Occurrence, identification and a potential management strategy of Fusarium species causing wilt of potatoes in South Africa

Fusarium is a soilborne fungus which can live in soil for long periods of time. It is known to cause wilt, root rot and crown rot diseases in a diverse group of crop plants. Of all the diseases caused by Fusarium the most important are the vascular wilts. Pathogens that cause wilting usually enter their host plant through young roots and then grow into and up the water-conducting vessels of the root and stem. The vessels become blocked and water supply to the leaves is limited. This results in the potato plant being weak resulting in yellowing of leaves, browning of stems and production of smaller tubers. Fusarium is diverse and widely distributed and can be isolated from agricultural soils and plant material. The study was done to determine the occurrence of this pathogen in the South African potato industry. Samples of plant material showing wilt symptoms were collected from nine potato production regions. Fungal isolations were made from tubers using a Fusarium selective medium, i.e Peptone PCNB Agar. The isolates were examined morphologically and those resembling Fusarium were further identified using molecular techniques. DNA sequence analysis of the translation elongation factor 1-α gene was done on the isolates. DNA-based techniques have increasingly become the tool of choice for understanding the genetic diversity and phylogeny of Fusarium species. The pathogenicity of the isolates from all the regions was also investigated on potato cultivar Caren. The DNA results confirmed Fusarium as the pathogen causing Fusarium wilt on potatoes. Two species of Fusarium were identified; namely F. oxysporum and F. solani. F. oxysporum was more prevalent and occurred in all regions compared to F. solani. F. oxysporum is best known for the plant pathogenic strain, which cause wilt, root rot and crown rot diseases on a wide variety of crops, often limiting crop production. It is also known to be phylogenetically diverse. In the pathogenicity test, the isolates were found to be virulent and one was highly virulent therefore confirming their ability to cause wilting of potatoes. The effect of silicon on Fusarium wilt of potatoes was investigated in this study to assess its effectiveness in the control of Fusarium wilt. An in vitro study using potassium silicate was done to determine if silicon can inhibit the growth of Fusarium at different concentrations. The results showed that at low concentrations of potassium silicate the growth of Fusarium was not inhibited, while at a high concentration, there was inhibition. Greenhouse pot trials were conducted to determine the effect of silicon soil amendments on Fusarium wilt of potatoes, tuber yield and the production of phenolics in the cell wall of potato peels. The levels of chlorogenic, caffeic and ferulic acids were also investigated. The following treatments were used: control, silicon ash (~99% Si), slag (30% Si), fly ash (50% Si) and lime (calcium carbonate) as a pH control. Treatments were divided into those inoculated with Fusarium and those without Fusarium. Results showed that for silicon treatments not inoculated with Fusarium, slag had the highest tuber yield, followed by lime, fly ash and silicon ash when compared to the control. Silicon treatments inoculated with Fusarium did not improve the yield as the control had the highest yield and the occurrence Fusarium wilt was not reduced in silicon treatments. In this regard silicon did not have an effect on Fusarium wilt because symptoms were visible in the silicon amended treatments. The results for phenolic acid content showed that ferulic acid levels were too low for analysis; for chlorogenic acid, concentrations were generally lower in the silicon treatments than in the treatments without silicon; and caffeic acid levels were higher in silicon treatments than treatments without silicon, as a result of increased production of as defence mechanism against invading pathogens. However, this is the first study on the effect of silicon on Fusarium wilt of potatoes and its influence on the production of phenolics. Further research is required to understand the role of silicon in potato pathosystems.