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Head Rot of Sunflower Caused by Rhizopus oryzae in New Mexico

Head rot was found in cultivated sunflower (Helianthus annuus) in eastern New Mexico in Tucumcari in 2007 and Clovis in 2007 and 2009 and in south-central New Mexico near Las Cruces in 2009. The disease was also observed in wild sunflower near Clovis in 2008. Disease incidence was 10 to 40% in cultivated sunflower and ~30% in wild sunflower. Heads were brown to dark brown with discoloration extending down the sepals and peduncles into the stems. The basal parts of the heads were shredded and had grayish, fluffy mycelial mats visible in the lumen, and kernels were mostly seedless. Three to five diseased heads were collected from cultivated sunflower in 2007 and 2009 and wild sunflower in 2008. Plant tissues from heads and peduncles were surface sterilized for 3 min in 0.5% NaOCl, rinsed once in sterile distilled water, cut into 0.5-cm pieces, and plated on acidified potato dextrose agar (PDA). Within 3 to 7 days, mycelial colonies with abundant aerial growth and black sporangia emerged and were identified as Rhizopus oryzae on the basis of the presence of pale brown sporangiospores with bluish stripes (3) and mycelial growth at 36°C on PDA (1). PCR amplification of the internal transcribed spacer (ITS) region of rDNA from two isolates, one from cultivated and one from wild sunflower, using primer pair ITS4/ITS5 (1) was followed by sequencing and showed a 99% homology with the sequence of the ITS region of rDNA from R. oryzae (GenBank No. FJ654430). Each isolate was tested for pathogenicity on inflorescences (5 to 6 cm in diameter) of sunflower cvs. Hysun 511 and Triumph 820 HO grown for 4 to 5 weeks in a growth chamber at 26°C with a 14-h photoperiod. To obtain inoculum, a sterile toothpick was passed through a culture of R. oryzae until a ~3-mm mycelial mat was collected at the tip. The toothpick was dabbed into the center of an inflorescence or into the peduncle. A cotton boll was placed over the inoculation and sprayed with sterile distilled water. Control inflorescences were dabbed with toothpicks with no mycelium mat. Each inoculated and noninoculated inflorescence was covered with a plastic bag that was sealed around the peduncle. Plants were kept in the growth chamber for 3 weeks. In each of two experiments, 13 plants were used per cultivar and inoculation type, with 5 plants inoculated per isolate, and 3 control plants. Symptoms observed on inoculated sunflower were similar to those on field infected sunflower. There was no difference between the two cultivars. On inoculated inflorescences, dark discoloration developed at the inoculation site and expanded over the inflorescences, and grayish mycelium with black sporangia was observed within 2 weeks. On inoculated peduncles, dark discoloration was also observed extending down the peduncle and up into the inflorescences. R. oryzae was reisolated from all inoculated heads. To our knowledge, this is the first report of R. oryzae causing head rot on sunflower in New Mexico. It is unknown what factors lead to head rot outbreaks. This disease has been reported in other U.S. regions and has been demonstrated to reduce sunflower yield and quality (2). The potential negative impact from Rhizopus head rot should be considered when determining whether to expand cultivation of this crop.

References: (1) G.-Y. Liou et al. Mycol. Res. 111:196, 2007. (2) C. E. Rogers et al. Plant Dis. Rep. 62:769, 1978. (3) T. Watanabe. Pictorial Atlas of Soil and Seed Fungi: Morphologies of Cultured Fungi and Key to Species. CRC Press, Boca Raton, FL, 2002.