Russell J. Tweddell

Effect of organic and inorganic salts on the growth and development of Fusarium sambucinum, a causal agent of potato dry rot

Potato dry rot, caused by Fusarium sambucinum , is a major postharvest disease of economic significance worldwide. Postharvest application of thiabendazole to control dry rot is becoming less effective since many strains of F. sambucinum have become resistant to this fungicide. Thus, alternative control strategies are needed. In vitro studies showed that several salts (0.2 M) inhibited completely mycelial growth and spore germination of F. sambucinum. Among these salts, sodium benzoate, sodium metabisulfite, potassium sorbate, trisodium phosphate and aluminium salts were fungitoxic. In vivo studies showed that aluminium chloride in curative application and sodium metabisulfite, sodium carbonate and sodium bicarbonate in preventive application significantly reduced the development of dry rot in potato tuber. Results from this study demonstrate that selected salts can be used to control potato dry rot.

Pythium attrantheridium sp. nov.: taxonomy and comparison with related species.

Pythium attrantheridium sp. nov. is a new species isolated from cavity spot lesions of carrots as well as apple and cherry seedlings from various locations widely distributed in Canada and the USA. This fungus is closely related to the heterothallic P. intermedium, but is distinguished by: (1) unique molecular characteristics; (2) unique morphological characteristics; and (3) mating incompatibility with P. intermedium. The ITS region of the nuclear rDNA of all strains of P. attrantheridium studied is different from that of all other known Pythium spp. The oogonia attract a large number of antheridia when compatible mating types contact each other. The positive mating type produces zoospores unlike those of P. intermedium. Thus, biological, morphological and molecular data support the recognition of a new species.

Ultrastructural alterations of Erwinia carotovora subsp. atroseptica caused by treatment with aluminum chloride and sodium metabisulfite.

ABSTRACT Aluminum and bisulfite salts inhibit the growth of several fungi and bacteria, and their application effectively controls potato soft rot caused by Erwinia carotovora. In an effort to understand their inhibitory action, ultrastructural changes in Erwinia carotovora subsp. atroseptica after exposure (0 to 20 min) to different concentrations (0.05, 0.1, and 0.2 M) of these salts were examined by using transmission electron microscopy. Plasma membrane integrity was evaluated by using the SYTOX Green fluorochrome that penetrates only cells with altered membranes. Bacteria exposed to all aluminum chloride concentrations, especially 0.2 M, exhibited loosening of the cell walls, cell wall rupture, cytoplasmic aggregation, and an absence of extracellular vesicles. Sodium metabisulfite caused mainly a retraction of plasma membrane and cellular voids which were more pronounced with increasing concentration. Bacterial mortality was closely associated with SYTOX stain absorption when bacteria were exposed to either a high concentration (0.2 M) of aluminum chloride or prolonged exposure (20 min) to 0.05 M aluminum chloride or to a pH of 2.5. Bacteria exposed to lower concentrations of aluminum chloride (0.05 and 0.1 M) for 10 min or less, or to metabisulfite at all concentrations, did not exhibit significant stain absorption, suggesting that no membrane damage occurred or it was too weak to allow the penetration of the stain into the cell. While mortality caused by aluminum chloride involves membrane damage and subsequent cytoplasmic aggregation, sulfite exerts its effect intracellularly; it is transported across the membrane by free diffusion of molecular SO2 with little damage to the cellular membrane.

Antagonist microorganisms with the ability to control Pythium damping-off of tomato seeds in rockwool.

A total of 237 microorganisms were isolated from five different greenhouse tomato growing media. Of those, 40 microorganisms reduced the in vitro mycelial growth of both Pythiumaphanidermatum and Pythiumultimum. The ability of these microorganisms to control damping-off was then tested in rockwool. As a result, Pseudomonas corrugata strains 1 and 3, Pseudomonas fluorescens subgroup F and G strains 1, 2, 3, 4 and 5, Pseudomonas marginalis, Pseudomonas putida subgroup B strain 1, Pseudomonas syringae strain 1 and Pseudomonas viridiflava significantly reduced damping-off caused by P. ultimum or P. aphanidermatum. Pseudomonas marginalis was the only microorganism that significantly reduced damping-off caused by both pathogens.

Fatty acid profiles of polar and neutral lipids of ten species of higher basidiomycetes indigenous to eastern Canada

Neutral and polar lipid contents of ten species of edible mushrooms indigenous to Eastern Canada belonging to the families Agaricaceae, Amanitaceae, Boletaceae, Coprinaceae, Ganodermataceae, and Lycoperdaceae were analysed. The total lipid content of the species analysed ranged from 3.1% (Ganoderma applanatum) to 16% (w/w) d.w. (Amanita vaginata) and averaged 8.6% (w/w) d.w. Polar lipids accounted for more than 50% of the total lipids in most species and differences were observed between neutral and polar lipid contents according to the species analysed. In both lipid fractions, high proportions of unsaturated fatty acids (FAs) ranging from 62.7 to 82.3% (polar lipids) and 59.8 to 82.5% (neutral lipids) of the total FAs were observed. Analysis of FA profiles showed that both neutral and polar lipids were mainly composed of linoleic (18:2 Delta9c,12c), oleic (18:1 Delta9c), and palmitic (16:0) acids. Significant differences (P<0.05) in the contents of specific FAs were observed between mushroom species. Among the 44 FAs detected in the species analysed, the occurrence of cis-11-heptadecenoic (17:1 Delta11c) acid is reported for the first time in basidiomycetes, while elaidic acid (18:1 Delta9t) is reported for the first time in fungi.

Identification of Pythium species associated with cavity-spot lesions on carrots in eastern Quebec

In this study, 248 isolates of Pythium spp. were recovered from cavity-spot lesions of carrot and identified by DNA sequencing. The isolates came from carrots harvested in different fields in the area of the city of Québec. The majority of the isolates belonged to Pythium sulcatum and P. sylvaticum, which represented 93% of the isolates collected. Pythium macrosporum, P. debaryanum, P. dissotocum, P. attrantheridium, P. rostratum, and P. acanthicum were also found to be associated with cavity-spot lesions. Among these species, only P. acanthicum failed to cause cavity-spot lesions when inoculated on carrots. Based on mycelial growth, 3%, 40%, 80%, and 100% of P. sylvaticum, P. sulcatum, P. macrosporum, and P. dissotocum isolates, respectively, were tolerant to metalaxyl while all isolates of P. attrantheridium, P. debaryanum, P. rostratum, and P. acanthicum were sensitive. In the area of the city of Québec, P. sulcatum and P. sylvaticum were by far the two most important species associated with the disease. This reinforces the concept that Pythium spp. causing cavity spots in carrot are diverse and may vary according to geographical areas.

Identification of soils suppressive against Helminthosporium solani, the causal agent of potato silver scurf

Abstract Silver scurf, caused by the fungus Helminthosporium solani, is an important disease affecting potato tubers. Control of the disease has been hampered by the development of H. solani strains resistant to thiabendazole. Currently, there is no alternative to thiabendazole for the efficient control of the disease. In this study, 45 selected soils from the province of Quebec were tested in greenhouse assays for their effect on silver scurf development. The results showed that soil influenced significantly silver scurf development and that specific soils displayed an interesting level of suppressiveness against potato silver scurf. Investigations into the cause(s) of soil suppressiveness revealed on the one hand, significant negative correlations between silver scurf severity and soil N-NO3 and available Fe contents, and on the other hand, absence of significant correlations between silver scurf severity and the different soil microbial populations. Investigations also revealed the presence of microorganisms antagonistic to the pathogen in the most suppressive soils. The antagonist microorganisms isolated in those soils were Bacillus cereus, Cellulomonas fimi, Kocuria varians, Pseudomonas putida, Rhodococcus erythropolis and Rhodococcus globerulus. Considering that few microorganisms were previously reported to display antagonism against H. solani, these results open the way to new avenues of investigation towards achieving biocontrol of silver scurf.

A bioactive (1→3)-, (1→4)-β-d-glucan from Collybia dryophila and other mushrooms

Polysaccharides from higher Basidiomycete mushrooms, mainly β-d-glucans, are considered to be potent bioactive fungal compounds. In this study a β-glucan (1.237 × 106 Da) consisting of (1→ 3) and (1→ 4) glucosidic linkages, named Collybia dryophila polysaccharide (CDP), was extracted from the wild mushroom C. dryophila. CDP was shown to strongly inhibit nitric oxide production in activated macrophages suggesting that this polysaccharide displays a potential anti-inflammatory activity. In addition it was shown that polysaccharides similar to CDP (CDP-like) are present in Lentinus edodes and different wild mushrooms collected in northeastern North America.

Role of Lipid Composition and Lipid Peroxidation in the Sensitivity of Fungal Plant Pathogens to Aluminum Chloride and Sodium Metabisulfite

ABSTRACT Aluminum chloride and sodium metabisulfite have shown high efficacy at low doses in controlling postharvest pathogens on potato tubers. Direct effects of these two salts included the loss of cell membrane integrity in exposed pathogens. In this work, four fungal potato pathogens were studied in order to elucidate the role of membrane lipids and lipid peroxidation in the relative sensitivity of microorganisms exposed to these salts. Inhibition of mycelial growth in these fungi varied considerably and revealed sensitivity groups within the tested fungi. Analysis of fatty acids in these fungi demonstrated that sensitivity was related to high intrinsic fatty acid unsaturation. When exposed to the antifungal salts, sensitive fungi demonstrated a loss of fatty acid unsaturation, which was accompanied by an elevation in malondialdehyde content (a biochemical marker of lipid peroxidation). Our data suggest that aluminum chloride and sodium metabisulfite could induce lipid peroxidation in sensitive fungi, which may promote the ensuing loss of integrity in the plasma membrane. This direct effect on fungal membranes may contribute, at least in part, to the observed antimicrobial effects of these two salts.

Control of greenhouse tomato root rot [Pythium ultimum] in hydroponic systems, using plant-growth-promoting microorganisms

Twenty-eight microorganisms showing in vitro antagonistic activity against Pythium ultimum were tested for their ability to reduce root rot [P. ultimum] on mature tomato plants grown in a greenhouse under hydroponic conditions. Of those, Penicillium brevicompactum, Penicillium solitum strain 1, Pseudomonas fluorescens subgroup G strain 2, Pseudomonas marginalis, Pseudomonas putida subgroup B strain 1, Pseudomonas syringae strain 1, and Trichoderma atroviride were shown to strongly reduce root rot severity, to improve the anchorage, and to increase the marketable yields of the plants grown in rockwool infested with P. ultimum. Experiments conducted in an organic medium containing peat, pine sawdust, and compost (v / v / v ; 60:30:10) also revealed the capability of most of these microorganisms to reduce root rot severity and to improve the anchorage of the tomato plants. However, Pseudomonas marginalis was the only microorganism that significantly improved fruit production of infected plants grown in organic medium. Moreover, Penicillium brevicompactum, Penicillium solitum strain 1, Pseudomonas fluorescens subgroup G strain 2, Pseudomonas marginalis, Pseudomonas putida subgroup B strain 1, and T. atroviride were shown to stimulate the growth of healthy tomato seedlings, suggesting that they act as PGPR (plant-growth-promoting rhizobacteria) or PGPF (plant-growth-promoting fungi). This study led to the selection of potential biocontrol agents against root rot of tomato caused by P. ultimum in hydroponic systems. This may open the way for new alternatives for the biological control of Pythium diseases in hydroponic systems that not only protect the crop but also have a beneficial effect on the plant growth and development in the absence of pathogens.