Author
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Gulya Jr, Thomas |
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Submitted to: Book Chapter
Publication Type: Book / Chapter Publication Acceptance Date: 10/1/2003 Publication Date: 1/1/2004 Citation: Gulya Jr, T.J. 2004. A seedling bioassay to detect the presence of plasmopara halstedii in soil. In: Spencer-Phillips, P., Jeger, M., editors. Advances in Downy Mildew Research-Volume 2. London: Kluwer Academic Publishers. p. 233-240. Interpretive Summary: Sunflower downy mildew is caused by a soilborne fungus that can only infect sunflower seedlings when the soil is waterlogged. Thus, the incidence of this disease is frequently underestimated because many fields do not receive heavy rains immediately after planting, and thus the plants escape infection. To accurately determine the incidence of this sunflower disease, one should determine if it is present in the soil, rather than waiting to observe infected plants in the field. Since no such test exists, it was the objective of this study to develop a greenhouse test which could show the presence of sunflower downy mildew. The test itself is rather simple, consisting of planting sunflower seeds in flats filled with soil mixed with sand, and then flooding the flats with distilled water for four consecutive days. Within two weeks, the developing sunflower seedlings will show symptoms of downy mildew if the fungus was present in the soil. While our initial results were very encouraging, we soon realized that some details of the test procedure needed to be modified. In order for the downy mildew fungus to mature complete, the soil samples need to be stored at room temperature for at least three months. We are continuing to refine the test procedure to make it as simple and efficient as possible. This test could be used by university diagnostic labs to verify whether downy mildew is in the soil prior to planting. This information would be useful to growers who could then decide whether a fungicide seed treatment or mildew-resistant hybrid was needed in a particular field. Technical Abstract: A greenhouse seedling test is under development which will detect and quantify the presence of the fungus causing sunflower downy mildew in soil, regardless of the crop planted. The test procedure consists of planting sunflower seeds in field soil mixed with sand, and flooding the flats with distilled water to produce a soil environment suitable for downy mildew infection. Initial trials in 2002 with soil stored for 4 months were very successful, with downy mildew detected in 92% of 62 fields sampled. Subsequent attempts using summer-collected soil samples without a storage period produced drastically lower disease incidence. An experiment in progress to assess the impact of temperature and storage period on stored soil suggests that room temperature storage for 3 to 4 months may be necessary for the fungal oospores in the soil to achieve maximum germination. Other parameters of the seedling bioassay will be studied including the use of tap versus distilled water, the effect of mixing varying amounts of sand with the soil sample, and the use of a soil flooding step prior to planting seed. |
