Arbuscular mycorrhizal fungi increase organic carbon decomposition under elevated CO2

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 (CO(2)) 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 CO(2) 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.

Warming and elevated ozone induce tradeoffs between fine roots and mycorrhizal fungi and stimulate organic carbon decomposition

Climate warming and elevated ozone (eO₃) are important climate change components that can affect plant growth and plant-microbe interactions. However, the resulting impact on soil carbon (C) dynamics, as well as the underlying mechanisms, remains unclear. Here, we show that warming, eO₃, and their combination induce tradeoffs between roots and their symbiotic arbuscular mycorrhizal fungi (AMF) and stimulate organic C decomposition in a nontilled soybean agroecosystem. While warming and eO₃ reduced root biomass, tissue density, and AMF colonization, they increased specific root length and promoted decomposition of both native and newly added organic C. Also, they shifted AMF community composition in favor of the genus Paraglomus with high nutrient-absorbing hyphal surface over the genus Glomus prone to protection of soil organic C. Our findings provide deep insights into plant-microbial interactive responses to warming and eO₃ and how these responses may modulate soil organic C dynamics under future climate change scenarios.

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.

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

BACKGROUND

The 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).

RESULTS

Populations 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 (Phi ST = 0.257, significant at P < 0.05) but not for locus pP42F (Phi ST = 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.

CONCLUSION

The 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.

Managing the Race Structure of Phytophthora parasitica var. nicotianae with Cultivar Rotation

Deployment of tobacco cultivars with single-gene, complete resistance to race 0 of the tobacco black shank pathogen, Phytophthora parasitica var. nicotianae, has resulted in a rapid increase in the occurrence of race 1 of the pathogen in North Carolina. Cultivar-rotation studies were conducted in three fields to assess how different levels and types of resistance affected the race structure and population dynamics of the pathogen when deployed in fields initially containing single or mixed races of the pathogen. In a field with both races present, a high level of partial resistance in cv. K 346 was most effective in reducing disease and decreasing the proportion of race 1 in the pathogen population. The deployment of complete resistance in cv. NC 71 resulted in intermediate levels of disease control and race 1 became the predominate race. The cv. K 326, with a low level of partial resistance, had the highest levels of disease, and race 0 was the dominant race recovered. In a field where no race 1 was detected initially, disease incidence was high with the use of partial resistance. Complete resistance was very effective in suppressing disease, but race 1 was recovered after only one growing season. By the end of the third growing season, race 1 was recovered from most treatments where single-gene resistance was deployed. A high level of partial resistance was most effective in suppressing disease in a field where race 1 initially was the predominant race. A rotation between cultivars with single-gene resistance and cultivars with a high level of partial resistance should provide the most effective approach to black shank management. This rotation will reduce disease incidence and minimize race shifts in the pathogen and, over time, should prolong the usefulness of the Ph gene for black shank control in commercial production of tobacco.

Origin of the Black Shank Resistance Gene, Ph, in Tobacco Cultivar Coker 371-Gold

Flue-cured tobacco (Nicotiana tabacum) cultivar Coker 371-Gold (C 371-G) possesses a dominant gene, Ph, that confers high resistance to black shank disease, caused by race 0 of the soil-borne pathogen Phytophthora parasitica var. nicotianae. The origin of this gene is unknown. Breeding lines homozygous for the Ph gene were hybridized with NC 1071 and L8, flue-cured and burley genotypes known to possess qualitative resistance genes from Nicotiana plumbaginifolia and N. longiflora, respectively. The F₁ hybrids were out-crossed to susceptible testers and the progenies evaluated in field black shank nurseries and in greenhouse disease tests with P. parasitica var. nicotianae race 0. Results showed that Ph was allelic to Php from N. plumbaginifolia in NC 1071. Testcross populations of hybrids between burley lines homozygous for Ph and L8, possessing Phl from N. longiflora, showed that Ph and Phl integrated into the same tobacco chromosome during interspecific transfer. Nevertheless, the two loci were estimated to be 3 cM apart. Random amplified polymorphic DNA (RAPD) analyses of the testcross progenies confirmed that recombination between the two loci was occurring. Forty-eight RAPD markers linked to Ph in doubled haploid lines were used in cluster analyses with multiple accessions of N. longiflora and N. plumbaginifolia, breeding lines L8, NC 1071, and DH92-2770-40, and cultivars K 326, Hicks, and C 371-G. A cladogram or region tree confirmed the data obtained from field and greenhouse trials, that Ph, transferred from C 371-G to DH92-2770-40, and Php in NC 1071 were allelic and originated from N. plumbaginifolia.

Contrasting Warming and Ozone Effects on Denitrifiers Dominate Soil N2O Emissions

Nitrous oxide (N₂O) in the atmosphere is a major greenhouse gas and reacts with volatile organic compounds to create ozone (an air pollutant) in the troposphere. Climate change factors such as warming and elevated ozone (eO₃) affect N₂O fluxes, but the direction and magnitude of these effects are uncertain and the underlying mechanisms remain unclear. We examined the impact of simulated warming (control + 3.6 °C) and eO₃ (control + 45 ppb) on soil N₂O fluxes in a soybean agroecosystem. Results obtained showed that warming significantly increased soil labile C, microbial biomass, and soil N mineralization, but eO₃ reduced these parameters. Warming enhanced N₂O-producing denitrifers ( nirS- and nirK-type), corresponding to increases in both the rate and sum of N₂O emissions. In contrast, eO₃ significantly reduced both N₂O-producing and N₂O-consuming ( nosZ-type) denitrifiers but had no impact on N₂O emissions. Further, eO₃ offsets the effects of warming on soil labile C, microbial biomass, and the population size of denitrifiers but still increased N₂O emissions, indicating a direct effect of temperature on N₂O emissions. Together, these findings suggest that warming may promote N₂O production through increasing both the abundance and activities of N₂O-producing microbes, positively feeding back to the ongoing climate change.

Fitness of Races 0 and 1 of Phytophthora parasitica var. nicotianae

Deployment of tobacco (Nicotiana tabacum) varieties with complete resistance to race 0 of Phytophthora parasitica var. nicotianae has led to a rapid increase in the field populations of race 1 in North Carolina. In a field study, population levels of race 1 decreased relative to race 0 when cultivars with partial resistance to both races were planted, suggesting that race 1 isolates were less fit than race 0 isolates. Experiments were conducted to quantify differences in aggressiveness and survivability of the two races. Tobacco varieties with low, moderate, or high levels of partial resistance were inoculated with 60 pathogen isolates, and symptom development was monitored for 3 weeks. Race 0 isolates were more aggressive than race 1 isolates on cultivars with moderate or high levels of partial resistance; incubation periods were shorter and root rot severity was greater with race 0 isolates. Isolates of race 1, however, caused greater stunting of plants with moderate and high levels of partial resistance than race 0 isolates. Field microplots were infested with either a single race or an equal mixture of each race. Soil samples were collected at the end of two growing seasons and again the following spring. Pathogen populations declined from 40 to 80% during winter months, but population declines for race 0 were lower than for race 1 in each treatment over each winter. Race shifts from race 1 to race 0 that were observed in the presence of cultivars with partial resistance appear to be primarily the result of differences in aggressiveness of the races, with a possible minor effect of enhanced overwintering survival of race 0 compared with race 1.

Shifts in the Composition and Activities of Denitrifiers Dominate CO2 Stimulation of N2O Emissions

Elevated atmospheric CO₂ (eCO₂) often increases soil N₂O emissions, but the underlying mechanisms remain largely unknown. One hypothesis suggests that high N₂O emissions may stem from increased denitrification induced by CO₂ enhancement of plant carbon (C) allocation belowground. However, direct evidence illustrating linkages among N₂O emissions, plant C allocation, and denitrifying microbes under eCO₂ is still lacking. We examined the impact of eCO₂ on plant C allocation to roots and their associated arbuscular mycorrhizal fungi and its subsequent effects on N₂O emissions and denitrifying microbes in the presence of two distinct N sources, ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N). Our results showed that the form of the N inputs dominated the effects of eCO₂ on N₂O emissions: eCO₂ significantly increased N₂O emissions with NO₃^--N inputs but had no effect with NH₄⁺-N inputs. eCO₂ increased plant biomass N more with NH₄⁺-N than with NO₃^--N inputs, likely reducing microbial access to available N under NH₄⁺-N inputs and/or contributing to higher N₂O emissions under NO₃^--N inputs. eCO₂ enhanced root and mycorrhizal N uptake and also increased N₂O emissions under NO₃^--N inputs. Further, eCO₂ enhancement of N₂O emissions under NO₃^--N inputs concurred with a shift in the soil denitrifier community composition in favor of N₂O-producing (nirK- and nirS-type) over N₂O-consuming (nosZ-type) denitrifiers. Together, these results indicate that eCO₂ stimulated N₂O emissions mainly through altering plant N preference in favor of NH₄⁺ over NO₃^- and thus stimulating soil denitrifiers and their activities. These findings suggest that effective management of N sources may mitigate N₂O emissions by negating the eCO₂ stimulation of soil denitrifying microbes and their activities.

Sources of Inoculum and Management for Rhizoctonia solani Damping-off on Tobacco Transplants under Greenhouse Conditions

Damping-off and target spot are important diseases of tobacco transplants produced under greenhouse conditions. Identification of sources of inoculum for these diseases caused by Rhizoctonia solani is an important first step in disease management. Control strategies based on sanitation and the eradication of primary inoculum were studied. Potting mix and Styrofoam trays used in transplant production were assayed to determine if they were sources of primary inoculum. Eleven sources of potting mix were sampled over a 2-year period. None of the mixes contained viable inoculum of R. solani. R. solani was isolated from previously used trays after 1 year of storage by removing and plating pieces of Styrofoam from individual tray cells on alkaline water agar (AWA). Sclerotia and melanized hyphae of R. solani were observed in the cracks present in the cells of the trays. Dry heat (70 to 80°C for 2 h) and chemical (sodium hypochlorite and sodium chloride) treatments reduced the levels of inoculum on trays up to 45% compared to controls, but only methyl bromide and steam treatments (80°C for 0.5 to 2.0 h) eradicated inoculum of R. solani from trays. Elimination of primary inoculum from previously used trays effectively controlled target spot and stem rot diseases caused by R. solani.

Initial Cellular Interactions Between Thielaviopsis basicola and Tobacco Root Hairs

ABSTRACT Cellular events that occur during the initial interactions between Thielaviopsis basicola and root hairs of tobacco (Nicotiana tabacum) were examined microscopically. Time-course documentation of the infection process indicated a dynamic interaction between T. basicola and the living host cell. Upon root hair contact and recognition, the vegetative apex of T. basicola rapidly differentiated to form infection structures, and the host cell responded cytologically. Penetration was achieved by threadlike hyphae that subsequently developed distal swellings, and intracellular hyphae of sickle-shaped morphology advanced from the distal swelling and colonized the cell. Streaming of the host cytoplasm became aggregated near the infection site prior to penetration and accumulated around the infecting hyphae as long as the host cell was viable. Substantial callose deposition, in the form of a bell-shaped collar around infection structures, resulted from the cytological activity at the infection site. Penetration of dead root hairs was common, but did not lead to the development of infection structures or to a sustained association with the host tissue; T. basicola exited dead root hairs and resumed vegetative growth. The establishment of the parasitic relationship by T. basicola was characteristic of hemibiotrophic fungi in that, initially, biotrophic infection led to tissue colonization, and host cell survival was limited under parasitism.

Inheritance of Resistance to Race 0 of Phytophthora parasitica var. nicotianae from the Flue-Cured Tobacco Cultivar Coker 371-Gold

Black shank, caused by Phytophthora parasitica var. nicotianae, is a widespread and severe disease of tobacco throughout the southeastern United States. Partial resistance derived from the cigar tobacco cultivar Florida 301 has been the primary means of reducing losses to the disease for many years. The recently released tobacco cultivar, Coker 371-Gold (C 371-G), was found to provide an additional source of resistance to P. parasitica var. nicotianae. Although the resistance in C 371-G is being used widely by breeders, the origin and inheritance of this resistance mechanism was unknown. Two populations of doubled haploid lines derived from C 371-G were used to determine that C 371-G possesses a single, dominant gene designated Ph, which confers a very high level of resistance to race 0 of P. parasitica var. nicotianae. A greenhouse inoculation procedure was developed that provided an efficient means of screening for the presence of this resistance gene prior to selection in the field, and confirmed that Ph provides complete resistance to race 0 but no resistance to race 1 of P. parasitica var. nicotianae. Because Florida 301 resistance is effective against both races of the pathogen that occur in the major tobacco growing areas of the United States, combination of these two sources of resistance should provide enhanced protection of new tobacco cultivars to P. parasitica var. nicotianae.

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.

Morphological and molecular characterization of Phytophthora glovera sp. nov. from tobacco in Brazil

A root rot disease of cultivated tobacco called yellow stunt has been observed in the burley tobacco production areas of Brazil since the early 1990s. Root infecting fungi and straminipiles were isolated from the roots of diseased tobacco plants, including a semi-papillate, homothallic, slow growing Phytophthora species. Pathogenicity trials confirmed that Phytophthora sp. caused root rot and stunting of burley and flue-cured tobaccos. Morphological characteristics of the asexual and sexual stages of this organism did not match any reported Phytophthora species and were very different from the widely known tobacco black shank pathogen P. nicotianae. Phylogenetic analysis based on sequences of the internal transcribed spacer rDNA, β-tubulin and translation elongation factor 1-α regions indicated that this organism represents a previously unreported Phytophthora species that is significantly supported in clade 2 and most closely related to P. capsici. However P. glovera differs from P. capsici in a number of morphological characters, most significantly P. glovera is homothallic and produces both paragynous and amphigynous antheridia while P. capsici is heterothallic and produces only amphigynous antheridia. In this paper we confirmed pathogenicity of this species on tobacco and describe the morphological and molecular characteristics of Phytophthora glovera sp. nov.

Occurrence of Race 3 of Phytophthora nicotianae in North Carolina, the Causal Agent of Black Shank of Tobacco

Black shank, caused by the oomycete Phytophthora nicotianae, causes significant annual yield losses in tobacco. Race 3 of P. nicotianae is reported here for the first time from North Carolina. It was identified from a North Carolina tobacco field with a history of tobacco varieties with Phl gene resistance and numerous field sites with no known deployment of varieties with the Phl gene. Race 3 was originally described from cigar-wrapper tobacco in Connecticut in the 1970s, but has not been reported in any other location since. Race 3 was defined as overcoming the Phl gene from Nicotiana longiflora but not the Php gene from N. plumbaginifolia. Stem and root inoculations were conducted on a set of host differentials to determine the virulence of North Carolina isolates. Stem inoculation was unable to distinguish between races 0 and 3 of P. nicotianae and is not a reliable method of identifying these virulence types. Race 1 gave a unique phenotype using stem inoculation. Root inoculation was the only reliable means of distinguishing between races 0 and 3. This is the first report of race 3 in North Carolina and the first report of damage to seedlings from root inoculations and to plants containing the Phl gene in naturally infested soil.

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.

Identification and Quantification of Ascospores as the Primary Inoculum for Collar Rot of Greenhouse-Produced Tobacco Seedlings

Collar rot, caused by Sclerotinia sclerotiorum, is a severe disease of tobacco seedlings grown in greenhouses. A semiselective medium was adapted and used to detect the presence and quantity of ascospores in commercial greenhouses. Petri dishes of the semiselective medium were placed inside and outside of greenhouses in four counties during the transplant production period in 1995 and 1996. Ascospores were present throughout the production period each year (February to April) and were confirmed to be the primary inoculum for the disease. Significant differences were observed in the number of ascospores trapped within and between counties. Peak numbers of ascospores were trapped between 10 and 12 a.m., and higher numbers of ascospores were trapped outside than inside houses. In general, distribution of ascospores inside houses was uniform unless a high concentration of apothecia was present very close to one section of the greenhouse. The semiselective medium and trapping technique used in this study may allow development of a forecasting system for collar rot of tobacco based on the presence and level of pathogen inoculum.

An Assessment of the Genetic Diversity in a Field Population of Phytophthora nicotianae with a Changing Race Structure

One hundred fifty-three isolates of Phytophthora nicotianae that were collected over a 4-year period from a single field were subjected to amplified fragment length polymorphism (AFLP) analysis to investigate the effect of different types of resistance in tobacco (Nicotiana tabacum) on genetic diversity in the pathogen population. No race 1 isolates were detected in the field prior to initiating the study, but the race was present in multiple plots by the end of the 4-year period. There were 102 race 0 isolates and 51 race 1 isolates characterized. Seventy-six of the 153 isolates had a unique AFLP profile, whereas the remaining 77 isolates were represented by 27 AFLP profiles shared by at least two isolates. Isolates of both races were found in both the unique and shared AFLP profile groups. Twenty-three of the AFLP profiles were detected in multiple years, indicating a clonal component to the pathogen population. Race 1 isolates that were detected over multiple years were always obtained from the same plot. No race 1 profile was found in more than one plot, confirming the hypothesis that the multiple occurrences of the race throughout the field were the result of independent events and not pathogen spread. Three identical race 0 AFLP profiles occurred in noncontiguous plots, and in each case, the plots contained the same partially resistant variety. Cluster analysis provided a high level of bootstrap support for 41 isolates in 19 clusters that grouped primarily by race and rotation treatment. Estimates of genetic diversity ranged from 0.365 to 0.831 and varied depending on tobacco cultivar planted and race. When averaged over all treatments, diversity in race 1 isolates was lower than in race 0 isolates at the end of each season. Deployment of single-gene resistance initially decreased genetic diversity of the population, but the diversity increased each year, indicating the pathogen was adapting to the host genotypes deployed in the field.

Evaluation of Tobacco Germplasm for Seedling Resistance to Stem Rot and Target Spot Caused by Thanatephorus cucumeris

Stem rot and target spot of tobacco, caused by Rhizoctonia solani and its teleomorph Thanatephorus cucumeris, respectively, can cause serious problems in production of tobacco (Nicotiana tabacum) seedlings. Previous screens for genetic resistance in tobacco have been limited. The objective of this study was to evaluate 97 genotypes composing several classes of tobacco and related Nicotiana spp. for seedling resistance to stem rot and target spot. Significant differences in disease incidence initially were observed among the genotypes for both stem rot and target spot; however, resistance to target spot was not observed when disease pressure was high. Partial resistance to stem rot was observed in several genotypes in repeated tests. These accessions may be useful as a source of resistance to R. solani in future breeding efforts.

Evaluation of Wounds as a Factor to Infection of Cabbage by Ascospores of Sclerotinia sclerotiorum

A semi-selective medium was used to examine the aerobiology of ascospores of Sclerotinia sclerotiorum in five commercial cabbage fields in eastern North Carolina. Ascospores were present in all five fields from 26 September to 30 November. However, numbers of ascospores varied greatly depending on location, sampling date, and time. In general, peak ascospore deposition occurred between 11:00 A.M. and 1:00 P.M., with the number of colonies recovered ranging from 3 to 55/dish (9 cm in diameter). Peak ascospore numbers at all locations were found from mid- to late October, but a second, smaller peak was also evident at each location in late November. Information obtained was employed to evaluate the role of wounding in infection of cabbage by ascospores of S. sclerotiorum in controlled environmental chambers. A method for production and release of ascospores of S. sclerotiorum was employed in controlled-environment chambers for the inoculation of cabbage plants with one of three representative foliar wounds: a bruise, a cut, or a non-lethal freeze. Wounding treatments were applied to 7-week-old cabbage plants, misting was added to maintain continuous leaf wetness, and ascospores were released from apothecia twice daily for four consecutive days. Spore trapping with a semi-selective medium indicated that inoculum was evenly distributed within the chambers and deposition was similar to levels recorded in the field. At 31 days after inoculation, disease incidence ranged from 0% on the control to 96% on the freeze treatments. Freeze-treated plants showed the highest disease severity throughout the entire incubation period. Mean area under the disease progress curve of severity values were 0, 0.2, 34 and 60 for the control, cut, bruise, and freeze treatments, respectively. Results indicate that freeze and bruise injuries are important factors associated with infection of cabbage by S. sclerotiorum.

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.