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Occurrence of Phoma macdonaldii, the Causal Agent of Sunflower Black Stem Disease, in Sunflower Fields in China

Phoma macdonaldii Boerma is the pathogen of sunflower black stem disease, causing dark black, oval to long lesions on stems of sunflower plants. Infection during early growth stages can reduce yield by 10 to 30% (3). This fungal disease is distributed mainly in North and South America and Europe. In China, the first case was reported in Xinjiang in 2008 (1), and was believed to be introduced as a result of hybrid sunflower seeds being imported from abroad. The Chinese government included this fungus into its quarantine pests list in 2010 (2). Since China imports a great number of sunflower seeds to grow in its Northern provinces from epidemic areas such as the United States, Argentina, and France, monitoring the disease occurrence in planting areas became crucial. During 2010 and 2011 growing seasons, surveys were conducted in 37 commercial farms or individual households in 12 counties of five areas (Xinjiang, Inner Mongolia, Ningxia, Hebei, and Beijing). A total of 185 suspicious samples of sunflower black stem disease were collected and all were found from imported hybrid seed fields. The presence of P. macdonaldii was confirmed as following: 4 mm² tissue pieces cut from lesion margins were disinfected with 1% NaOCl, plated on APDA (acid potato dextrose agar, 4.5 to 5.0 pH adjusted with lactic acid), and incubated at 25°C with 12L:12D photoperiod. After 3 days of incubation, colonies were opalescent or ivory in color, and fluffy or flocculent in appearance. After 4 to 6 days, a large number of spherical or oblate black-brown pycnidia were formed separately or in clusters with thin wall and papillate ostiole in diameter of 135 to 324 μm (average 178 μm). A light pink or opalescent gelatinous substance (pycnidiospores) exuded from the ostiole. Pycnidiospores were single celled, oval or kidney-shaped and hyaline both with and without oil balls, and 1.5 to 3.0 μm × 3.0 to 6.5 μm (average 2.0 × 4.7 μm). Sequences of ITS1-5.8S- ITS2 rDNA fragment of all isolates (GenBank Accession No. JQ979487, JQ979488) were identical and had 100% homology with P. macdonaldii isolates from Xinjiang (HM003206) and Australia (DQ351823, DQ351825) and 99% homology with isolates from the former Yugoslavia (DQ351821, DQ351822) in GenBank. Pathogenicity studies of the isolate were performed by injecting 10 × 10⁶/ml spore suspension into the hypocotyl of four true leaves of sunflower seedlings with a syringe. Sterile water was injected as control. After being inoculated in a plastic bag in the shade at room temperature for 48 h, the plastic bag was removed and the seedlings were grown under natural light. Symptoms of black stem disease were observed in all P. macdonaldii inoculated seedlings and the fungus was reisolated from the lesions for confirmation. The current survey found that 105 of 185 suspicious samples were P. macdonaldii positive and were all from four counties in Xinjiang, suggesting that the disease has not spread to other areas since its introduction. The monitoring of sunflower black stem disease is continuing, as is the research for measuring P. macdonaldii adaptability in China and the development of rapid molecular detection technology.

References: (1) W. M. Chen et al. J. Yunnan Agric. Univ. 23:609, 2008. (2) J. Luo et al. Australas. Plant Pathol. 40:504, 2011. (3) E. Miric et al. Aust. J. Agr. Res. 50:325, 1999.