Variability among Ralstonia solanacearum isolates and screening of chili germplasm for resistance

Abstract

Chili (Capsicum annuum) belongs to family solanaceae and is one of the most common cultivated crops of the world. Chili has been cultivated on 63.6 thousand hectares in Pakistan with a production of 147.2 thousand tons in 2012-13.Bacterial wilt caused by Ralstonia solanacearum is a highly challenging and destructive disease of solanaceous crops. R. solanacearum is a soil and water borne bacterium which enters the plant roots, multiplies through the xylem, and collapses the host. The bacterial wilt infects more than 450 plant species belonging to 54 different botanical families. Major hosts include tomato, hot pepper, sweet pepper and potato. Eighty countries are affected with a loss of $ 1 billion each year. As the information regarding distribution of bacterial wilt in different agro-ecological zones of Pakistan, pathogenic variability among different isolates, genetic diversity of the bacterium and response of different chili cultivars towards the pathogen is lacking therefore the present studies were carried out. Major chili growing areas from different agro-ecological zones were surveyed for the determination of incidence, prevalence, biovars identification, pathogenic variability and genetic diversity of different isolates of Ralstonia solanacearum. Chili germplasm comprising 28 varieties was screened for their relative resistance or susceptibility against the highly virulent strain (RsBd 6) of the bacterium in the glasshouse. The maximum incidence of 22% of bacterial wilt was recorded in district Badin while the incidence was the minimum (4.4%) in district Loralai. The disease incidence was found to be the maximum (19.5%) in Indus delta and was the minimum in Western dry mountains (5%). Maximum incidence was observed in Sindh province followed by Punjab (11.4%) and was the minimum in Baluchistan (4.9%). The xxi overall incidence in the country was 9.95%. The prevalence was found to be the maximum in Sindh (100%) followed by Punjab (84%) with an overall prevalence of 75.8% in the country. A total of 114 isolates of R. solanacearum from eight agro-ecological zones were isolated, confirmed by immunoStrips and characterized by employing different biochemical tests. The biovars were identified on the basis of sugar consumption. Of the 114 isolates, 92 were distinguished as biovar 3 while 22 were recognized as biovar 4. Both the biovar 3 and biovar 4 were diagnosed from Sindh and Punjab while only biovar 3 was distinguished from Baluchistan and Khyber Pakhtoonkhwa. The isolates varied in pathogenicity when tested on highly susceptible cv. California wonder; 21.9% isolates were found highly virulent, 29.8% virulent, 25.4% weakly virulent and 22.8% were avirulent. The isolates were then confirmed through PCR by using specific primers and running on 1% agarose gel visualized under UV light. The screening of chili germplasm against the bacterium revealed that two cultivars viz. Skyline II and Hifly were highly resistant. Sanam was the only cultivar which was identified as resistant. Five cultivars were categorized as moderately resistant. The cultivars Maxi and Talhari were found highly susceptible to the pathogen while rest of the varieties was either susceptible or moderately susceptible. It is concluded that bacterial wilt caused by R.solanacearum is prevalent throughout the country in all the agroecological zones with varying intensities warranting stringent surveillance and control measures. As variations have been observed in the virulence of R. solanacearum isolates, management strategies should be followed accordingly. Resistant and moderately resistant cultivars have been identified, therefore, recommended for cultivation.