Morphologic and Pathometric Characterization of the Asian Soybean Rust (Phakopsora pachyrhizi) on Kudzu (Pueraria lobata) in Argentina.

Abstract

Asian soybean rust (ASR) is a very important disease caused by Phakopsora pachyrhizi. The disease has emerged as a major threat to soybean production in South America since 2001. During the 2003-2004 growing season, P. pachyrhizi spread rapidly throughout most soybean-growing areas of northwestern and northeastern Argentina (1). One widespread naturalized host in the northeastern part of the country is kudzu (Pueraria lobata). Plants of severely infected kudzu were sampled during January 2005 in Cerro Azul (29°29'S Misiones Province) to quantify P. pachyrhizi infection and morphologically characterize the fungus in leaves. The number of lesions, uredinia per cm2, and uredinia per lesion were recorded from the undersides of 50 leaflets that were visually showing rust symptoms. The average number of lesions and uredinia per cm2 was 14 (4 to 22), and 24 (5 to 78), respectively. The number of uredinia per lesion was 3 (1 to 10). Twenty leaflets from the lower canopy averaged 55 (42 to 78) uredinia per cm2. The average size of urediniospores was 18.4 μm wide (12.5 to 22.5) and 22.7 μm long (17.5 to 26.3). Although important epidemics of ASR have not been registered on soybean crops in January (2) because of adverse conditions, the fungus was observed on kudzu plants. To our knowledge, this is the first report of morphologic and pathometric characterization of P. pachyrhizi on kudzu in Argentina.