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United States Department of Agriculture

Agricultural Research Service

Title: Epidemiology of Sclerotinia Head Rot in Wild Sunflower Species

Authors
item Rashid, Khalid - AGRIC & AGRI-FOOD CANADA
item Seiler, Gerald

Submitted to: Proceedings Sunflower Research Workshop
Publication Type: Proceedings
Publication Acceptance Date: April 1, 2003
Publication Date: April 1, 2003
Citation: Rashid, K.Y., Seiler, G.J. 2003. Epidemiology of Sclerotinia head rot in wild sunflower species. Proceedings Sunflower Research Workshop. http://www.sunflowernsa.com/research_statistics/research_workshop/documents/104.pdf

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 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. The light brown paper bags provided the most favorable conditions for infection and disease development resulting in the highest disease infection, compared to pollinating and plastic bags. There were minor differences in the reactions of the two perennial species to the various inoculation treatments. These results demonstrate that high epidemics of Sclerotinia infection in wild sunflower species can be achieved by inoculating the plants between the third and fifth week of flowering, and covering the plants with a light brown paper bags for two weeks. 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: Sunflower head rot and mid-stem infections caused by Sclerotinia sclerotiorum are destructive diseases affecting sunflower in North America. These infections are caused by airborne ascospores of the fungus produced by germination of sclerotia under saturated soil conditions. Tolerance to Sclerotinia wilt in several populations of wild sunflower species has been reported. The lack of proper methodology for testing and assessing head rot infections attests to the difficulty in assessing the reaction of the wild species to head rot. Hence, the objective of this study was to study the epidemiology of head rot in two perennial wild sunflower species and to develop methodology for assessing the reaction of wild species to head rot. Forty-eight populations each of wild perennials Helianthus maximiliani, and H. nuttallii collected from southern Manitoba, Canada and established in a nursery at the Agriculture and Agri-Food Canada Research Station at Morden, Manitoba were used in the study. Each population was divided into four designated plots, with each plot artificially inoculated with one of four types of inoculum; ascospores collected from apothecia produced by sclerotia 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 left uncovered. Fifteen milliliters 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 at early flowering (week 1), 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 dry up very quickly, with no typical Sclerotinia infection symptoms clearly identified. 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 a source of inoculum. The light brown paper bags provided the most favorable conditions for infection and disease development, resulting in the highest disease indices with this type of covering compared to pollinating and plastic bags. There were minor differences in the reactions of the two perennial species to the various inoculation treatments. These results demonstrate that high epidemics of Sclerotinia infection in wild sunflower species can be achieved by inoculating the plants between the third and fifth week of flowering, and covering the plants with a light brown paper bags for two weeks.

Last Modified: 4/20/2014
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