2013 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.
Soil sampling and soil bioassays to map the distribution of the charcoal rot fungus (Macrophomina phaseolina) and Verticillium wilt fungus (V. dahliae) were completed in 2011 and this portion of the project was finished. Macrophomina was more prevalent than Verticillium in both years of the study. The annual survey coordinated by the National 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. With the discovery of new downy mildew races virulent on the widely used Pl6 resistance gene, we have continued to annually collect DM samples. In 2012, we made race determinations on 150 samples, bringing the four year total to 400. Nine virulent races (those able to overcome the Pl6 gene) accounted for 30% of all samples. Fifty percent of the 2012 DM samples were virulent races. Over the 4 years of the study, 30% of the 400 DM samples were of nine races virulent on the Pl6 gene. Four resistance genes, available in 12 USDA inbreds, remain immune to all DM races. A Master’s student at North Dakota State University, under co-supervision of an ARS scientist, is currently testing ~ 200 accessions of wild Helianthus annuus for resistance to these virulent races, and resistance is very common in accessions from Texas. These resistant wild H. annuus accessions will be crossed with elite USDA lines to quickly transfer genes and germplasm releases should be made within two to three years.