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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

2012 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 2011 from five states (ND, SD, MN, KS, and NE) and race determined in greenhouse tests. Of 40 samples tested, 30% were races that overcame the Pl6 resistance gene, and these new, virulent races were found in ND, SD, MN and NE, but not in KS. Over the past three years, 17% of 250 samples were identified as one of nine race that overcome the Pl6 reisstance gene. 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 total 17 in the United States.

Soil samples collected by cooperators in seven states (ND, SD, MN, KS, NE, CO, TX) were bioassayed for the presence of Verticillium dahliae in a commercial lab and for Macrophomina (charcoal rot) in a USDA lab in Jackson, TN. Ten percent of the 212 soil samples tested positive for Verticillium. The fungus was most common in TX (67% of fields positive), and absent from all sampled fields in CO, KS and MN. Regarding charcoal rot, 27% of the soil samples were positive for Macrophomina, with TX again having the highest incidence (83%). Macrophomina was more prevalent than Verticillium in both years of the study. The annual survey coordinated by the Nat. Sunflower Association records more Verticillium than Macrophomina, suggesting the charcoal rot either does not manifest disease symptoms or that surveyors fail to recognize the disease. A yield loss study was conducted to determine the impact of charcoal rot on sunflower. None of the eight sites in six states had enough heat/drought stress to induce disease symptoms despite being inoculated, so no data was obtained.


Last Modified: 8/30/2014
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