Fungal Root Diseases Research Articles

Effects of increased nitrate availability on the control of plant pathogenic fungi by the soil bacterium Bacillus subtilis

In wet soils, low oxygen conditions often develop that favour disease development by many soil-borne plant pathogens. The introduction of a biocontrol agent, to suppress disease development, would require that the agent remains metabolically active under such conditions. Denitrifying bacteria can maintain this metabolic activity by switching to nitrate respiration. In the rhizosphere, plant roots not only supply carbon as an electron donor, but also cause a localised lowering of oxygen concentrations, conditions favourable for nitrate respiration. Two strains of Bacillus subtilis, showing strong inhibition of a number of pathogenic fungi on agar plates, and the capacity to grow under anoxic and anaerobic conditions when provided with nitrate, were used to study the possible involvement of nitrate respiration in fungal disease control. The effect of the addition of nitrate on the activity of these antagonistic strains was studied under anoxic conditions using the sealed plate method of Fiddaman and Rossal [Fiddaman, P.J., Rossal, S., 1995. Plant Pathol. 44, 695–703]. The assay tests the activity, measured as a reduction in fungal growth, of antifungal volatiles (AFV) produced by the bacteria. The in vitro experiments showed that antagonism by the B. subtilis strains towards Fusarium oxysporum varied under anoxic conditions, depending on the nitrate availability and agar used as a growth medium. AFV activity was increased by the presence of nitrate in the medium at concentrations of 10 mM or more. Nitrate respiration may therefore have an important role in the control of fungal root diseases by allowing denitrifying soil-borne bacteria to remain metabolically active in wet soils with low oxygen concentrations.

Molecular and biochemical aspects of rhizobacterial ecology with emphasis on biological control

The rhizosphere is the narrow zone of soil surrounding the root that is subject to influence by the root. Rhizobacteria are plant-associated bacteria that are able to colonize and persist on roots. An understanding of the ecology of a microorganism is a fundamental requirement for the introduction of a microbial inoculant into the open environment. This is particularly true for biological control of root pathogens in the rhizosphere, where one is actively seeking to alter the ecological balance so as to favour growth of the host plant and to curtail the development of pathogens. Some strains of plant growth-promoting rhizobacteria can effectively colonize plant roots and protect plants from diseases caused by a variety of root pathogens and growth promotion of plants through direct stimulation of growth hormone. Such beneficial or plant health-promoting strains are emerging as promising biocontrol agents. They are suitable as soil inoculants either individually or in combination and may be compatible with current chemical pesticides. Considerable progress has been achieved using molecular genetic techniques to elucidate the important microbial factors or genetic traits involved in the suppression of fungal root diseases. Strategies utilizing molecular genetic techniques have been developed to complement the ongoing research ranging from the characterization and genetic improvement of a selected biocontrol agent to the measurement of its persistence and dispersal. Finally, biocontrol is considered as part of a disease control strategy like integrated pest management which offers a successful approach for the deployment of both agro-chemicals and biocontrol agents.

Asymptotic analysis of an epidemic model with primary and secondary infection

For many biological systems, the behavior of interest is contained in the evolution of transients rather than in the stability of equilibria. These include systems in which perturbations and interruptions occur on a time scale much shorter than the equilibration time, and those in which any final equilibrium is sensitive to initial conditions. In this article, we examine a model of fungal root disease in a crop involving primary and secondary infection mechanisms. This system is subject to regular interruptions in the form of harvesting and sowing. Using an asymptotic approach in which certain parameter values are assumed to be small, the model can be broken down into a set of simpler subsystems respresenting recognizable biological mechanisms. These linear models can be solved to give closed-form analytical solutions for transient evolution. From this information, it is possible to construct an annual map of disease severity in the crop, and determine the parameter values under which the infection will bulk up or fade out.

Interactions between earthworms, beneficial soil microorganisms and root pathogens

The capacity of earthworms and soil microorganisms to influence plant growth is well known. However, the interactions between them have been little studied. Recent work in southern Australia has examined these associations with a view to managing both earthworms and beneficial microorganisms to promote cereal and pasture productivity. In this paper we examine the role of earthworms as vectors of beneficial soil bacteria and their capacity to influence the population dynamics and impact of microorganisms on soil and plants. The earthworms Aporrectodea trapezoides and Aporrectodea rosea are the most common species in agricultural ecosystems in southern Australia. They have been shown to spread Pseudomonas corrugata 2140R, a biocontrol agent for Take-all (a fungal disease of wheat roots caused by Gaeumannomyces graminis var. tritici) through soil with corresponding colonization of the roots of wheat seedlings at soil depths of 3–9 cm. In addition, A. trapezoides can spread Rhizobium through the soil and cause increased root colonization and nodulation of legume roots. Despite evidence that populations of soil microorganisms are affected by earthworm activity at a small spatial scale (e.g. in casts or tunnel walls), there is little evidence to demonstrate that earthworms affect populations at larger spatial scales. Take-all and Rhizoctonia bare patch are the most serious fungal root diseases of wheat in southern Australia. Laboratory trials have shown that, in the presence of the worms A. trapezoides and A. rosea, symptoms of these diseases in wheat seedlings are substantially reduced. In a further trial, the presence of A. trapezoides was associated with a 100-fold decrease in population size of Rhizobium recoverable from soil after 40 days.

Microbial recolonization and suppression of Rhizoctonia solani in a bedding-plant potting mix amended with recycled mix before aerated-steam treatment

Some South Australian bedding-plant nurseries add recycled potting mix to fresh potting mix before aerated-steam treatment in an attempt to decrease fungal root diseases. To test the hypothesis that this addition of recycled potting mix affects microbial colonization and, consequently, the disease-suppressiveness of mixes treated with aerated steam, recycled potting mix was added to or withheld from fresh potting mix. The mixes were then treated with aerated steam at 60°C for 35 min or not treated. Their recolonization by actinomycetes, heat-resistant organisms, pseudomonads, and fungi was followed for 25 days. The addition of recycled potting mix frequently increased the numbers of actinomycetes and heat-resistant microbes recovered from the mixes, whether treated with steam or not, but decreased the numbers of aerobic bacteria. Fungal populations were rarely influenced by the addition of recycled potting mix. Pseudomonad populations dropped to undetectable levels after the aerated-steam treatment and failed to reach levels comparable to those in treatments without steam during the next 25 days. Organisms from each group were tested for in vitro antagonism to Rhizoctonia solani Kuhn (AG-8). The proportion of inhibitory organisms varied with the microbial group, the mix treatment, and the time since mix treatment. Where aerated-steam treatment was applied, the addition of recycled potting mix rarely affected the development of Rhizoctonia disease symptoms on seedlings of Celosia argentea. The potential for improving the suppressiveness of aerated-steam-treated nursery mixes via microbial inoculation is discussed.

Effects of Single and Combined Infection of Arrowleaf Clover with Bean Yellow Mosaic Virus and a Phytophthora sp. on Reproduction and Colonization by Pea Aphids (Homoptera: Aphididae) 1

Reproduction and colonizing behavior of pea aphids, Acyrthosiphon pisum (Harris), were studied on reproductive arrowleaf clover, Trifolium vesiculosum Savi, using four treatments: healthy, infected with bean yellow mosaic virus (BYMV), infected with a Phytophthora sp. that caused root rot, or infected with both BYMV and the Phytophthora sp. Reproduction of aphids in no-choice experiments was significantly lower on plants in the two treatments with the root rot disease than on healthy or BYMV-infected plants. The highest level of reproduction was observed on healthy plants. Aphid colonization also was lower on fungus-infected plants whether BYMV was present or not. Results of these studies suggest that a complex relationship exists between aphid vectors, virus disease, and fungal root disease in arrow leaf clover. Presence of fungal root rot disease may influence reproduction and movement of alate aphids and epidemiology of aphid-transmitted viruses during both fall and spring periods of alate aphid activity, when plants are in vegetative or reproductive stages, respectively.

Factors contributing to poor lucerne persistence in southern Queensland

Lucerne disease surveys made in southern Queensland have shown the presence of seven fungal root and crown diseases. The two most wide spread and serious diseases are Phytophthora root rot (Phytophthora megasperma) and Colletotrichum crown rot (Colletotrichum trifolii). The general disease survey did not reveal the presence of bacterial wilt (Corynebacterium insidiosum) in Queensland. Studies made on the survival of lucerne populations for 2.5 years at three sites in Queensland have shown that disease was the major cause of all detected plant deaths.