Development of an in vitro selection technique of potato cultivars for improved resistance to fusarium oxysporum

Abstract

Fusarium oxysporum W.C. Snyder & Hans. causes dry rot, stem-end rot and wilt of potatoes (Solanum tuberosum L.) Screening for Fusarium resistance is important and an effective screening method is essential. The aim of the study was therefore to develop an in vitro selection and/or screening technique to select and/or screen potato somaclones with improved resistance to Fusarium oxyspornm. Techniques for the in vitro production, cultivation and regeneration of five South African potato cultivars, Late Harvest, BPI, Vanderplank, Up-to-Date and Kimberley Choice, have been established and optimized. It was important to develop an optimal in vitro system which could enable one to add selective agents (toxin and/or culture filtrates) to screen the potato cultivars for better resistance against F. oxysporum. A correlation was found between fusaric acid production and virulence of isolates of F. oxyspornm. The toxin, fusaric acid, seemed to play a major role in the development of dry rot in potato tubers of susceptible cultivars. Virulence, Vegetative compatibility's and RAPD analysis have classified F. oxyspornm isolates that cause dry rot on potatoes into groups that coincide with races. All three techniques proved useful in differentiating among isolates of F. oxysporom and due to cost effectively RAPD analysis may be the more widely applicable technique especially for screening large samples of isolates. The toxicity of F. oxysporum culture filtrate and fusaric acid on cell and callus cultures of the different resistant and susceptible cultivars have been described. It was found that an in vitro relationship existed between susceptibility of potatoes to F. oxysporum, and toxicity of fusaric acid and culture filtrates of F. oxysporum. Results suggested that fusaric acid could be used as selective agent. It is possible that approaches described in the study will have an impact on the production of novel disease - resistant potato plants particularly when conventional breeding methods have failed. The extent to which it can be exploited will, however, depend on whether the recovered undesirable heritable trials could impede the breeding process. Plants regenerated from callus cultures resistant to F. oxysporum culture filtrate and/or fusaric acid have been selected and evaluated for agronomic potential as well as field resistance against F. oxysporum. The main advantage of the developed in vitro screening techniques remains the large number of individuals which can be screened in a small space in a relatively short period of time, right through the year and the potential source of novel breeding material for trails. Results obtained in this study warns that the applying of in vitro selection for the improved resistance of potato cultivars to F. oxysporum by adding only one characteristic without altering the remaining genotype should be researched carefully. It can be anticipated that in vitro selection and screening for disease resistance as a breeding tool will continue to expand in future. This method is however dependent on linkage to the convential breeding program. This linkage will ensure that the screened and developed tissue culture material has the agronomic performance that is essential.