Mason Kearns and Dr. Brad Geary, Plant and Wildlife Sciences
The purpose of our project was first to apply polymerase chain reaction (PCR) technology in quantifying the infection levels of a disease in potato, and also to determine the result of varying potassium nutrition on fungal infection levels. The method currently used to assay infection is the root colonization assay (RCA) which produces less accurate results than PCR quantification. We hypothesized that a Potassium concentration of 80mg/L would be most beneficial for resisting the pathogen. The project began in January 2009, finished in May 2009 and was presented at the 2009 annual American Phytopathological Society meeting in Portland Oregon in the form of an oral presentation.
The potatoes were grown in hydroponic solution so that we could maintain every aspect of plant nutrition constant between plants in the study. The plants were allowed to grow in a pretreatment solution with a minimal amount of potassium for 2 weeks in order to obtain sufficient biomass for the study. Plants were then grown in solutions with 0, 10, 80, and 160 mg/L and infected with spores of Colletotrichum coccodes (the disease). After infection, the plants were allowed to grow for 3 weeks. After the three week period, plants were removed from the hydroponic solution and analyzed.
PCR analysis consisted of taking a sample of each plant’s roots, and extracting total DNA. Markers were designed that would amplify fungal DNA and Potato DNA separately. By comparing the amplification of Fungal vs. Potato DNA, we were able to create infection coefficients for each plant from each treatment group.
Plants were sent to Washington state university for the root colonization assay. This consisted of taking a small sample of root tissue and placing it on a sterile Petri dish containing water agar, then waiting for fungal mycelia to develop on the tissue. The amount of disease present is correlated with the amount of visible fungal material on the root.
Data from both analyses coincided, and it was found after three repetitions of the study that 80mg Potassium per Liter was optimal for plant growth and resistance to C. coccodes (fig. 1).
The differences in the infection coefficients of two week old plants did not vary significantly; suggesting that at two weeks growth, disease infection may depend little upon K concentration.
A study that incorporates longer infection times and host-pathogen gene expression would provide a greater understanding of how potassium is used as a defense against black dot.
We have undertaken to adapt this experimental design in order to better understand the effects of Phosphorous nutrition on infection severity. We plan on submitting the results of this study to the Journal of Potato Research.