In September 2009, sunflower (Helianthus annuus L.) plants (cv. Mycogen 8C451) from a University of Illinois field research trial in Fayette County, Illinois exhibited silvery gray girdling lesions on the lower stems and premature death. When lower stems and roots were split open, the pith tissue was compressed into layers. Black microsclerotia (90 to 180 μm) were present on the outside of the lower stem tissue and in the stem vascular tissue. Five pieces (approximately 1 cm long) of symptomatic stem tissue from five different affected plants (25 pieces total) were soaked in a 0.5% solution of NaOCl for 30 s, rinsed with sterile distilled water, and placed on potato dextrose agar (PDA; Becton, Dickinson, and Company, Franklin Lakes, NJ). Gray hyphae grew from all of the stem pieces, which subsequently turned black and formed black microsclerotia (75 to 175 μm). On the basis of plant symptoms and size and color of the microsclerotia, the disease was diagnosed as charcoal rot caused by Macrophomina phaseolina (Tassi) Goid (2). To confirm that the isolated fungus was M. phaseolina, DNA was extracted from the pure culture, and PCR amplification of a subunit rDNA and internal transcribed spacer (ITS) region with primers EF3RCNL and ITS4 was performed (3). The Keck Biotechnology Center at the University of Illinois, Urbana sequenced the PCR product. The resulting nucleotide sequence shared the highest homology (99%) with sequences of M. phaseolina when compared with the subunit rDNA and ITS sequences in the nucleotide database (http://www.ncbi.nlm.nih.gov). A greenhouse experiment was conducted to confirm pathogenicity; the greenhouse temperature was approximately 27°C and sunflower plants (cv. Cargill 270) were grown in pots and watered daily to maintain adequate soil moisture for growth. Sterile toothpicks were infested with M. phaseolina and placed through the stems (10 cm above the soil surface) of five 40-day-old sunflower plants that were approximately at growth stage R4 (1,4). Five sterile, noninfested toothpicks were similarly placed through sunflower plants to act as controls. Parafilm was used to hold the toothpick in the stem and seal the stem injury. Thirty-five days after inoculation, the mean lesion length on stems inoculated with M. phaseolina was 595 mm and no lesions developed on the control plants. M. phaseolina-inoculated plants also began to wilt and die. Cultures identical to the original M. phaseolina isolate were reisolated from stem lesions of the M. phaseolina-inoculated plants. This is the first report of charcoal rot on sunflower in Illinois to our knowledge. Sunflower is currently not a major crop grown in Illinois, but on-going research is focused on evaluating sunflower as a potential late-planted crop to follow winter wheat. If sunflower production increases in Illinois, growers may need to take precautions to manage charcoal rot.
References: (1) L. K. Edmunds. Phytopathology 54:514, 1964. (2) T. Gulya et al. Page 263 in: Sunflower Technology and Production. American Society of Agronomy, Madison, WI, 1997. (3) N. S. Lord et al. FEMS Microbiol. Ecol. 42:327, 2002. (4) A. A. Schneiter and J. F. Miller. Crop Sci. 21:901, 1981.