Crop Protection 25 (2006) 195–201 Pathogenic diversity of Sclerotium rolfsii isolates from Mexico, and potential control of southern blight through solarization and organic amendments H.E. Flores-Moctezuma, R. Montes-Belmont à , A. Jime´nez-Pe´rez, R. Nava-Jua´rez Centro de Desarrollo de Productos Bio´ticos. Instituto Polite´cnico Nacional. Apartado postal 24. C.P. 62731 Yautepec, Morelos, Mexico Received 30 April 2004; received in revised form 5 April 2005; accepted 5 April 2005 Abstract Pathogenic diversity and cultural control of Sclerotium rolfsii are two fields of knowledge not well understood. In this work, two onion isolates of S. rolfsii from the States of Morelos and Guanajuato, Mexico were inoculated to 51 plant species and disease severity levels were determined. Subsequently, 12 out of 51 plant species were selected for the determination of pathogenic reaction to 20 isolates of S. rolfsii from different regions of Mexico. Onion isolates from Morelos and Guanajuato produced variable levels of disease severity for half of the plants tested. Five plant species were susceptible or highly susceptible to all isolates. The remaining plants tested showed differential reactions to individual isolates, ranging from highly resistant to highly susceptible. Three organic soil amendments alone or combined with solarization were tested for control of S. rolfsii on microplots containing onion seedlings in naturally infested soil. A combination of solarization and incorporation of Parthenium hysterophorus into soil was effective in reducing damage and sclerotial numbers of S. rolfsii. However, the amendment alone was associated with increased damage and high sclerotial populations. r 2005 Elsevier Ltd. All rights reserved. Keywords: Sclerotium rolfsii; Pathogenic diversity; Organic amendments 1. Introduction Sclerotium rolfsii is a soilborne fungal pathogen that causes southern blight disease on a wide range of agricultural and horticultural crops, weeds and forest trees. The fungus is distributed around the world in tropical and subtropical regions, and it is quite common in the southern US, as well as in Central and South America. It has also been reported from Africa, Asia, Australia and parts of Europe (Aycock, 1966). So far, no worldwide compilation of the host plants of S. rolfsii has been published. In the USA (including Hawaii), about 500 host plant species have been reported (Farr et al., 1989). In New Zealand, 44 plant species have been recorded as hosts (Broadhurst, 1995). In Mexico, virtually no information on the potential host range of this important pathogen is known. Genetic diversity of Sclerotium rolfsii has been tested in two ways, mycelial compatibility groups (MCG) and the analysis of restriction fragment length polymorph- isms (RFLP) of the internal transcribed spacer region (ITS) of RNA genes (rDNA) (Punja and Sun, 2001). Harlton et al. (1995) found 49 MCG and 12 RFLP-ITS groups in a worldwide collection of isolates. They did not find a correlation between MCG groups and pathogenicity. Some subspecific RFLP-ITS grouping patterns were correlated with MCGs; however, isolates within an MCG sometimes showed different RFLP-ITS patterns and certain patterns were dispersed among different MCGs. In Japan, all S. rolfsii isolates tested belonged to three RFLP-ITS groups. Two of these ARTICLE IN PRESS www.elsevier.com/locate/cropro 0261-2194/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2005.04.007 à Corresponding author. Fax: +527353941896. E-mail address: rbelmont@ipn.mx (R. Montes-Belmont).