Submitted to: Sunflower International Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 7/15/2004
Publication Date: 8/29/2004
Citation: Rashid, K.Y., Seiler, G.J. 2004. Epidemiology and resistance Sclerotinia head rot in wild sunflower species. International Sunflower Conference Proceedings. v. 2. p. 751-754.
Interpretive Summary: White mold (Sclerotinia head rot) has become a serious problem in sunflower production areas of North America. It caused an estimated $70 million loss in North Dakota alone in 1999. The infection is caused by airborne spores of the fungus produced by germination of the hard black fruiting body under saturated soil conditions. Tolerance to white mold head rot in several populations of wild sunflower species has been reported, but proper screening methodology is lacking due to the difficulty of assessing the reaction of the wild species. The objective of this study was to study the disease progression of head rot in the wild Maximilian and Nuttall's perennial sunflower species and to develop a methodology for assessing their reaction to head rot. Forty-eight populations of each wild perennial species collected from the southern Manitoba, Canada were established in a nursery at the Agriculture and Agri-Food Canada Research Station at Morden, Manitoba and used in the present study. Each population was divided into four designated plots, with each plot artificially inoculated with one of four types of inoculum: spores collected from the fruiting bodies produced in the laboratory; fresh ground mycelia grown in agar in the laboratory; ground infected millet seed; and one uninoculated plot. Each plot in a population was inoculated with one type of inoculum and was covered with either a light brown paper bag, a thin transparent plastic bag, a perforated pollinating bag, or no bag. One-half ounce of water was sprayed into each bag using a hand-held sprayer at the second and third day after inoculation to create a humid environment to enhance the infection process. Since wild sunflower accessions are multi-branched and flower over a period of several weeks during the growing season, the inoculation process was done during the first week of flowering, mid-flowering (week 3), and late flowering (week 5). Most infections observed were to the stems and peduncles below the heads and not directly to the heads. The heads of wild sunflower species are very small and quickly dry without any typical white mold head rot symptoms clearly exhibited. The paper bag covering resulted in the highest infection levels. The combination of ground infected millet inoculum and the paper bag covers resulted in 88% infection in 2002 and 55% in 2003. Fifteen wild sunflower populations remained healthy under the various artificial inoculation methods in both years of this study. The ground millet inoculum resulted in the highest disease indices for both wild sunflower species, followed by the ascospore inoculation and fresh ground mycelia as source of inoculum. Future research will use a wetting agent to help disperse the inoculum on the small heads, which may help to obtain a better head rot infection.
Technical Abstract: Field trials were conducted in 2002 and 2003 to study the epidemiology of Sclerotinia infections on wild sunflower heads and stems, to establish methodology for assessing wild sunflower germplasm, and to identify sources of resistance. Forty-eight populations of two wild perennial sunflowers, Helianthus maximiliani Schrader and H. nuttallii Torrey and Gray from Canada were evaluated using artificial inoculation with ascospores, fungal mycelia, and ground infected millet seed at three-week intervals starting at flowering time. Plants were covered for 14 days after inoculation with light brown paper bags, sunflower pollination bags, and thin plastic bags. The ground Sclerotinia-infected millet inoculum resulted in the highest level of infection followed by ascospores and fungal mycelia. The paper bag covering resulted in the highest infection levels. The combination of ground infected millet inoculum and the paper bag covers resulted in 88% infection in 2002 and 55% in 2003. Fifteen wild sunflower populations remained healthy under the various artificial inoculation methods in both years of this study.