Indian Phytopath. 68 (1) : 73-77 (2015) Assessing genetic diversity in Peronosclerospora sorghi causing downy mildew on maize and sorghum Y. SIREESHA and R. VELAZHAHAN* Department of Plant Pathology, Centre for Plant Protection Studies, Tamil Nadu Agricultural University, Coimbatore 641 003, Tamil Nadu, India ABSTRACT: Downy mildew, caused by Peronosclerospora sorghi (Weston & Uppal) Shaw, is an economically important disease on maize (Zea mays L.) and sorghum [Sorghum bicolor (L.) Moench] in India. The isolates of P. sorghi infecting sorghum and maize are difficult to distinguish from each other using morphologic criteria. However, there is wide variability in pathogenic and molecular characters among isolates of P. sorghi from maize, sorghum and wild sorghum. Therefore, studies were undertaken to assess the genetic variability in isolates of P. sorghi causing downy mildew on maize and sorghum by RAPD method during 2013 at Tamil Nadu Agricultural University, Coimbatore. A total of 65 reproducible RAPD bands were obtained with 15 selected primers on 11 isolates of P. sorghi tested. Each primer yielded on an average of 6 RAPD fragments and molecular size of amplicons ranged from 250 bp to 2.0 kb. The 11 isolates were grouped into two main clusters (A and B) on the basis of cluster analysis of the pooled data for all 15 primers using unweighted pair group method with arithmetic mean (UPGMA). All the maize and sorghum isolates were grouped together in cluster A and cluster B respectively, confirming the genetic diversity among the isolates infecting both maize and sorghum. Keywords: Downy mildew, genetic diversity, maize, Peronosclerospora sorghi, RAPD, sorghum RESEARCH ARTICLE *Corresponding author: velazhahan@hotmail.com Downy mildew, caused by Peronosclerospora sorghi (Weston & Uppal) Shaw, is an important disease of maize (Zea mays L.) and sorghum [ Sorghum bicolor (L.) Moench] worldwide (Hau et al ., 1995; Adenle and Cardwell, 2000). The fungus produces both local and systemic infection in both the crops (Bonman et al., 1983) and cause significant yield losses under favourable environmental conditions. Soil-borne oospores or aerially disseminated conidia cause systemic infection, whereas local infection is caused by conidia (Cohen and Sherman, 1977; Tuleen et al., 1980; Ramalingham and Rajasab, 1981; Bock et al., 1998). The local infection acts as a source of inoculum for subsequent systemic infection on young plants (Cohen and Sherman, 1977). Although P. sorghi can produce symptoms on both maize and sorghum, it does not complete sexual reproduction on maize and hence no oospores are formed (Perumal et al ., 2008). Olanya and Fajemisin (1993) reported existence of two pathotypes of P. sorghi, one capable of infecting both maize and sorghum and the other specific to maize. The P. sorghi infecting sorghum and maize are difficult to distinguish from each other morphologically (Bock, 1995). However, occurrence of pathogenic and molecular variability among the isolates of P. sorghi from maize, sorghum and wild sorghum has been well documented (Bock et al., 2000; Perumal et al., 2006; Mathiazhagan et al., 2008). Such variabilities among pathogen populations are more likely to overcome genetic resistance (McDonald and Linde, 2002). Genetic variabilities among isolates of P. sorghi have been elucidated by molecular analysis using restriction fragment length polymorphism (RFLP) of PCR-amplified internal transcribed spacer (ITS) region of ribosomal DNA (Mathiyazhagan et al ., 2008), random amplified polymorphic DNA (RAPD) (Ladhalakshmi et al., 2009), amplified fragment length polymorphism (AFLP) (Perumal et al., 2006) and simple sequence repeat (SSR) (Perumal et al., 2008). Genetic markers generated by RAPD have proven to be useful for studying genetics and phylogenetic relationships in several fungi (Shi et al., 1996). The RAPD has the potential to detect polymorphism throughout the genome as compared to other techniques. This technique can produce a spectrum of amplified products, characteristic of the template DNA due to arbitrary priming at a relatively low annealing temperature and at multiple locations. The DNA polymorphism in these markers arises from differences in DNA sequences caused by nucleotide pair substitutions, deletions, inversions and translocations (Waugh and Powell, 1992). Therefore, present study was undertaken to assess the genetic diversity in isolates of P. sorghi from sorghum and maize using RAPD method. MATERIALS AND METHODS Collection of fungal isolates and extraction of DNA Five isolates of P. sorghi from sorghum (MLMU, EMPI, PNGR, SMTR and KMPI) and six isolates from maize (EB, NA, PNM, DPTM, TEPM and TPM) were collected from Tamil Nadu, India (Table 1). The DNA was isolated from the conidia of P. sorghi as per the method of McDermott et al. (1994). Briefly, conidia were collected