Sporulation Capacity and Longevity of Puccinia horiana Teliospores in Infected Chrysanthemum Leaves

Puccinia horiana Henn., a quarantine-significant fungal pathogen and causal agent of chrysanthemum white rust (CWR), was first discovered in the United States in 1977 and later believed to have been eradicated. Recently, however, the disease has sporadically reappeared in the northeastern US. Possible explanations for the reappearance include survival of the pathogen in the local environment, and reintroduction from other locations. To determine the possibility that the pathogen might be overwintering in the field, we undertook the study described here. Results from the study showed that P. horiana teliospores, imbedded in infected leaves, were capable of sporulating 2 weeks after inoculation, and this capacity continued until the leaf became necrotic and desiccated. This is the first report of the extreme susceptibility of P. horiana teliospores to leaf necrosis and desiccation and suggests that field infections following winter are unlikely to originate from teliospores. Teliospore germination on excised leaves was shown to be inhibited by light. Accepted for publication 3 April 2013. Published 23 August 2013.

The infection process of Puccinia xanthii f. sp. ambrosiae-trifidae on Ambrosia trifida

Puccinia xanthii Schwein. f. sp. ambrosiae-trifidae S.W.T. Batra is an obligate parasitic rust fungus of Ambrosia trifida Linn. Field investigations in Liaoning Province, China, showed that it is an effective biocontrol agent of this alien invasive weed. Its infection of the plant was observed by light microscopy combined with Coomassie Brilliant Blue R-250 staining. We report the infection process, including teliospore germination and basidiospore formation on the host leaf surface, penetration of host tissue, and development of fungal hyphae within the host tissue. Fresh teliospores began to germinate from the germ pore within 1 h under suitable conditions and soon produced basidiospores or secondary basidiospores. Basidiospores falling on host leaves germinated from the end of the basidiospore opposite to the apiculus. Appressoria of germ tubes tended to orient along leaf epidermis cell ridges or at junctions near stomata rather than fixing randomly on the leaf surface. These germ tubes grew for short or longer distances before forming appressoria. The rust fungus directly penetrated the host epidermis by infectious pegs rather than through stomata. Within host tissues, the rust fungus formed intraepidermal vesicles, primary hyphae, intracellular hyphae, and M-haustoria. The intricate infectious structures formed by P. xanthii f. sp. ambrosiae-trifidae on or in host tissues suggest that the rust fungus is a suitable organism for researching the interaction between the pathogen and host plant.

The modes of teliospore germination of Ustilago trichophora (Link) Kunze ex Körnicke occurring in Poland

Ustilago trichophora affects <em>Echinochloa crus-galli </em>and other species of <em>Echinochloa </em>genus. <em>U. trichophora </em>is a member of the order Ustilaginales, of which the germinating teliospores form a promycelium with sporidia. Investigations by Ingold (1996) proved that spores coming from various parts of the world germinate in a different way. The aim of the investigation was to determine the mode of germination of <em>U. trichophora </em>spores found in Poland.

Factors Influencing Epidemiology and Management of Blackberry Rust in Cultivated Rubus laciniatus

The blackberry rust pathogen Phragmidium violaceum was first observed in Oregon in spring 2005 on both commercially cultivated Rubus laciniatus (Evergreen blackberry) and naturalized R. armeniacus (Himalayan blackberry). Several commercial plantings suffered severe economic losses. In 2006 to 2008, all five spore stages of this autoecious, macrocyclic rust pathogen were observed annually, and asexual perennation of the pathogen on old leaves or in leaf buds was not evident in the disease cycle. In field experiments, teliospore germination and infection by basidiospores occurred mostly during April. On potted “trap” plants exposed for periods of 1 week under dense collections of dead leaves bearing teliospores, basidiospore infection was associated with wetness durations of >16 h with mean temperatures >8°C. Trap plants placed under the bundles of collected leaves frequently developed spermagonia, whereas only 1 of 630 trap plants placed in a production field of R. laciniatus became diseased, an indication that the effective dispersal distance of basidiospores may be limited. In growth chambers programmed for constant temperatures of 5, 10, 15, 20, 25, and 30°C, a minimum of six continuous hours of leaf wetness was required for infection by urediniospores, with >9 h required for moderate infection (>4 pustules/cm2) at 15 and 20°C. With diurnal temperature regimes averaging 5, 10, 15, 20, or 25°C, urediniospore germination and infection was highest in the range of 5 to 15°C; similarly, in the diurnal environment, >9 h of leaf wetness was required to attain moderate infection. In the field, lime sulfur applied as a delayed dormant treatment significantly suppressed teliospore germination and basidiospore infection. Over two seasons, one application of myclobutanil, a demethylation-inhibitor fungicide, applied in early May near the time of spermagonial appearance provided effective suppression of the summer epidemic.

Puccinia jaceae var. solstitialis Teliospore Priming on Yellow Starthistle

Following the introduction of Puccinia jaceae var. solstitialis to California for biological control of yellow starthistle (Centaurea solstitialis, Asteraceae), teliospores, pycnia, and multiple urediniospore generations have been observed in the field. Because urediniospores have a relatively short life span in the field, functioning teliospores are expected to be necessary for the permanent establishment of P. jaceae var. solstitialis in California. To determine if conditions in California were conducive to this, teliospore emergence and priming were evaluated in the field. A factorial experiment in the laboratory with five incubation times and three incubation temperatures was used to determine teliospore priming requirements. Teliospore production coincided with plant senescence in August and September at two sites in 2 years; fewer teliospores were produced in 2006, suggesting inconsistent teliospore production may limit population growth and contribute to local extinctions in some areas. When teliospores were primed in the field, germination was low through the fall and abruptly peaked in January during both years. In the laboratory, teliospore germination increased as incubation time increased from 2 to 6 weeks and temperatures decreased from 12 to 4°C. A degree-hour model derived from laboratory data accurately predicts when teliospores are primed for germination in the field. Based on the results obtained in this study, it is apparent that teliospore germination can occur over a range of priming conditions. However, lower temperatures and longer incubation periods are superior in breaking teliospore dormancy.

Light and electron microscopy of teliospores and teliospore germination in the rust fungus Coleosporium ipomoeae

A combination of light, scanning, and transmission electron microscopy was used to examine teliospores and teliospore germination in the rust fungus Coleosporium ipomoeae (Schw.) Burrill, a parasite of the wild morning glory Ipomoea coccinea L. Telia developed on abaxial surfaces of infected leaves and appeared as orange, waxy crusts usually associated with uredinia. Mature teliospores were cylindrical to slightly clavate in shape and thin-walled. The presence of chitin in the spore wall was demonstrated using wheat germ agglutinin gold labeling. Teliospores were surrounded by an electron-dense extracellular material. Each spore possessed a large prominent nucleus containing synaptonemal complexes indicative of prophase I meiotic nuclei. Following hydration, the nucleus of each spore completed meiosis and the spore was divided into four uninucleate compartments by the formation of three transverse septa. Each compartment gave rise to a germ tube into which the nucleus and cytoplasm migrated. Germ tubes developed into long slender sterigmata that grew through the extracellular material within the telium to become exposed on the leaf surface. A basidiospore then developed at the tip of each sterigma. Once the nucleus moved from the sterigma into the spore, a septum formed to delimit the spore from the tip of the sterigma.Key words: fungal spores, transmission and scanning electron microscopy, high pressure freezing.

Survival of Teliospores of Tilletia indica in Arizona Field Soils

The survival of teliospores of the Karnal bunt of wheat pathogen, Tilletia indica, was determined in field plots in Tucson, AZ. Two methods were used to test viability during a 48-month period in which 21-μm-pore-size polyester mesh bags of teliospore-infested soil were buried in irrigated and nonirrigated field plots at two sites. One method determined the total number of viable teliospores in a soil sample, regardless of whether or not they could be extracted from the soil using a sucrose centrifugation technique. The total number of viable teliospores declined over time in both irrigated and nonirrigated field plots and in the same soils in the laboratory. Based on nonlinear regressions, total number of viable teliospores decreased from 55.7% at time zero to 9.7 and 6.7% for nonirrigated and irrigated field soils, respectively, in 48 months. Total number of viable teliospores in soil in the laboratory decreased from 55.7 to 34.0% after 48 months. The second method determined germination percentages of teliospores extracted from the soil samples by means of a sucrose centrifugation technique. Based on linear regressions of transformed data, germination of teliospores extracted from irrigated and nonirrigated field soils, and control (laboratory) soil, significantly decreased over time. The rate of decrease in germination was significantly greater for teliospores from irrigated field plots than from nonirrigated plots and the laboratory soil. At time zero, 55.7% of teliospores germinated, and by 48 months, average germination of teliospores extracted from soil in nonirrigated plots had decreased to 13.6% compared with 4.4% in irrigated plots and 36.8% for teliospores in the laboratory control. Regression over time of total number of viable teliospores accounted for more of the overall variability than did regression over time of germination percentages of extracted teliospores. Neither field site nor soil depth had any effect on total number of viable teliospores or on teliospore germination percentages.