Disease of <i>Armoracia rusticana</i> in Serbia

Abstract

Leaf spot diseases caused by <i>Alternaria</i> species have been reported worldwide in plants in the Brassicaceae family. However, there is little information on <i>Alternaria</i> species causing diseases in horseradish. In the present study, 89 <i>Alternaria</i> spp. isolates from <i>Armoracia rusticana</i>, sampled from nine districts in Serbia, were characterized based on their morphology, physiology, and molecular markers. Morphological characterization and molecular analyses based on <i>ITS</i>, <i>GAPDH</i>, <i>Alt a 1</i>, and <i>PM-ATP</i> sequences identified three distinct species associated with the disease: <i>Alternaria brassicae</i>, <i>A. brassicicola</i>, and <i>A. alternata</i>. For all species, growth and sporulation rates at 0, 5, 10, 15, 20, 25, 30, 35, and 40°C showed a quadratic response, with <i>A. alternata</i> having the widest temperature optimum (20 to 30°C) while <i>A. brassicicola</i> had higher optimum temperatures (20 to 25°C) than <i>A. brassicae</i> (15 to 20°C). To gain a better understanding of the pathogenicity of these species, the influence of leaf age, host susceptibility, and ability to infect artificially wounded and nonwounded leaves were tested. The pathogenicity test identified <i>A. brassicicola</i> and <i>A. brassicae</i> as the main causal agents of horseradish leaf spot disease. Results indicated that young and intact leaves of horseradish and cabbage were less susceptible to infection and also suggested the potential for cross-infection between these two hosts. Haplotype networks showed haplotype uniformity for <i>A. brassicae</i>, two haplotype groups of <i>A. brassicicola</i>, and eight haplotype groups of <i>A. alternata</i> in Serbia and suggest the possible association of some haplotypes with the geographic area. This study is the first to investigate Alternaria leaf spot disease on <i>A. rusticana</i> in Serbia and is the first record of <i>A. brassicicola</i> on horseradish in this country.