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Berner D.K.,U.S. Department of Agriculture | Smallwood E.L.,U.S. Department of Agriculture | Vanrenterghem M.,French National Institute for Agricultural Research | Cavin C.A.,U.S. Department of Agriculture | And 6 more authors.
Biological Control | Year: 2015

Systemic disease of Cirsium arvense caused by Puccinia punctiformis depends on teliospores, from telia that are formed from uredinia, on C. arvense leaves. Uredinia result from infection of the leaves by aeciospores which are one main source of dispersal of the fungus. However, factors governing aeciospore spread, germination, infection, and conversion to uredinia and telia have not been extensively investigated. In this study, effective spread of aeciospores from a source area in a field was fitted to an exponential decline model with a predicted maximum distance of spread of 30. m from the source area to observed uredinia on one leaf of one C. arvense shoot. However, the greatest number of shoots bearing leaves with uredinia/telia was observed within 12 m of the source area, and there were no such shoots observed beyond 17 m from the source area. Aeciospore germination under laboratory conditions was low, with a maximum of about 10%. Temperatures between 18 °C and 25 °C were most favorable for germination with maximum germination at 22. °C. Temperature and dew point data collected from the Frederick, MD airport indicated that optimum temperatures for aeciospore germination occurred in late spring from about May 18 to June 20. Dew conditions during this period were favorable for aeciospore germination. A total of 122 lower leaves, 2 per shoot, on 61 C. arvense shoots were individually inoculated in a dew tent in a greenhouse by painting suspensions of aeciospores onto the leaves. Of these inoculated leaves, 47 produced uredinia within an average of 21.2 ± 6.9 days after inoculation. Uredinia were also produced, in the absence of dew, on 17 non-inoculated leaves of 12 shoots. These leaves were up to 4 leaves above leaves on the same shoots that had been individually and separately inoculated. Results of PCR tests for the presence of the fungus in non-inoculated leaves that were not bearing uredinia, showed that 44 leaves above inoculated leaves on 27 shoots were positive for the presence of the fungus. These leaves were up to 5 leaves above inoculated leaves on the same shoot. Uredinia production and positive PCR results on leaves above inoculated leaves on the same shoot indicated that aeciospore infection was weakly systemic. In other tests in which all leaves of plants were spray-inoculated with aeciospores, uredinia were produced by 10. days after inoculation and converted to telia and sole production of teliospores in about 63. days after inoculation. Successful systemic aeciospore infections in late spring would be expected to result in uredinia production in excess of a 1:1 ratio of aeciospore infections to uredinia and ultimately telia production in late summer. In this manner, systemic aeciospore infections would promote increased density of telia that lead to systemic infections of roots in the fall. © 2015 Published by Elsevier Inc.

Berner D.K.,U.S. Department of Agriculture | Smallwood E.L.,U.S. Department of Agriculture | Cavin C.A.,U.S. Department of Agriculture | McMahon M.B.,U.S. Department of Agriculture | And 7 more authors.
Biological Control | Year: 2015

Canada thistle (Cirsium arvense) is one of the worst weeds in temperate areas of the world. A rust fungus, Puccinia punctiformis, was first proposed as a biological control agent for C. arvense in 1893. The rust causes systemic disease which ultimately kills C. arvense plants. In 2013 it was demonstrated in four countries, that inoculation of C. arvense rosettes in the fall with ground telia-bearing leaves can initiate epidemics of systemic rust disease with an average of 28% of inoculated rosettes producing a systemically diseased shoot the following spring. Other rosettes that emerged near inoculation points in spring were stunted and appeared diseased. To determine whether other rosettes were diseased, a chemiluminescence western slot blot test, applying polyclonal antibodies raised against P. punctiformis antigens, was developed to detect the fungus in roots. Rosettes were inoculated with telia-bearing leaves in the fall in Maryland, USA and Veroia, Greece. Roots of asymptomatic rosettes that emerged adjacent to inoculation points the following spring were tested for the presence of the fungus with the slot blot test. Rosettes that had diseased shoots were recorded. Based on the slot blot tests, 50-60% of the asymptomatic rosettes adjacent to inoculation points were positive for presence of the rust and likely to be systemically diseased. To demonstrate that systemic disease leads to C. arvense decline, C. arvense shoot densities were measured annually at 10 sites, in three countries, that had been inoculated with telia-bearing leaves in the fall between 2008 and 2012. Changes in C. arvense shoot densities over time were calculated. Average reductions in C. arvense density across the 10 sites were 43.1. ±. 10.0% at 18 months after inoculation, 63.8. ±. 8.0% at 30. months after inoculation, and 80.9. ±. 16.5% at 42. months after inoculation, and 72.9. ±. 27.2% at 54. months after inoculation; the 54. month reduction was, however, based on only two sites. © 2015.

Berner D.,U.S. Department of Agriculture | Lagopodi A.L.,Aristotle University of Thessaloniki | Kashefi J.,USDA ARS European Biological Control Laboratory | Mukhina Z.,All Russia Research Institute for Biological Means of Plant Protection | And 5 more authors.
Biological Control | Year: 2014

Russian thistle (Salsola tragus, tumbleweed, RT) is a problematic invasive weed in the United States (U.S.) and is a target of biological control efforts. The facultative saprophytic fungus Colletotrichum salsolae (CS) kills RT plants in greenhouse tests and is specific to Salsola spp., which are not native in the U.S. However, the effectiveness of CS in controlling RT has not been previously demonstrated. The objectives of this study were to determine in field tests: (1) disease progress of CS in time; (2) the relationship of disease progress to rainfall and temperature; (3) the effect of CS on RT plant density. Field tests were established in Serres and Kozani, Greece and Taman and Tuzla, Russia with isolates of the pathogen collected in the respective countries. Solid inoculum was prepared by asceptically inoculating sterile mixtures of grain and grain hulls with axenic cultures of CS. Spore suspensions used in Russia were prepared by blending pure sporulating cultures of CS with distilled water and diluting the suspension to 106 conidia per ml. Six field plots, each subdivided into 36 subplots, of an RT infested field in Serres, Greece were inoculated on October 23, 2006 by placing about 300g of solid inoculum in the center of each plot. Four field plots, similarly subdivided, in each of two fields at Kozani, Greece were inoculated in the same way on October 1, 2010. RT density was counted and recorded in each sub-plot prior to inoculation and in September in each of 2years following inoculation. Disease incidence and/or severity in each sub-plot were recorded at about 7-days intervals after inoculation. Rainfall and temperature data, from inoculation until 40-55days after inoculation, were collected and recorded at Serres and at the Kozani airport meteorological station. Disease progressed rapidly at both sites and was correlated with cumulative rainfall. By 2years after inoculation, RT had been eliminated from the Serres site and one field in Kozani. In the other Kozani field, RT density declined to 0-25% from original densities of about 80% in large areas of the field. RT plants in Taman and Tuzla, Russia were inoculated either with 250g of grain inoculum or with a suspension of 106 conidia sprayed onto each plant until runoff. The proportion of diseased tissue reached 1.0 by 55days after inoculation in both sites. Non-inoculated plants that were near inoculated plants became diseased quickly and reached the same disease severity as inoculated plants. Disease severity was correlated with cumulative rainfall but not temperature. This pathogen and inoculation procedure offers a low-cost solution to RT infestations. Since CS is specific to Salsola spp., this effective biological control is also environmentally friendly. © 2014.

Berner D.,U.S. Department of Agriculture | Smallwood E.,U.S. Department of Agriculture | Cavin C.,U.S. Department of Agriculture | Lagopodi A.,Aristotle University of Thessaloniki | And 6 more authors.
Biological Control | Year: 2013

Canada thistle (Cirsium arvense, CT) is one of the worst weeds in temperate areas of the world. The rust fungus Puccinia punctiformis was first proposed as a biological control agent for CT in 1893. The rust causes systemic disease, is specific to CT, and is in all countries where CT is found. Despite a 120-year lapse since biological control with the rust was proposed, establishment of epiphytotics of the rust have previously been unsuccessful due to incomplete understanding of the disease cycle. In this study, newly-emerging rosettes in the fall are proposed as the physical and temporal infection courts for basidiospores, from germinating teliospores, to systemically infect CT and give rise to systemically diseased shoots the following spring. To test this hypothesis, rosettes of CT were inoculated in the fall with either telia-bearing leaves collected in mid-summer or with greenhouse-produced teliospores. Field sites were located near Kozani, Greece, Moscow, Russia, Christchurch, New Zealand, and Ft. Detrick, Maryland, USA. Telia-bearing leaves, which were used as inoculum in 12 of 13 field sites, were collected near each field site from CT shoots in close proximity to systemically diseased CT shoots producing aeciospores in the spring. Aeciospore infections of the leaves of these nearby shoots gave rise to uredinia which turned to telia in mid- to late-summer. Temperature and dew conditions at inoculation in the fall at each site were very favorable for teliospore germination. Rosettes inoculated in the fall were marked with flags, and systemically diseased shoots emerging near these flags the following spring were recorded. In 11 of the sites in these countries, individual rosettes were inoculated 2, 4, 6, or 8 times with telia-bearing leaves. Proportions of rosettes giving rise to systemically diseased shoots, out of the number of rosettes inoculated, were analyzed. Inoculations in all 13 sites produced systemically diseased shoots. A separate study on the phenology of CT showed that the maximum rate of leaf abscission occurred at the time of maximum emergence of new CT rosettes in the fall. This period coincided with an annually occurring period of sustained dew and favorable temperatures for teliospore germination. In nature, abscising telia-bearing leaves likely come into contact with a receptive rosette during favorable conditions for teliospore germination in the fall. This study demonstrates that epiphytotics of systemic rust disease of CT can be routinely established, by mimicking the natural disease cycle. © 2013.

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