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United States Department of Agriculture

Agricultural Research Service

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Research Project: Monitoring the Distribution of Downy Mildew, Verticillium and Charcoal Rot on Sunflower: a Multiple States Approach

Location: Sunflower Research

2011 Annual Report


1a.Objectives (from AD-416)
New downy mildew races were found in 2009 which overcome the commonly used resistance genes in most commercial hybrids, but the true distribution of these new races is unknown. We propose to enlist pathologists in neighboring states (SD, NE, KS, CO) to systematically sample sunflower fields, collect samples of diseased plants, and have the downy mildew samples typed to race in greenhouse tests in Fargo. Secondly, we have identified charcoal rot in a Minnesota sunflower field, and this is the first report. With increased incidence of charcoal rot on other hosts in the Northern Great Plains, we need to determine the distribution of the causal fungus. The most accurate way to do this is via a soil bioassay and not be assessing disease incidence, which is highly dependent upon weather conditions. The same soil samples will be sent to a commercial lab to isolate Verticillium.


1b.Approach (from AD-416)
We will use university personnel in North Dakota, South Dakota, and Nebraska to survey 200 fields in early summer for downy mildew and to collect soil samples. The soil samples will be shipped to an ARS lab in Tennessee for a Macrophomina assay and to a commercial lab to isolate Verticillium, and the fields testing positive will be surveyed late in the growing season to assess the severity of charcoal rot and Verticillium wilt. Downy mildew samples will be used in greenhouse tests to determine races of the pathogen.


3.Progress Report

Downy mildew samples were collected in 2010 from five states (ND, SD, MN, KS, and NE) and race determined in greenhouse tests. Of 160 samples tested, 10% were races that overcame the Pl6 resistance gene, and these new, virulent races were found in ND, MN and NE, but not in KS or NE. Ninety percent of commercial hybrids claiming "downy mildew resistance" were susceptible to these races. Four released USDA germplasms continue to be resistant to all downy mildew races, which now number 17 in the United States.

Soil samples collected by cooperators in six states (ND, SD, MN, KS, NE, CO) were bioassayed for the presence of Verticillium dahliae in a commercial lab and for Macrophomina (charcoal rot) in a USDA lab in Jackson, TN. Twenty percent of the 203 soil samples tested positive for Verticillium, and inspections of positive fields confirmed disease in all cases. Regarding charcoal rot, 41% of the soil samples were positive for Macrophomia, but in field inspections failed to find charcoal rot in positive fields.

Soil sampling and pathogen identification will continue in 2011. Since the manifestation of charcoal rot is influenced by hot, dry conditions, a yield loss study is underway in 2011. Eight sites, ranging from TX to ND, will have 12 oilseed hybrids in an inoculated/non-inoculated trial to determine both the potential yield impact and the influence of climate on charcoal rot.

Progress with cooperators has and will be monitored via phone and email, and in 2011 by one site visit with each cooperator.


Last Modified: 11/27/2014
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