INDUCTION OF SYSTEMIC ACQUIRED RESISTANCE AGAINST Ralstonia solanacearum IN IRISH POTATO

Abstract

Irish potato (Solanum tubesorum) is an economically important crop throughout the world with large-scale production and consumption. Irish potato is rich in carbohydrates, vitamins and minerals, especially iron, magnesium and potassium. Irish potato production has been plagued by constraints such as lack of certified seeds, pests and diseases. Bacterial wilt caused by Ralstonia solanacearum is the major bacterial disease of potato resulting to 50 - 100% crop losses. Management of R. solanacearum is difficult since it has a wide host range and the fact that it is a soil borne pathogen. Several methods including biocontrol, chemical control, cultural practices, host resistance and phytosanitation have been used to manage R. solanacearum. However, there is no method that has been found to effectively control the disease but a combination of different methods have been reported to reduce the impact of the disease. Inducing systemic acquired resistance is another alternative to control bacterial wilt. Systemic acquired resistance is the response of a plant to a pathogen attack and confers long-lasting protection against a broad spectrum of microorganisms. Thus, this study determined the induction of systemic acquired resistance against bacterial wilt in Irish potato through injection of attenuated pathogen and non-pathogens. The ability of Escherichia coli, Saccharomyces cerevisiae and attenuated R. solanacearum to induce systemic acquired resistance against bacterial wilt of Irish potato was tested in the greenhouse. Attenuation assay was conducted using four levels of formaldehyde concentrations and incubation duration for attenuation of R.solanacearum using randomized complete block design. Randomised complete block design was also used to evaluate in-vivo effect of different treatments and varieties on R. solanacearum in the greenhouse. Bacterial wilt symptoms were observed and a rating scale of 0 - 6 was used to score for disease severity. Leaves of the treated plants were collected for salicylic acid quantification in the laboratory. Disease severity, colony count, amount of salicylic acid and tuber yield data were analysed using statistical analysis software version 9.4 to determine induction of systemic acquired resistance. Significant means were separated using Least Significant Difference test at 5% probability level. Result from this study showed that there was statistically significant effect (p < 0.05) of formaldehyde concentrations and incubation duration for attenuation of R. solanacearum. The results from attenuation of R. solanacearum showed that colony count ranged from 0.67 colony forming units (in 0.6% formaldehyde concentration and incubation duration of attenuation of two hours) to 233 colony forming units (in 0% formaldehyde concentration and incubation duration of attenuation of two hours). There was significant (p < 0.05) treatment effect on disease severity, salicylic acid levels and tuber yield of Irish potato. Higher disease severity was recorded in S. cerevisiae treatments (80%) followed by E. coli treatments (40%) while least disease severity was recorded in attenuated R. solanacearum treatments. Attenuated R. solanacearum induced the highest level of salicylic acid (3409.64 mg/kg of Irish potato leaves) while R. solanacearum induced the lowest (1222.74 mg/kg of Irish potato leaves). Irish potato plants with attenuated R. solanacearum treatment had the highest yield of tubers (0.5772 kg/plant) while those treated with S. cerevisiae treatments had the lowest (0.4840 kg/plant). This study concluded that attenuated R. solanacearum and E. coli triggered systemic acquired resistance in Irish potato plants thereby increasing the levels of salicylic acid, which reduced the negative impact of the disease in the plants. Induction of systemic acquired resistance is a viable method for induction of resistance against bacterial wilt in Irish potato. The study recommends the identification of the specific molecules involved in induction of resistance from attenuated R. solanacearum and E. coli and their utilization in the control of bacterial wilt in order to mitigate the huge loses that are caused by this disease.