The Development of Infection Structures of Sderotium cepivorum on Onion

Growth of Sderotium cepivorum mycelium on root tissue differed from that on stem tissue. Hyphae grew along the lines of the longitudinal epidermal cell walls often producing side branches which resulted in a distinctive pattern of growth. Penetration occurred mainly between anticlinal wall junctions with occasional direct penetration through the periclinal wall. Growth on the surface of the stem resulted in the formatíon of donne shaped infection cushions, arising from repeated dichotomous branching of hyphal tips. Penetration of the stem tissue occurred solely from these structures. The results of experiments using artificial membranes and surface replicas indicated that the stimulus for attempted penetration was chemical in nature but that the nature of the infection structure produced was determined by the relative strength of the tissue under attack.     

The effect of temperature on the infection of onions by Sclerotium cepivorum

Studies on Sclerotium cepivorum infection of salad onions in artificially infested soil showed that under field conditions infection was greatest in late spring and summer and declined to a low level in the late autumn and winter months. Change of infection levels was found to be correlated with the effect of soil temperature in the field on the germination of sclerotia. This was substantiated in laboratory studies which demonstrated that sclerotium germination, mycelial growth and seedling infection were all markedly influenced by temperature.     

Germination and Infectious Potential of Microconidia of Sclerotium cepivorum

Observation with the scanning electron microscope showed that microconidia of Sclerotium cepivorum are able to germinate on onion plants. A young hypha penetrated the plant cuticle. Cytochemical techniques gave evidence of phenoloxydase and esterase activities in microconidia and thus confirmed their infectious potential. Massive production of microconidia was induced by immersion of the fungus in water. These elements suggest that in high moisture conditions microconidia of S. cepivorum might play a crucial role in survival and dissemination of the fungus.     

Effect of Bacillus subtilis on Growth and Protection of Onion against White Rot

The effect of isolates of Bacillus subtilis applied as seed treatments on plant growth and white rot on three onion cultivars was studied for two seasons on muck soil of Fraser valley of British Columbia. The isolates of B. subtilis provided significant season-long protection against onion white rot on all three cultivars. The most effective isolate (BACT 2) protected the onion cultivar Autumn spice against onion white rot in both years. plant height, number of leaves per plant, and bulb weight were not affected by treating seeds with the Bacillus isolates. Bulb neck diameter of the cultivar Autumn Spice was significantly reduced by two isolates in both seasons.     

Germination of sclerotia of Sclerotium cepivorum Berk. under aseptic conditions

The response of sclerotia of Sclerotium cepivorum to a number of plant extracts and nutrient solutions was tested, aseptically, on agars, silica gels, filter paper discs, and on columns of soil. There was no specific response of sclerotia to extracts of Allium species in any of these experiments, whereas in unsterile soil consistently high levels of germination were obtained only with Allium extracts. It was shown that sclerotia are subject to the mycostatic influence of unsterile soil. With S. cepivorum soil mycostasis appears to be specifically reversed by Allium species or by water extracts made from them.     

Differentiation of the causal pathogen of onion white rot Sclerotium cepivorum isolates by using the APIZYM system

The APIZYM system of detection of enzymes was proven to be useful in the differentiation of 15 European and Egyptian isolates of S. cepivorum, the incitant of onion white rot. The tested isolates produced alkaline phosphatase, esterase (c4), esterase lipase (c8), leucine arylamidase, valinearylamidase, trypsine, α-chymatrypsin, acid phosphatase, naphthol-AS-B1-phosphohydrolase, ß-galactosidase, ß-glucutronidase, α-glucosidase, ß-glucosidase and N-acetyl-ß-glucosaminidase and did not produced lipase (c14), crystine arylamidase, trypsine, ß-glucutronidase, α-mannosidase and α-fucosidase. According to enzyme activity, isolates can be divided into four groups (G). The differences between groups were in the activity of the enzymes α-chymotrypsin and α-glucosidase. The tested European isolates and the Egyptian isolates No.6 of the pathogen were in G1 and G2; however the rest of the Egyptian isolates were in G3 and G4.     

Induced tolerance of Sclerotium cepivorum to antibiotics in the presence of onion exudates

The onion white rot pathogen Sclerotium cepivorum was cultured on agar media containing 2% malt extract and one of the antifungal antibiotics, endomycin, griseofulvin, venturicidin and cycloheximide at concentrations that reduced but did not prevent growth of mycelium. When onion seeds or agar discs impregnated with diffusates from onion bulbs were placed on the antibiotic media, radial growth of the fungus was greatly increased, and there was a profuse development of aerial mycelium. Gaseous diffusates from onion tissue and from impregnated agar discs were also effective. On the antibiotic media, tomato, cabbage and radish seeds did not stimulate the growth of S. cepivorum and the onion exudates did not stimulate the growth of four other fungi. This and other evidence is considered to show that the stimulation of growth of S. cepivorum was not caused by any direct effect on the antibiotics but by a tolerance of the fungus to them, which was specifically induced by an exudate from its host plant. The phenomenon may be related to the reported reversal by onion extracts of the inhibitory effects of soil mycostasis on germination of sclerotia of the fungus.     

Experiments on the control of white rot (Sclerotium cepivorum Berk.) in onions

Experiments-made during 1943 and 1944 on the use of mercurous chloride (calomel) for the control of Sclerotium cepivorum in spring-sown onions are described. The application of 4% calomel dust to the seed drill at sowing time gave better results than seed treatment. 1 lb. of dust per 25 yd. of seed drill gave good disease control in bulb onions, and 1 lb./so yd. may suffice for salad onions.
The relation between disease control and the effective mercury content of the treated soil is discussed.     

君子兰软腐病病原体侵染途径与致病规律的研究

本文报导了君子兰(Clivia miniata)软腐病二株病原体的分离及确定;鉴定了病原体的分类地位,一株为链球菌属的一个新种,定名为君子兰链球菌(Streptococcus miniatus Zhou,Ping et al shao sp.nov),另一株为环状芽孢杆菌(Bacillus subtilis);并对其二病原体的侵染途径与致病规律进行了研究。     

Molecular characterization of European and Egyptian isolates of Sclerotium cepivorum,the incitant of onion white rot

Abstract

The tested European and Egyptian isolates of Sclerotium cepivorum were able to infect Giza 6 onion cultivar causing white rot disease with a different degrees of disease severity (ranging from sever to weak). The pattern of esterase isozymes produced by the tested isolates of the pathogen showed two main bands (arrows) which were different in density. Such differences in density of bands were present in every run and therefore appear to be indicators for differences among the tested isolates. Analysis of the protein pattern of the tested isolates of the pathogen indicated that the tested isolates had major proteins of a molecular weight of 52, 36, 23 and 16 kDa. Variation between isolates was detected by presence of bands of low molecular weight. Isolate Nos. 1, 4, 5, 7, 8, 9, 10 and 13 had a band at 17 kDa, whereas isolate Nos. 2, 3, 6, 11, 12, 14, and 15 had a band at 20 kDa. Using RAPD analysis to evaluate the genetic diversity of the tested isolates indicated that the tested field population of the pathogen was genetically heterogeneous but shared a number of common bands with molecular weights ranging from 650 to 2500 bp. Based on the DNA banding pattern the tested isolates can be assigned to seven genetically different groups. All tested isolates produced a band at 2500 bp except isolate No. 7. No correlation was exibited between patterns esterase isozmes, protein and DNA patterns of S. cepivorum isolates and their virulence or geographical origin.     

Pathogenic characterization and study of the mycelial groups of compatibility in Sclerotium cepivorum Berk

We have studied the pathogenicity and the formation of mycelial compatibility groups of (MCG) in 12 isolates of the fungus Sclerotium cepivorum Berk from the states of Táchira, Mérida, Trujillo in Venezuela and Pamplona in Colombia. The pathogenicity was evaluated according to severity and virulence. In vitro confrontation tests were used to check the presence of compatible and incompatible interactions expressed by MCG when different isolates of the fungus were put together in a petri dish. When pathogenicity was evaluated, we found high levels of disease with isolates T-9, C-1 and TR-10. The isolate T-9 was the most virulent and agressive, as the symptoms began at day 12 and at day 19 and 100% of the nine inoculated plants died. Three responses were observed when confronting S. cepivorum isolates: a reaction of compatibility or anastomosis and two reactions of incompatibility. Two MCG were found, represented by isolates T-8 (Táchira) and TR-3 (Trujillo), which showed incompatibility when faced with other isolates. These results provide a basis upon which the genetic characterization of S. cepivorum can be established.     

Biodegradation of Lignin by White Rot Fungi

A review is presented related to the biochemistry of lignocellulose transformation. The biodegradation of wood constituents is currently understood as a multienzymatic process with the mediation of small molecules; therefore, this review will focus on the roles of these small molecular compounds and radicals working in concert with enzymes. Wood rotting basidiomycetous fungi penetrate wood and lead to more easily metabolized, carbohydrate constituents of the complex. Having a versatile machinery of enzymes, the white rot fungi are able to attack directly the "lignin barrier." They also use a multienzyme system including so-called "feed back" type enzymes, allowing for simultaneous transformation of both lignin and cellulose. These enzymes may function separately or cooperatively.     

Biological and fungicidal antagonism of Sclerotium cepivorum for controlling onion white rot disease

The action of some microbial isolates and Topsin-M against the most pathogenic isolate (Sc₂) of Sclerotium cepivorum causing onion white rot was tested. Bacillus subtilis B₄, B. subtilis B₅, Trichoderma koningii and Trichoderma harzianum were the most antagonistic isolates of the causal fungus. Mycelia of sclerotial germination of S. cepivorum (Sc₂) were completely inhibited in vitro by application of 2.0 g ?L^?1 Topsin-M. In pots, disease incidence was decreased to 8.33 % by the use of Topsin-M followed by T. koningii (29.17 %) compared with 95.83 % for the control, i.e., a remarkable reduction in severity was obtained. Under field conditions, disease incidence was decreased to 2.78 % by Topsin-M and to 11.11 % by T. harzianum. Both agents caused a sharp reduction in disease severity, reaching 1.39 % and 9.72 %, respectively, with 11.80 % being achieved by T. koningii and B. subtilis B₅. A close link between the biological action and enhancement of the enzyme activities of polyphenol oxidase and peroxidase with ability of onion to resist S. cepivorum was found, indicating induction of systemic acquired resistance. Accordingly, chlorophyll, root and foliage lengths, foliar, bulb dry matter and bulb productivity were also enhanced upon application of this biological control strategy.     

Factors affecting the survival of sclerotia of Sclerotium cepivorum in the Fraser Valley of British Columbia

Sclerotia of Sclerotium cepivorum buried in muck soil in the Fraser Valley decayed with time. The rate of decay of sclerotia was influenced by local environmental conditions. A mixture of soil with sclerotia increased their survival but there was no difference in the rates of decay in three different soils. The decay was greatest during winter when Fraser Valley fields are often flooded. Sclerotial decay was also affected by pretreatment of the sclerotia. Dried sclerotia decayed significantly (P < 0.05) faster than sclerotia which had not been dried, a phenomenon which is apparently due to changes in micro-organisms on the sclerotia. Dried sclerotia which had been incubated in moist soil had fewer bacteria and more fungi than sclerotia which had been incubated in soil without being dried. The increase in fungi on the dried sclerotia was due to a dramatic increase in Trichoderma spp.     

Dibenzyl Sulfide Metabolism by White Rot Fungi

Microbial metabolism of organosulfur compounds is of interest in the petroleum industry for in-field viscosity reduction and desulfurization. Here, dibenzyl sulfide (DBS) metabolism in white rot fungi was studied. Trametes trogii UAMH 8156, Trametes hirsuta UAMH 8165, Phanerochaete chrysosporium ATCC 24725, Trametes versicolor IFO 30340 (formerly Coriolus sp.), and Tyromyces palustris IFO 30339 all oxidized DBS to dibenzyl sulfoxide prior to oxidation to dibenzyl sulfone. The cytochrome P-450 inhibitor 1-aminobenzotriazole eliminated dibenzyl sulfoxide oxidation. Laccase activity (0.15 U/ml) was detected in the Trametes cultures, and concentrated culture supernatant and pure laccase catalyzed DBS oxidation to dibenzyl sulfoxide more efficiently in the presence of 2,2′-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) than in its absence. These data suggest that the first oxidation step is catalyzed by extracellular enzymes but that subsequent metabolism is cytochrome P-450 mediated.