Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2012
Publication Date: 12/1/2012
Citation: Caesar, A.J., Lartey, R.T., Caesar, T. 2012. Disease Notes: First report of Pilidium concavum causing root lesions of meadow hawkweed in France. Plant Disease. 96(12):1830-1830.
Interpretive Summary: This paper is concerned with describing the pathogenicity in its European native range of a plant pathogen of meadow hawkweed, an invasive pasture weed which is economically important in north America. A broad goal is to document the diversity of plant pathogens on this important host, to provide information on pathogens co-occurring with a rust of hawkweed, Puccinia hieracii, being studied for use as a biological control agent, to understand if the smaller, sparser stands of this invasive weed that occur in Europe might be due to a complex of plant pathogens if not overwhelmingly the rust alone. There is almost no existing data of diseases (other than obligate pathogens, for which is there is very little) of native populations of species that are invasive weeds in North America. Not only is knowledge of diseases of species invasive in North America of economic importance, but also ecologically and scientifically important, since the habitat of the native populations is being restricted by development.
Technical Abstract: Hieracium caespitosum (meadow hawkweed)is an exotic, invasive weed among a complex of hawkweed species infesting nearly 500,000 hectares of pasture and wildlands in north America, primarily in the pacific northwest). Economic losses can be up to $222 per hectare. 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 such plant pathogens as rusts and smuts as potential biological control agents. Searches for plant pathogens have included Europe, a portion of the native range of the weed. In the small stands of H. caespitosum that occur in northern France, plants with chlorosis and significant stunting were found during these searches. Typically 10-30% 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. Roots with these symptoms were thoroughly washed, cut into ca 0.25-0.5 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 X 1.6 µm. These traits essentially match published descriptions of Pilidium concavum, described as a pathogen of numerous plants usually causing leaf spots and stem necrosis. To corroborate this identification, the internal transcribed spacer region of rDNA was amplified by PCR using primers ITS1 and ITS4 and sequenced. There was 100% DNA homology with the sequences of 6 isolates of P. concavum in Genbank. To confirm pathogenicity, 5 each of 30 day-old H. caespitosum plants were either sprayed with, or their roots soaked for 1 hr in, a 1 X 106 per ml suspension of conidia prepared from 10-day old cultures of the fungus grown on V-8 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 20-25º C for 72-96 hrs. Sprayed plants were then uncovered and assessed for the appearance of lesions over the next 7-10 days. No foliar symptoms occurred. Plants inoculated by soaking roots were assessed for chlorosis and stunting over 4 months and P. concavum was isolated from root lesions on such plants, similar to those observed in the field, confirming Koch’s postulates. Although reported to cause root deterioration of strawberry, this the first report of a root disease of H. caespitosum caused by P. concavum.