Distribution of Puccinia punctiformis in above-ground tissue of Cirsium arvense (Californian thistle)

Cirsium arvense (Californian thistle) is a problematic weed in agricultural systems throughout New Zealand and the rust fungus Puccinia punctiformis is a potential biological control agent for this weed. Puccinia punctiformis can systemically infect thistles but the movement of the pathogen in planta is not fully understood. This research determined the level of infection in planta caused by P. punctiformis at a single time point. The concentration of P. punctiformis DNA in planta was determined to ascertain the location of the fungus within naturally field-infected C. arvense. Quantitative polymerase chain reaction was undertaken on above-ground symptomatic and asymptomatic C. arvense tissue at various locations within leaves (top, middle and bottom) and the main stem. All C. arvense shoots had detectable amounts of P. punctiformis but the concentration was 100× greater in symptomatic compared with asymptomatic shoots. In general, the concentration of fungus progressed up the leaves with a significant effect between locations (P<0.001). Puccinia punctiformis was found in planta but broadscale disease of C. arvense does not occur and the reason for this is unknown.

Insect-Transmitted Urediniospores of the Rust Puccinia punctiformis Cause Systemic Infections in Established Cirsium arvense Plants

The rust fungus Puccinia punctiformis has potential as a biological control agent for creeping thistle Cirsium arvense, because systemically infected shoots usually die before flowering. The mechanism of rust transfer as well as the spore type responsible for systemic infections have been a source of controversy. One possibility of successful transmission is the use of the weevil Ceratapion onopordi as a vector. Our results from a garden experiment show that urediniospores transmitted by the weevil are able to induce systemic infections in established thistle clones. Furthermore, the weevil origin and the date of rust treatment significantly influenced the number of rust-infected shoots. The earlier a shoot was treated, the higher the probability of rust transmission. These results challenge the current belief that teliospores passing through the soil and infecting root buds are the major cause of systemic infections in the field. Further research on biological control of creeping thistle should therefore concentrate on the application of urediniospores to enhance systemic rust infections.

First Report of Silybum marianum as a Host of Puccinia punctiformis

Silybum marianum (L.) Gaertn. (milk thistle) is a problematic invasive weed in the western United States. The rust fungus, Puccinia punctiformis (F. Strauss) Rohl., is found throughout the world as a pathogen of Cirsium arvense (L.) Scop. (Canadian thistle). Recently, plants of S. marianum grown from surface-disinfested seeds in our quarantine greenhouse were parasitized by a rust. Apparently, an isolate of P. punctiformis collected from C. arvense in Turkey that was present in the greenhouse had spread to adjacent S. marianum plants and caused infection without applying any artificial dew period. Ribosomal internal transcribed spacer region sequences from fungal spore DNA isolated from the two hosts were identical. Initial signs on S. marianum were abundant, fragrant spermogonia on large leaves. These signs occur on secondary shoots of C. arvense and are indicative of systemic fungal infection (1). As the fungus infection developed on S. marianum, uredinia and urediniospores were produced. Sori on older leaves also produced teliospores. Urediniospores from infected leaves were harvested and sprayed uniformly on eight 17-day-old plants of S. marianum grown in isolation from P. punctiformis. The spore suspension consisted of 4 mg urediniospores suspended in 40 ml distilled water. Inoculated plants were incubated for 18 h in a dew chamber at 20°C in the dark and transferred to a greenhouse (20 to 25°C, 30 to 50% relative humidity, and natural light). After 13 days, uredia with urediniospores developed on four of the plants. Using the same procedure, inoculations were repeated on plants of S. marianum and S. eburneum Coss. & Durieu (the only other species described in the genus) with urediniospores of a domestic isolate of the fungus from C. arvense in Maryland. Of 51 inoculated plants of S. marianum, 23 became infected and produced uredinia. None of the 12 inoculated plants of S. eburneum showed symptoms of infection. In nature, C. arvense and S. marianum occupy different ecological areas. C. arvense is found predominately in humid temperate habitats, while S. marianum is found in habitats with a dry Mediterranean climate. Life cycles of each host are also different. C. arvense is a perennial that emerges in spring and dies back in winter, while S. marianum is a winter annual that emerges in fall and dies in late spring. Because of the differences in life cycles combined with the different geographical distribution, P. punctiformis from C. arvense may rarely encounter susceptible S. marianum plants in the field. Since fungal spores can be produced routinely on artificially inoculated plants, there might be potential to use P. punctiformis for biological control of S. marianum. To our knowledge, this is the first report of S. marianum as a host for P. punctiformis. Reference: (1) A. H. R. Buller. Puccinia sauveolens and its sexual process. Page 345 in: Researches on Fungi. Vol VII. The Sexual Process in the Uredinales, Toronto, Canada, 1950.