H. David Shew

Arbuscular Mycorrhizal Fungi Increase Organic Carbon Decomposition Under Elevated CO2

A Fungal Culprit to Carbon Loss In some ecosystems, such as in the layer of soil containing plant roots, fungi, and bacteria, increased levels of CO2 should stimulate more efficient aboveground photosynthesis, which in turn should promote increased sequestration of organic carbon in soil through the protective action of arbuscular mycorrhizal fungi. However, in a series of field and microcosm experiments performed under elevated levels of CO2 thought to be consistent with future emissions scenarios, Cheng et al. (p. 1084; see the Perspective by Kowalchuk) observed that these fungi actually promote degradation of soil organic carbon, releasing more CO2 in the process. Counter to expectations, fungi associated with plant roots diminish the carbon pool in soil ecosystems under elevated levels of carbon dioxide. The extent to which terrestrial ecosystems can sequester carbon to mitigate climate change is a matter of debate. The stimulation of arbuscular mycorrhizal fungi (AMF) by elevated atmospheric carbon dioxide (CO2) has been assumed to be a major mechanism facilitating soil carbon sequestration by increasing carbon inputs to soil and by protecting organic carbon from decomposition via aggregation. We present evidence from four independent microcosm and field experiments demonstrating that CO2 enhancement of AMF results in considerable soil carbon losses. Our findings challenge the assumption that AMF protect against degradation of organic carbon in soil and raise questions about the current prediction of terrestrial ecosystem carbon balance under future climate-change scenarios.

Genetic diversity of Rhizoctonia solani AG-3 from potato and tobacco in North Carolina

Anastomosis group 3 (AG-3) of Rhizoctonia solani (teleomorph = Thanatephorus cucumeris) is frequently associated with diseases of potato (AG-3 PT) and tobacco (AG-3 TB). Although isolates of R. solani AG-3 from these two Solanaceous hosts are somatically related based on anastomosis reaction and taxonomically related based on fatty acid, isozyme and DNA characters, considerable differences are evident in their biology, ecology, and epidemiology. However, genetic diversity among field populations of R. solani AG-3 PT and TB has not been documented. In this study, the genetic diversity of field populations of R. solani AG-3 PT and AG-3 TB in North Carolina was examined using somatic compatibility and amplified fragment length polymorphism (AFLP) criteria. A sample of 32 isolates from potato and 36 isolates from tobacco were paired in all possible combinations on PDA plus activated charcoal and examined for their resulting somatic interactions. Twenty-eight and eight distinct somatic compatibility groups (SCG) were identified in the AG-3 PT and AG-3 TB samples, respectively. AFLP analyses indicated that each of the 32 AG-3 PT isolates had a distinct AFLP phenotype, whereas 28 AFLP phenotypes were found among the 36 isolates of AG-3 TB. None of the AG-3 PT isolates were somatically compatible or shared a common AFLP phenotype with any AG-3 TB isolate. Clones (i.e., cases where two or more isolates were somatically compatible and shared the same AFLP phenotype) were identified only in the AG-3 TB population. Four clones from tobacco represented 22% of the total population. All eight SCG from tobacco were associated with more than one AFLP phenotype. Compatible somatic interactions between AG-3 PT isolates occurred only between certain isolates from the same field (two isolates in each of four different fields), and when this occurred AFLP phenotypes were similar but not identical.

Phylogeography of the Solanaceae-infecting Basidiomycota fungus Rhizoctonia solani AG-3 based on sequence analysis of two nuclear DNA loci

BackgroundThe soil fungus Rhizoctonia solani anastomosis group 3 (AG-3) is an important pathogen of cultivated plants in the family Solanaceae. Isolates of R. solani AG-3 are taxonomically related based on the composition of cellular fatty acids, phylogenetic analysis of nuclear ribosomal DNA (rDNA) and beta-tubulin gene sequences, and somatic hyphal interactions. Despite the close genetic relationship among isolates of R. solani AG-3, field populations from potato and tobacco exhibit comparative differences in their disease biology, dispersal ecology, host specialization, genetic diversity and population structure. However, little information is available on how field populations of R. solani AG-3 on potato and tobacco are shaped by population genetic processes. In this study, two field populations of R. solani AG-3 from potato in North Carolina (NC) and the Northern USA; and two field populations from tobacco in NC and Southern Brazil were examined using sequence analysis of two cloned regions of nuclear DNA (pP42F and pP89).ResultsPopulations of R. solani AG-3 from potato were genetically diverse with a high frequency of heterozygosity, while limited or no genetic diversity was observed within the highly homozygous tobacco populations from NC and Brazil. Except for one isolate (TBR24), all NC and Brazilian isolates from tobacco shared the same alleles. No alleles were shared between potato and tobacco populations of R. solani AG-3, indicating no gene flow between them. To infer historical events that influenced current geographical patterns observed for populations of R. solani AG-3 from potato, we performed an analysis of molecular variance (AMOVA) and a nested clade analysis (NCA). Population differentiation was detected for locus pP89 (ΦST= 0.257, significant at P < 0.05) but not for locus pP42F (ΦST= 0.034, not significant). Results based on NCA of the pP89 locus suggest that historical restricted gene flow is a plausible explanation for the geographical association of clades. Coalescent-based simulations of genealogical relationships between populations of R. solani AG-3 from potato and tobacco were used to estimate the amount and directionality of historical migration patterns in time, and the ages of mutations of populations. Low rates of historical movement of genes were observed between the potato and tobacco populations of R. solani AG-3.ConclusionThe two sisters populations of the basidiomycete fungus R. solani AG-3 from potato and tobacco represent two genetically distinct and historically divergent lineages that have probably evolved within the range of their particular related Solanaceae hosts as sympatric species.

Genetic structure of populations of Rhizoctonia solani AG-3 on potato in eastern North Carolina

A polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method was developed to identify and differentiate genotypes of Rhizoctonia solani anastomosis group 3 subgroup PT (AG-3 PT), a fungal pathogen of potato. Polymorphic co-dominant single-locus PCR-RFLP markers were identified after sequencing of clones from a genomic library and digestion with restriction enzymes. Multilocus genotypes were determined by a combination of PCR product and digestion with a specific restriction enzyme for each of seven loci. A sample of 104 isolates from one commercial field in each of five counties in eastern North Carolina was analyzed, and evidence for high levels of gene flow between populations was revealed. When data were clone-corrected and samples pooled into one single North Carolina population, random associations of alleles were found for all loci or pairs of loci, indicating random mating. However, when all genotypes were analyzed, the observed genotypic diversity deviated from panmixia and alleles within and between loci were not randomly associated. These findings support a model of population structure for R. solani AG-3 PT on potato that includes both recombination and clonality.

Expression of the bacteriophage T4 lysozyme gene in tall fescue confers resistance to gray leaf spot and brown patch diseases

Tall fescue (Festuca arundinacea Schreb.) is an important turf and forage grass species worldwide. Fungal diseases present a major limitation in the maintenance of tall fescue lawns, landscapes, and forage fields. Two severe fungal diseases of tall fescue are brown patch, caused by Rhizoctonia solani, and gray leaf spot, caused by Magnaporthe grisea. These diseases are often major problems of other turfgrass species as well. In efforts to obtain tall fescue plants resistant to these diseases, we introduced the bacteriophage T4 lysozyme gene into tall fescue through Agrobacterium-mediated genetic transformation. In replicated experiments under controlled environments conducive to disease development, 6 of 13 transgenic events showed high resistance to inoculation of a mixture of two M. grisea isolates from tall fescue. Three of these six resistant plants also displayed significant resistance to an R. solani isolate from tall fescue. Thus, we have demonstrated that the bacteriophage T4 lysozyme gene confers resistance to both gray leaf spot and brown patch diseases in transgenic tall fescue plants. The gene may have wide applications in engineered fungal disease resistance in various crops.

Detecting migrants in populations of Rhizoctonia solani anastomosis group 3 from potato in North Carolina using multilocus genotype probabilities

ABSTRACT The relative contribution of migration of Rhizoctonia solani anastomosis group 3 (AG-3) on infested potato seed tubers originating from production areas in Canada, Maine, and Wisconsin (source population) to the genetic diversity and structure of populations of R. solani AG-3 in North Carolina (NC) soil (recipient population) was examined. The frequency of alleles detected by multilocus polymerase chain reaction-restriction fragment length polymorphisms, heterozygosity at individual loci, and gametic phase disequilibrium between all pairs of loci were determined for subpopulations of R. solani AG-3 from eight sources of potato seed tubers and from five soils in NC. Analysis of molecular variation revealed little variation between seed source and NC recipient soil populations or between subpopulations within each region. Analysis of population data with a Bayesian-based statistical method previously developed for detecting migration in human populations suggested that six multilocus genotypes from the NC soil population had a statistically significant probability of being migrants from the northern source population. The one-way (unidirectional) migration of genotypes of R. solani AG-3 into NC on infested potato seed tubers from Canada, Maine, and Wisconsin provides a plausible explanation for the lack of genetic subdivision (differentiation) between populations of the pathogen in NC soils or between the northern source and the NC recipient soil populations.

Soil Microbial Responses to Elevated CO2 and O3 in a Nitrogen-Aggrading Agroecosystem

Climate change factors such as elevated atmospheric carbon dioxide (CO2) and ozone (O3) can exert significant impacts on soil microbes and the ecosystem level processes they mediate. However, the underlying mechanisms by which soil microbes respond to these environmental changes remain poorly understood. The prevailing hypothesis, which states that CO2- or O3-induced changes in carbon (C) availability dominate microbial responses, is primarily based on results from nitrogen (N)-limiting forests and grasslands. It remains largely unexplored how soil microbes respond to elevated CO2 and O3 in N-rich or N-aggrading systems, which severely hinders our ability to predict the long-term soil C dynamics in agroecosystems. Using a long-term field study conducted in a no-till wheat-soybean rotation system with open-top chambers, we showed that elevated CO2 but not O3 had a potent influence on soil microbes. Elevated CO2 (1.5×ambient) significantly increased, while O3 (1.4×ambient) reduced, aboveground (and presumably belowground) plant residue C and N inputs to soil. However, only elevated CO2 significantly affected soil microbial biomass, activities (namely heterotrophic respiration) and community composition. The enhancement of microbial biomass and activities by elevated CO2 largely occurred in the third and fourth years of the experiment and coincided with increased soil N availability, likely due to CO2-stimulation of symbiotic N2 fixation in soybean. Fungal biomass and the fungi∶bacteria ratio decreased under both ambient and elevated CO2 by the third year and also coincided with increased soil N availability; but they were significantly higher under elevated than ambient CO2. These results suggest that more attention should be directed towards assessing the impact of N availability on microbial activities and decomposition in projections of soil organic C balance in N-rich systems under future CO2 scenarios.

Effect of Soluble Silica on Brown Patch and Dollar Spot of Creeping Bentgrass

Abstract Disease pressure on cool season turfgrasses is very high in the transition zone due to the heat and humidity of prolonged summers. Creeping bentgrass (Agrostis stolonifera L.) is susceptible to both brown patch (Rhizoctonia solani Kuhn) and dollar spot (Sclerotinia homoeocarpa F.T. Bennett) fungi. The objective of this study was to assess the potential of soluble silica for reducing the severity or incidence of disease on creeping bentgrass. Two mature stands of creeping bentgrass, maintained as putting greens, were used. Both greens were constructed to USGA specifications and planted with either “Penncross” or “Cato” and “Crenshaw” blend creeping bentgrass. Plots were treated biweekly with silica at rates of 0, 25, and 50 kg ha−1. Turf quality was unaffected by silica application. Differences in disease incidence between the silica treatments and the control were observed in 1995 but not in 1996. There was no effect of treatment on tissue Si, suggesting that creeping bentgrass may be a Si excluder. Based on these results, silica may provide some degree of protection from dollar spot and brown patch. However, the level of control is unlikely to satisfy expectations for putting green turf quality.

A Foliar Blight and Tuber Rot of Potato Caused by Phytophthora nicotianae: New Occurrences and Characterization of Isolates

Phytophthora spp. are pathogenic to many plant species worldwide, and late blight, caused by Phytophthora infestans, and pink rot, caused by P. erythroseptica, are two important diseases of potato. Another Phytophthora sp., P. nicotianae, was recovered from pink-rot-symptomatic tubers collected from commercial fields in Nebraska, Florida, and Missouri in 2005, 2006, and 2007, respectively. P. nicotianae also was recovered from foliage obtained from commercial potato fields in Nebraska and Texas exhibiting symptoms very similar to those of late blight. Isolates of P. cactorum also were recovered from foliar infections in a commercial potato field in Minnesota in 2005. Natural infection of potato foliage by P. cactorum and infection of wounded potato tuber tissue via inoculation with zoospores of P. capsici are reported here for the first time. Isolates of P. nicotianae, regardless of origin, were primarily of the A1 mating type. All isolates of P. nicotianae and P. cactorum were sensitive to the fungicide mefenoxam. Optimum growth of P. nicotianae, P. erythroseptica, and P. cactorum in vitro occurred at 25°C; however, only P. nicotianae sustained growth at 35°C. Regardless of the tissue of origin, all isolates of P. nicotianae and P. cactorum were capable of infecting potato tubers and leaves. However, isolates of P. nicotianae were less aggressive than P. erythroseptica isolates only when tubers were not wounded prior to inoculation. Pink rot incidence varied significantly among potato cultivars following inoculation of nonwounded tubers with zoospores of P. nicotianae, ranging from 51% in Red Norland to 19% in Atlantic. Phytophthora spp. also differed significantly in their ability to infect potato leaves. Highest infection frequencies were obtained with P. infestans and levels of infection varied significantly among P. nicotianae isolates. The rate of foliar lesion expansion was similar among isolates of P. nicotianae and P. infestans. Whereas P. infestans infections yielded profuse sporulation, no sporulation was observed with foliar infections of P. nicotianae.

Tuber Rot of Potato Caused by Phytophthora nicotianae: Isolate Aggressiveness and Cultivar Susceptibility

A study was undertaken in 2008 and 2009 to examine potato (Solanum tuberosum) cultivar susceptibility, the potential of other host species to act as sources of inoculum for potato infections, and other aspects of potato-Phytophthora nicotianae interactions. Twelve isolates of P. nicotianae collected from five leaf, one petiole, and six tuber infections of potato from five states, as well as isolates from a variety of other host species, were evaluated for ability to cause tuber rot of potato via inoculation studies. Additionally, the susceptibility of 27 potato cultivars commonly grown in the United States to tuber infection by P. nicotianae was determined. Eighty-three percent of the isolates recovered from potato were highly aggressive, infecting tubers at nearly four times greater incidences than isolates originating from nonpotato hosts. With the exception of two tobacco isolates, zoospores of all isolates recovered from nonpotato hosts were able to infect potato tubers. Russet cultivars were significantly less susceptible to P. nicotianae than red and white cultivars in 2008, and red cultivars in 2009. Umatilla Russet was the most resistant cultivar in both years, whereas Red Norland and Dakota Rose were the most susceptible in both years. Results of a survey for P. nicotianae conducted in four states from 2008 through 2010 confirmed previous observations of naturally occurring infections of potato in Missouri, Nebraska, and Texas, as well as infections of potato in Michigan (documented for the first time). All isolates recovered in the survey were sensitive to mefenoxam (EC50 < 1.0 μg/ml).