Hieracium caespitosum (meadow hawkweed) is an exotic invasive weed belonging to a complex of hawkweed species infesting nearly 500,000 hectares of pasture and wildlands in North America, primarily in the Pacific Northwest (1). Economic losses can be up to $222 per hectare (2). Despite prolonged effort, no promising insects have been found as agents of biological control of H. caespitosum. This has led to a greater emphasis on seeking plant pathogens such as rusts and smuts as potential biological control agents. Searches for plant pathogens have included Europe, which contains a portion of the native range of the weed. In the small stands of H. caespitosum that occur in northern France, plants were found with chlorosis and significant stunting. Typically, 10 to 30% of plants in affected stands exhibited such symptoms. Excavation and examination of roots revealed discrete necrotic lesions along the length of roots and decayed root tips, roughly resembling symptoms of corky root. Ten roots with these symptoms were thoroughly washed, cut into ~0.25- to 0.5-cm-long pieces and plated on potato dextrose agar (PDA) and acidified PDA. On these media, colonies were light-salmon-beige, woolly, zonate, with older mycelium sparser with the dense occurrence of dark, hemispherical conidiomata. The conidiomata exuded pink spore masses. The spores were hyaline and falcate with acute apices, 6.2 × 1.6 μm (n = 200). These traits match published descriptions of Pilidium concavum, described as a pathogen of numerous plants usually causing leaf spots and stem necrosis (3). To corroborate this identification, the internal transcribed spacer region of rDNA was amplified by PCR using primers ITS1 and ITS4 and sequenced. BLAST analysis of the 575-bp fragment (GenBank Accession No. JX047867) showed 100% homology with the sequences of six isolates of P. concavum in GenBank, including Accession No. AY487094. To confirm pathogenicity, five each of 30-day-old H. caespitosum plants were either sprayed with, or their roots soaked for 1 hr in a 1 × 106 per ml suspension of conidia prepared from 10-day-old cultures of the fungus grown on V8 agar. An equal number of control plants were either sprayed with sterile distilled water (SDW) or their roots were soaked for an hour in SDW. To plant soaked roots, a hole was made in the pre-moistened pasteurized potting mix in each pot and inoculum or SDW was poured around them before they were covered with soil. The experiments were repeated twice. Following leaf inoculations, plants were covered with a plastic bag and placed in the greenhouse in partial shade at 20 to 25°C for 72 to 96 h. Sprayed plants were then uncovered and assessed for the appearance of lesions over the next 7 to 10 days. No foliar symptoms occurred. Plants inoculated by soaking roots showed chlorosis and stunting by 4 months post-inoculation. P. concavum was isolated from root lesions on all inoculated, symptomatic plants, confirming Koch's postulates. Although reported to cause root deterioration of strawberry (4), to our knowledge, this the first report of a root disease of H. caespitosum caused by P. concavum.
References: (1) C. A. Duncan et al. Weed Technol. 18:1411, 2004. (2) L. Frid. Economic Impacts of Invasive Plants in BC Final Project Report. British Columbia: Invasive Plant Council, 2009. (3) J. A. von Arx. Plant Pathogenic Fungi. J. Cramer, 1987. (4) J. L. Maas. Compendium of Strawberry Diseases. The American Phytopathological Society, St. Paul, MN, 1998.