Stemphylium Leaf Blight of Garlic (Allium sativum) in Spain: Taxonomy and In Vitro Fungicide Response

Abstract

The most serious aerial disease of garlic is leaf blight caused by <i>Stemphylium</i> spp. Geographical variation in the causal agent of this disease is indicated. <i>Stemphylium</i> vesicarium has been reported in Spain, whereas <i>S. solani</i> is the most prevalent species recorded in China. In this study, <i>Stemphylium</i> isolates were obtained from symptomatic garlic plants sampled from the main Spanish production areas. Sequence data for the ITS1-5.8S-ITS2 region enabled assignation of the isolates to the <i>Pleospora herbarum</i> complex and clearly distinguished the isolates from <i>S. solani</i>. Conidial morphology of the isolates corresponded to that of <i>S. vesicarium</i> and clearly discriminated them from <i>S. alfalfae</i> and <i>S. herbarum</i> on the basis of the size and septation pattern of mature conidia. Conidial morphology as well as conidial length, width and length:width ratio also allowed the Spanish isolates to be distinguished from <i>S. botryosum</i> and <i>S. herbarum</i>. Control of leaf blight of garlic is not well established. Few studies are available regarding the effectiveness of chemical treatments to reduce <i>Stemphylium</i> spp. incidence on garlic. The effectiveness of nine fungicides of different chemical groups to reduce <i>Stemphylium</i> mycelial growth <i>in vitro</i> was tested. Boscalid + pyraclostrobin (group name, succinate dehydrogenase inhibitors + quinone outside inhibitors), iprodione (dicar-boximide), and prochloraz (demethylation inhibitors) were highly effective at reducing mycelial growth in <i>S. vesicarium</i> with EC₅₀ values less than 5 ppm. In general, the effectiveness of the fungicide was enhanced with increasing dosage.