Wendy E. Seel

Low Temperature–Short Duration Steaming of Soil Kills Soil-Borne Pathogens, Nematode Pests and Weeds

Agricultural soil samples containing survival structures of the fungal crop pathogens Verticillium dahliae, Sclerotinia sclerotiorum, Sclerotium cepivorum, Pythium ultimum, potato cyst nematodes Globodera rostochiensis and G. pallida and weeds Chenopodium album and Agropyron repens [Elymus repens] were treated in the laboratory with aerated steam at temperatures ranging from 40 to 80°C in a specially constructed apparatus. Steaming at 50 or 60°C for 3min, followed by an 8-min resting period in the steamed soil and immediate removal from the soil thereafter, resulted in 100% kill of all weeds, diseases and nematodes. When steamed at 45°C, there was a small but significant reduction in the survival of V. dahliae microsclerotia but no reduction in survival of S. cepivorum.

Suppression of host photosynthesis by the parasitic plant Rhinanthus minor.

BACKGROUND AND AIMS Parasitism is well understood to have wide-ranging deleterious effects on host performance in species thus far characterized. Photosynthetic performance reductions have been noted in the Striga-Zea mays association; however, no such information exists for facultative hemiparasitic plants and their hosts, nor are the effects of host species understood. METHODS Chlorophyll fluorimetry was used to study the effects of parasitism by the hemiparasite Rhinanthus minor on the grass Phleum bertolinii and the forb Plantago lanceolata, and the effects of host species on the photosynthetic apparatus of R. minor. KEY RESULTS Parasitism by Rhinanthus led to a significant decrease in the host, and total (host + parasite) biomass in Phleum; however, in Plantago, no significant repression of growth was noted. Maximum quantum yield (F(v)/F(m)) was reduced in parasitized Plantago, relative to control plants, but not in Phleum. F(v)/F(m) was significantly lower in R. minor parasitizing Phleum than Plantago, suggesting Phleum to be a superior host to Plantago for R. minor. Steady-state quantum yield (Phi(PSII)) was significantly depressed in parasitized Phleum, but only at low irradiances in Plantago. Phi(PSII) was very low for R. minor grown on Plantago, but not Phleum. CONCLUSIONS Shown here is the first evidence of the suppression of host photosynthesis by a facultative hemiparasitic plant, which has significant effects on total biomass production. Host identity is a significant factor in parasite success, with the forb Plantago lanceolata exhibiting apparent chemical as well as previously identified physical defences to parasitism. It is proposed that the electron transport rate (as denoted by Phi(PSII)) represents the limiting factor for biomass accumulation in this system, and that Plantago is able to suppress the growth of Rhinanthus by suppressing the electron transport rate.

Interactions between the hemiparasitic angiospermRhinanthus minor and its hosts: From the cell to the ecosystem

Parasitic plants can significantly influence the species to which they attach. The host response is variable however, and ranges from death of the host to no detectable effects in terms of both growth and physiology. The parasite can directly influence its hosts through resource abstraction, and indirectly by influencing inter- and intra-specific interactions. Abiotic factors interact with these direct and indirect effects to moderate the potential outcome of the host parasite interaction. This paper sets out to review a series of experiments that have been undertaken in our laboratory over a number of years that examine these effects and help us to understand mechanisms underpinning the variability in host response.

Evaluation of Alternaria alternata for biological control of Amaranthus retroflexus

Abstract Amaranthus retroflexus L. is a common annual weed worldwide. It can be found in a wide range of habitats and causes substantial yield reduction in many crops mainly through competition. Alternaria spp. are airborne molds that are considered to have potential for the biological control of weeds. The aim of this study was to assess the effect of spore concentration, host-plant growth stage, dew period, and temperature on the pathogenicity of three Alternaria alternata isolates against A. retroflexus. The pathogenicity of A. alternata increased with increasing spore concentration and length of dew period. A spore concentration of 107 spores ml−1 in a rapeseed oil emulsion and given a 24 h dew period caused 100% mortality of A. retroflexus plants at the four-leaf stage. Infection and mortality in older plants (>four-leaf stage) was lower. The highest levels of plant mortality were obtained at post-inoculum temperatures between 20 and 30 C. These experiments confirm the potential of A. alternata as a mycoherbicide under specific environmental conditions. Nomenclature: Alternaria alternata L.; Amaranthus retroflexus L. AMARE, redroot pigweed.

Effects of nitrogen availability and spore concentration on the biocontrol activity of Ascochyta caulina in common lambsquarters (Chenopodium album)

Abstract Common lambsquarters is an annual weed of many important crops. Ascochyta caulina is a plant pathogenic fungus that causes necrotic lesions on the leaves and stems of common lambsquarters. The objective of the present study was to estimate the effect of plant N supply on the biocontrol activity of A. caulina isolates against common lambsquarters. In greenhouse experiments replicated groups of common lambsquarters plants raised with different N supplies were sprayed with various isolates and concentrations of A. caulina 3 wk after planting. Height, number of leaves, total leaf area, fresh and dry weight, and tissue N concentration of common lambsquarters 4 wk after emergence increased significantly with increasing N supply. Disease development was positively related to increasing plant tissue N and also to increasing spore concentration. Fungal spore concentration also had a positive effect on the plant tissue N percentage. Ascochyta caulina isolate W90-1 caused a greater dry weight reduction in common lambsquarters than isolates I-001 and NW-6 did. Nomenclature: Ascochyta caulina (P. Karst) v.d. Aa & v Kest.; common lambsquarters, Chenopodium album L. CHEAL.

Biological Control of Weeds with Antagonistic Plant Pathogens

Many research programs have studied different aspects of the use of antagonistic plant pathogens in biological weed control strategies. The study of effects of individual environmental factors can be regarded as the first step in understanding limitations to the success of biological control methods. This review attempts to address the current advances of the basis and the progress of biocontrol methods, the link between environmental factors and plant infection development, and the use of formulation technology in biological weed control.

Effects of light availability on attachedRhinanthus minor (L.), an angiospermatic root hemiparasite

We studied the effect of light availability on the growth of an angiospermatic root hemiparasite,Rhinanthus minor. When attached to its host, height growth increased in response to shading, demonstrating thatR. minor was able to detect alterations in light quality and/or quantity. However, this reduced illumination did not affect its biomass, number of haustoria, or the amount of15N transferred from the hosts, compared with its performance under non-shaded conditions. Therefore,R. minor is unlikely to have difficulty in extracting host resources under shading. This result may have been mediated by a loweredR. minor transpiration rate in response to fluctuations in external conditions, including shading and water stress, compared with non-parasitic plants. Therefore, we suggest that, as long as the extent of resources diverted from host to parasite is not significantly altered by shading, growth of the attachedR. minor will be unaffected by reduced light availabilityper se.

Roles Played by Timing of Seedling Development and Host Identity in Determining Fitness of an Annual, Subarctic, Hemiparasitic Plant

Individually mapped plants of Euphrasia frigida, a hemiparasitic annual, were followed for one growing season at a subarctic site in northern Sweden. The strongest factor influencing seed-set was the date when the seedling stage ended, i.e., when plants produced their first noncotyledonous leaves, probably equalling date of attachment to a host. The advantage for early-developing plants was large even with just a 5-d difference in development. A positive effect was caused by presence of the perennial legume Astragalus alpinus, both on seedling development and plant performance, whereas the total cover of nonleguminous herbs or graminoids had no influence on the performance of E. frigida.

Effect of plant age, temperature and humidity on virulence of Ascochyta caulina on common lambsquarters (Chenopodium album)

Abstract Common lambsquarters is an important annual weed of many crops world-wide. Ascochyta caulina is a plant pathogenic fungus that, under natural conditions, causes necrotic spots on the leaves and stems of Chenopodium species. The objective of this study was to evaluate the effect of weed growth stage, relative humidity, dew period, and temperature on the infection of A. caulina isolates against common lambsquarters. In greenhouse experiments, replicated groups of common lambsquarters plants were sprayed with different isolates of A. caulina 2, 3, 4, 5, and 6 wk after emergence. Both disease severity and pathogen-induced dry weight reduction decreased with plant age. The efficacy of all isolates tested was reduced by high leaf-to-air vapor-pressure deficit. Disease severity was more responsive to relative humidity than temperature. However, a minimum dew period of 6 h was required to cause significant disease severity in common lambsquarters. Among all tested A. caulina isolates, W90–1 gave the highest disease scores under all conditions, with the exception of temperatures ≤15 C. Nomenclature: Ascochyta caulina (P. Karst) v.d. Aa & v Kest.; common lambsquarters, Chenopodium album L. CHEAL.