Biological control of Pythium damping‐off by seed‐treatment with pseudomonas putida: Relationship with ethanol production by pea and soybean seeds

The relationship between biocontrol activity of Pseudomonas putida strain N1R against Pythium ultimum on pea and soybean seeds and the reduction in ethanol evolution by imbibed seeds was investigated under different treatment conditions, including temperature and numbers of seed‐applied cells of the bacterium. Treatment with strain N1R increased emergence at all temperatures, except for soybean at 12 °C and reduced ethanol concentration in the spermosphere of imbibed seeds at several temperatures. The concentration of bacterial cells in the seed treatment suspension also significantly affected biocontrol efficiency and reduced ethanol production, especially in pea seeds. In contrast, the duration (0–7 h) of submergence of seeds in bacterial suspension had little effect on biocontrol activity of N1R, although submergence of soybean seeds reduced their emergence even in the absence of the pathogen or biocontrol agent. Competition for seed‐derived compounds, including ethanol, is suggested to be one possible mechanism of biocontrol of Pythium by strain N1R, which is not known to produce antifungal antibiotics.     

Non‐pathogenic association of L‐form bacteria (pseudomonas syringae pv. phaseolicola) with bean plants (Phaseolus vulgaris L.) and its potential for biocontrol of halo blight disease

L‐forms of the halo blight pathogen, Pseudomonas syringae phaseolicola, were maintained in a medium which suppressed cell wall synthesis. These L‐forms, unlike revertants (walled forms derived from unstable L‐forms) and cell walled (parent) organisms, did not elicit a hypersensitive response in tobacco leaves. Association of L‐forms with Phaseolus vulgaris was established by seed imbibition in L‐form suspensions compared with appropriate control treatments (5% mannitol or heat‐killed cells). Seedling emergence and plant growth was not affected by L‐form imbibition. The association was detected by agglutination assays using polyclonal antibody. The L‐form association was localized to the lower shoot tissue and was progressively lost with age of plants. Plants with associated L‐forms had vigour and shoot weights equivalent to controls and showed no disease symptoms. The cell walled form could not be isolated from plants showing positive agglutination. On challenge with the pathogen, plants associated with L‐forms showed significantly less disease symptoms than controls. Stem extracts, from associated plants, were inhibitory to in vitro cultures of both L‐forms and parent forms of Ps. syr. phaseolicola. These results indicate that L‐form associations confer induced systemic resistance to bean plants and might be developed as novel biocontrol systems.     

Use of disease progress curves to study the effects of the biocontrol agent Sporidesmium sclerotivorum on lettuce drop

At the end of the spring 1987 growing season, the mycoparasite Sporidesmium sclerotivorum was applied at 0, 0.2, 2 or 20 kg ha^‐1 to lettuce plants infected with Sclerotinia minor. Disease incidence was monitored in the same plots for five subsequent crops (three fall and two spring crops) without additional application of either pathogen or mycoparasite. Logistic growth curves were fitted to the data to describe disease progression over time for each inoculum level within each of the five crops. Within each crop, increasing the quantity of mycoparasite inoculum resulted in positive horizontal displacement of the curve with respect to time. As quantities of inoculum of S. sclerotivorum increased, inflection points of the disease progress curves increased at a decreasing rate. Thus, additional mycoparasite inoculum resulted in ever‐smaller increases in inflection point, and after a certain threshold level of mycoparasite inoculum (< 0.2 kg ha^‐1), increases in inflection point did not result in meaningful increases in harvestable lettuce. Maximum rates of disease increase were not different among the treatments within each crop, but were different between crops. Maximum rates of disease increase averaged 3.4, 3.4, 2.1, 3.6 and 1.5% day^‐1 for the fall 1987, spring 1988, fall 1988, spring 1989, and fall 1989, respectively. At all inoculum levels, the fall epidemics began later after planting than the spring epidemics.     

Temporal constancy in grasshopper assemblies (Orthoptera: Acrididae)

ABSTRACT.

• 1 Temporal constancy in the structure of grasshopper assemblies (about forty-five species each) from two types of North American grasslands was assessed; one site was followed 25 years and the other 7 years.
• 2 Densities and relative abundances varied but composition of assemblies based on ranks suggested significant structure when three or more species were included in the analysis.
• 3 Results compared favourably with other insect herbivore assemblies which have been examined; variability in population change was intermediate along the spectrum of organisms which have been studied.

     

The biogeochemistry of a north-temperate grassland with native ungulates: Nitrogen dynamics in Yellowstone National Park

Nutrient dynamics of large grassland ecosystems possessing abundant migratory grazers are poorly understood. We examined N cycling on the northern winter range of Yellowstone National Park, home for large herds of free-roaming elk (Cervus elaphus) and bison (Bison bison). Plant and soil N, net N mineralization, and the deposition of ungulate fecal-N were measured at five sites, a ridgetop, mid-slope bench, steep slope, valley-bottom bench, and riparian area, within a watershed from May, 1991 to April, 1992.Results indicated similarities between biogeochemical properties of Yellowstone grassland and other grassland ecosystems: (1) landscape position and soil water affected nutrient dynamics, (2) annual mineralization was positively related to soil N content, and (3) the proportion of soil N mineralized during the year was negatively related to soil C/N.Grazers were a particularly important component of the N budget of this grassland. Estimated rates of N flow from ungulates to the soil ranged from 8.1 to 45.6 kg/ha/yr at the sites (average = 27.0 kg/ha/yr), approximately 4.5 times the amount of N in senescent plants. Rates of nitrogen mineralization for Yellowstone northern range grassland were higher than those measured in other temperate grassland ecosystems, possibly due to grazers promoting N cycling in Yellowstone.     

Presence of double‐stranded RNAs and virus‐like particles in the entomopathogenic fungus Metarhizium anisopliae

Nucleic acids from 41 strains of Metarhizium anisopliae, obtained from different parts of the world were extracted and examined by electrophoresis. Strong bands of double‐stranded RNA (dsRNA) were detected in two isolates from Brazil, V215 and V291, which had, respectively, 13 and 9 distinct bands ranging in size from ca. 0.75 to 3.5 kb. Icosahedral virus‐like particles (VLPs) (ca. 33 nm in diameter) were observed by transmission electron microscopy in extracts of these isolates. The VLPs and dsRNA were both absent from a clone of the isolate V291 which had been subcultured successively on solid medium. Bioassays against the aphid Myzus persicae showed no detectable difference in virulence between the clone of V291 which contained dsRNA and the clone that did not.     

Differences between oligotrophic communities resulting from old-field succession in relation to bedrock

Dehesa ecosystems on granite and on slate clearly differ in soil characteristics. Our aim was to find out whether floristic composition differed too.we selected an equal number of plots, on both granite and slate, in different successional stages of development and at various ages after field abandonment (the youngest 15–20 years). Plots with rock-outcrops, which had never been cultivated, were also included in the study.The results showed that floristic differences between granite and slate communities were not detectable in the younger successional stages. However, they showed up with time and on stabilized grasslands became significant. But on plots with rock-outcrops the differences were not observable. This was undoubtedly influenced by the peculiar features of these plots, characterised by poor arid soils and surface parent material. In fact they did not show significant differences in the analysed soil parameters either.Species diversity and biomass showed a similar pattern of differences to floristic composition. The highest species diversity was found on plots with lower biomass (the youngest ones). The lowest biomass in conjunction with quite low species diversity was found on plots with rock-outcrops, which again is consistent with their peculiar features.     

The rapid and long-lasting growth of grasses following small falls of rain on stony downs in the arid interior of Australia

Abstract Stony downs consist of grassy areas that alternate with areas that have a substantial stone cover. The stone-covered areas are impermeable, and most rain falling on them runs off, substantially increasing the effective rainfall in adjacent grassy areas. As a result, 20–25 mm of rain on stony downs wetted the soil around the grass to a depth of 140–170 mm and allowed sustained grass response. This is much less than the 35–40 mm of rain required for the same response on red clay or grey clay plains. Grasses respond very rapidly after rain. Some have green shoots the day after rain, and all have responded by the second day. Ephemerals dry off in 4–6 weeks, but most tussock grasses still have some green foliage 8–10 weeks after rain. Deeper rooted tussock grasses remain green for so long because most of the moisture that reaches deeper roots after rain remains there. Most moisture loss is through the soil surface and is recognizable as a drying front that descends through the soil profile. Soil above the drying front is nearly air dry (<5% moisture) while soil below the front has substantial moisture (14–16%). By about a month after rain in summer, the drying front is at a depth of about 80–120 mm. This is near the tips of the roots of ephemeral grasses and the ephemerals then dry off rapidly. Only the tips of the leaves of deep rooted grasses like Mitchell grass (Astrebla spp.) dry off. Their leaves continue to remain mostly green during most of the second month after rain and they do not dry off completely until the third month when the drying front reaches the bottom of the main root system.     

Mortality rates of desert vegetation during high-intensity drought at Uluru-Kata Tjuta National Park,Central Australia

Precipitation variability and heatwaves are expected to intensify over much of inland Australia under most projected climate change scenarios. This will undoubtedly have impacts on the biota of Australian dryland systems. However, accurate modelling of these impacts is presently impeded by a lack of empirical research on drought/heatwave effects on native arid flora and fauna. During the 2018–2021 Australian drought, many parts of the continent's inland experienced their hottest, driest period on record. Here, we present the results of a field survey in 2021 involving indigenous rangers, scientists and national parks staff who assessed plant dieback during this drought at Ulur u-Kata Tjut a National Park (UKTNP), central Australia. Spatially randomized quadrat sampling of eight common and culturally important plants indicated the following plant death rates across UKTNP (in order of drought susceptibility): desert myrtle (Aluta maisonneuvei subsp. maisonneuvei) (91%), yellow flame grevillea (Grevillea eriostachya) (79%), Maitland's wattle (Acacia maitlandii) (67%), waxy wattle (A. melleodora) (65%), soft spinifex grass (Triodia pungens) (53%), mulga (A. aneura) (42%), desert oak (Allocasuarina decaisneana) (22%) and quandong (Santalum acuminatum) (0%). The sampling also detected that seedling recruitment was absent or minimal for all plants except soft spinifex, while a generalized linear mixed model (GLMM) indicated two-way interactions among species, plant size and stand density as important predictors of drought survival of adult plants. A substantial loss of biodiversity has occurred at UKTNP during the recent drought, with likely drivers of widespread plant mortality being extreme multi-year rainfall deficit (2019 recorded the lowest-ever annual rainfall at UKTNP [27 mm]) and record high summer temperatures (December 2019 recorded the highest-ever temperature [47.1°C]). Our findings indicate that widespread plant death and extensive vegetation restructuring will occur across arid Australia if the severity and frequency of droughts increase under climate change.