Nathan M. Kleczewski

Leaf Blight Disease on the Invasive Grass Microstegium vimineum Caused by a Bipolaris sp.

In 2009, a previously undescribed disease was found on the nonnative invasive annual grass Microstegium vimineum (Japanese stiltgrass). Diseased plants exhibited foliar lesions, wilting, and in some cases, death of entire plants. We identified the causal agent as a Bipolaris sp. similar to B. zeicola. We observed spores and associated structures characteristic of Bipolaris spp. growing from leaf lesions on field collected plants. Pure cultures of the fungus were made and spore suspensions were applied to laboratory-reared M. vimineum seedlings in growth chamber and greenhouse experiments. Initial symptoms appeared on seedlings in the growth chamber experiment within 72 h of inoculation, and seedlings exhibited characteristic lesions within 10 days. The fungus was reisolated from lesions, and the internal transcribed spacer (ITS) region was sequenced to confirm its identity. In the greenhouse experiment, inoculated plants displayed characteristic lesions, and relatively greater spore loads increased disease incidence. Disease reduced seed head production by 40% compared to controls. This is the first report of a Bipolaris sp. causing disease on invasive M. vimineum. Following further analysis, including assays with co-occurring native species, this Bipolaris sp. may be considered as a biocontrol agent for invasive M. vimineum.

Effects of soil type, fertilization and drought on carbon allocation to root growth and partitioning between secondary metabolism and ectomycorrhizae of Betula papyrifera

Paper birch (Betula papyrifera Marsh) seedlings were grown in a greenhouse in either subsoil or topsoil in factorial combination with two fertilization and drought regimes to investigate how different soil environments and nutrient availability drive belowground partitioning between growth, secondary metabolism and ectomycorrhizal (EM) associations, and impact drought tolerance of paper birch. Root and total seedling dry biomass, starch, soluble sugars, soluble phenolics, lignin and EM abundance were quantified. In unfertilized topsoil, total plant biomass and root biomass were approximately nine times higher than in unfertilized subsoil, but the root weight ratios did not differ between soils. Root soluble phenolics and lignin were higher in unfertilized subsoil than in unfertilized topsoil, whereas EM abundance was significantly higher in unfertilized topsoil than in unfertilized subsoil. In topsoil, fertilization decreased root biomass and EM abundance and increased root phenolics and lignin. In contrast, fertilization of subsoil increased root biomass but decreased root phenolics and lignin, while EM abundance was unaffected. In both soil types, fertilization reduced root weight ratios. Across soil types, EM abundance was negatively correlated with root soluble sugars, root phenolics and lignin, but this was driven mainly by the responses in the topsoil treatment. Our results show that soil fertility mediates carbon tradeoffs among defense, growth and EM associations.

Nitrogen-fixing bacteria, arbuscular mycorrhizal fungi, and the productivity and structure of prairie grassland communities

Due to their complementary roles in meeting plant nutritional needs, arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (N2-fixers) may have synergistic effects on plant communities. Using greenhouse microcosms, we tested the effects of AMF, N2-fixers (symbiotic: rhizobia, and associative: Azospirillum brasilense), and their potential interactions on the productivity, diversity, and species composition of diverse tallgrass prairie communities and on the productivity of Panicumvirgatum in monoculture. Our results demonstrate the importance of AMF and N2-fixers as drivers of plant community structure and function. In the communities, we found a positive effect of AMF on diversity and productivity, but a negative effect of N2-fixers on productivity. Both AMF and N2-fixers affected relative abundances of species. AMF shifted the communities from dominance by Elymus canadensis to Sorghastrum nutans, and seven other species increased in abundance with AMF, accounting for the increased diversity. N2-fixers led to increases in Astragalus canadensis and Desmanthus illinoense, two legumes that likely benefited from the presence of the appropriate rhizobia symbionts. Sorghastrum nutans declined 44xa0% in the presence of N2-fixers, with the most likely explanation being increased competition from legumes. Panicum monocultures were more productive with AMF, but showed no response to N2-fixers, although inference was constrained by low Azospirillum treatment effectivity. We did not find interactions between AMF and N2-fixers in communities or Panicum monocultures, indicating that short-term effects of these microbial functional groups are additive.

Ecological consequences of pathogen accumulation on an invasive grass

Release from natural enemies can contribute to the success of biological invasions. However, the advantage gained through enemy release may diminish over time as pathogens, parasites, or herbivores accumulate on invasive species. Pathogen accumulation may result from the development of novel host-pathogen interactions in the invasive range or the introduction of pathogens from the invaders home range or other areas. Previous studies suggest that pathogen accumulation may reduce the performance of invasive species but experimental demonstrations are lacking. Microstegium vimineum is a highly invasive annual grass with an expanding distribution in eastern North America. Bipolaris species and other fungal pathogens have recently been shown to cause foliar blight on invasive populations of Microstegium. Here we used experimental inoculations in controlled laboratory and greenhouse conditions and fungicide applications in the field to evaluate host relations of pathogens infecting invasive Microstegium and the ecological consequences of pathogen infection in nature. Four of seven native plant species tested, including three native grasses and one forb, became infected by at least one of the five pathogens isolated from Microstegium, suggesting that the native species might be sources or alternative hosts for Microstegium pathogens. Microstegium populations may also provide a reservoir of pathogens that could spillover to co-occurring native species and exacerbate the effects of invasions. All five pathogens caused foliar lesions on Microstegium, but there was over a four-fold difference in lesion area caused by different pathogens. Fungicide applications to infected field populations significantly reduced disease symptoms, increased invader biomass by up to 50% and increased seed head production by up to 200% compared to controls. Our results show that pathogen infection of invasive Microstegium caused significant declines in invasive host performance in natural populations, suggesting that the release of invasive species from natural enemies may be temporary.

Nutrient and water availability alter belowground patterns of biomass allocation, carbon partitioning, and ectomycorrhizal abundance in Betula nigra

In managed settings, seedlings are often fertilized with the objective of enhancing establishment, growth, and survival. However, responses of seedlings to fertilization can increase their susceptibility to abiotic stresses such as drought. Seedlings acclimate to variation in soil resources by reallocating carbon among different physiological processes and compartments, such as above versus belowground growth, secondary metabolism, and support of ectomycorrhizal fungi (EMF). We examined the effects of nutrient and water availability on carbon allocation to above and belowground growth of river birch (Betula nigra), as well as partitioning among root sugars, starch, phenolics, lignin, and EMF abundance. As nutrient availability increased, total plant biomass and total leaf area increased, while percent root biomass decreased. Root sugars, total root phenolics and EMF abundance responded quadratically to nutrient availability, being lowest at intermediate fertility levels. Decreased water availability reduced total leaf area and root phenolics relative to well-watered controls. No interactions between nutrient and water availability treatments were detected, which may have been due to the moderate degree of drought stress imposed in the low water treatment. Our results indicate that nutrient and water availability significantly alter patterns of carbon allocation and partitioning in roots of Betula nigra seedlings. The potential effects of these responses on stress tolerance are discussed.

Variation in Pathogenicity and Host Range of Bipolaris sp. Causing Leaf Blight Disease on the Invasive Grass Microstegium vimineum

Abstract Microstegium vimineum is a widespread invasive grass that poses significant threats to forests and disturbed areas throughout the United States. Often, the large-scale, rapid spread of Microstegium prohibits management by traditional methods. Control of Microstegium may be possible through the use of a pathogen (referred to here as Bipolaris Mv) that causes leaf blight on Microstegium. Members of the fungal genus Bipolaris are known pathogens of many plants, including important agronomic crops. However, little is known about the biology and host range of Bipolaris Mv. We used a series of growth chamber and light bank experiments to determine the variation in Bipolaris Mv from different geographic origins and its ability to cause foliar lesions and chlorosis on Microstegium. We used petri plate and soil infestation assays to determine the effects of Bipolaris Mv on Microstegium emergence from seed, biomass, and root necrosis. Finally, we tested the host range of these fungi on economically and ecologically important plant species. All isolates increased disease on Microstegium foliage relative to controls, although the effects varied among isolates. Isolates increased root necrosis by 97% in petri plate assays and by 4% in soil infestation trials compared to controls. Infestation of soils with Bipolaris Mv reduced emergence of Microstegium from seed by 31% compared to controls, but did not affect root or stand biomass. Bipolaris Mv produced lesions on a range of grasses, including corn, sorghum, rye, and wheat, although lesion size varied with isolate. These results indicate that Bipolaris Mv may be an effective pathogen on Microstegium, but its use as a bioherbicide may be impractical because of its effects on a wide range of grasses. Nomenclature: Microstegium (Marys grass, stiltgrass), Microstegium vimineum (Trin.) A. Camus; rimini rye, Secale cereale L.; sorghum, Sorghum bicolor (L.) Moench ssp. bicolor; wheat, Triticum aestivum L.; corn, Zea mays L.

Effect of soybean vein necrosis on yield and seed quality of soybean

Abstract Soybean vein necrosis virus (SVNV) rapidly became a widespread virus of soybean (Glycine max (L.) Merr.) in the USA and Canada within a few years of its initial detection in 2008; however, the economic impact of soybean vein necrosis (SVN) symptoms caused by virus infection remains unknown. Field studies were conducted in six states in the USA during 2013, 2014 and 2015 to determine the effect of SVN on soybean yield and seed quality. Quantitative parameters, including seeds per pod, pods per plant, yield and 100-count seed weight, were assessed from plants or seeds collected from research and commercial production fields. Qualitative parameters, including protein and oil concentration, were also obtained from samples collected in Indiana and Iowa. Results from all states suggest that yield is not impacted by SVN; however, seed quality was affected in four of seven location-years. In Iowa, oil concentration decreased by 0.11% as disease incidence increased by 1% (P = 0.04). In Indiana, SVNV infected plants exhibited decreased total oil content compared with asymptomatic plants (0.16% (P = 0.04); 0.67% (P > 0.01) in 2014 and 2015, respectively. These results suggest that SVN may change soybean seed quality, which may affect the marketability of soybeans for premium markets, specifically those interested in high oleic soybeans.