Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 17, 2010
Publication Date: July 7, 2010
Repository URL: http://www.plantmanagementnetwork.org/sub/php/brief/2010/charcoal/
Citation: Gulya, T.J., Mengistu, A., Kinzer, K., Balbyshev, N., Markell, S. 2010. First Report of Charcoal Rot of Sunflower in Minnesota, USA. Plant Health Progress. Available: http://www.plantmanagementnetwork.org/sub/php/brief/2010/charcoal/ Interpretive Summary: We have identified a disease of sunflowers called Charcoal Rot in Minnesota, 2009. This is the first time this disease has been detected in the state, although it has been previously identified in both North and South Dakota. Notably, the disease is more commonly found in hotter-drier climates, and was recovered from Minnesota after a relatively cool and wet summer. A brief survey for the pathogen was done examining the soil in nearby sunflower fields. Although no visible disease on sunflowers was found in any other fields, the pathogen was detected in the soil in over half the fields surveyed. We hypothesize that if a hot dry summer occurs in Minnesota in the future, this pathogen may be more common.
Technical Abstract: A field of oilseed sunflower (Helianthus annuus L. hybrid 'Pioneer 63M82') was observed with uneven maturation in west central Minnesota near Aldrich (Todd County) in late September, 2009. The field's soil type was sandy loam and cropping history was oats in 2008 preceded by four years of alfalfa. Most plants were green, but 37% of the plants had brown stems and wilted, senescent leaves. One-third of these prematurely dead plants (~12%) had 2 to 14 cm long (av. 8.5) silvery grey girdling lesions at the soil line. When cut open, every plant with such lesions had small black microsclerotia, but lacked horizontal pith compression typically associated with charcoal rot. Additionally, some stalks exhibited pink pith discoloration, later determined to be due to multiple Fusarium species. After plating on acidified PDA and incubation at 35 C, colonies and microsclerotia (but no pycnidia) typical of Macrophomina phaseolina (Tassi) Goid formed within 7 days. Microsclerotia recovered from plants and those in culture both generally measured between 75 and 175 microns. To access prevalence of charcoal rot in the region, 100 sunflower plants in five strips were visually inspected in 20 fields in a 20 km radius from the affected field. Additionally, random soil samples from 13 of those fields, plus a soil sample from the affected field were assayed for Macrophomina at the USDA-ARS Plant Pathology Lab in Jackson, TN. No charcoal rot symptoms were observed in any field. However, nine of 13 soil samples (69%) were positive for Macrophomia, with microsclerotial counts ranging from 1 to 4 per 1 g soil, which is low in comparison with counts from fields with other hosts in warmer areas of the Midwest. Grower estimated yield in the affected field was 2200 kg/ha, above the MN state average of 1700 kg/ha, so charcoal rot may have had minimal effect on yield. This area of MN received 106, 75 and 15 mm of rain in July, August and September, with average monthly temperatures of 18, 18 and 17 C. These conditions were unlikely to stress sunflower or provide conditions optimal for charcoal rot development. Charcoal rot on sunflower is typically seen in hot, arid climates such as Texas. It was first observed on sunflower in ND and SD, and was widespread on soybeans recently in IA, suggesting that Macrophomina may becoming more common in cooler growing areas of Midwestern U.S. However, in surveys conducted by the National Sunflower Association, charcoal rot has been recorded only 35 times in 1854 surveyed fields between 2001 and 2009 in eight states, and absent the last three years. This infrequency of observation could be due in part to the persistent cool and wet climate in recent years, which is unfavorable for disease development and symptom expression. However, with the multitude of Macrophomina hosts in the northern Great Plains and the high incidence of microsclerotia we detected in soil, high disease potential may exist, suggesting that in drier, hotter years the sunflower crop may be more affected.