The effect of species of Allium on soil bacteria in relation to germination of sclerotia of Sclerotium cepivorum Berk

Extracts made from Allium species were shown to exhibit marked antibiotic properties in a variety of tests. However, no evidence was obtained to suggest that intact Allium seedlings exude antibiotic compounds in sufficient quantity to cause inhibition of bacteria in the soil or in assay tests in the laboratory. Germination of sclerotia of Sclerotium cepivorum was induced by Allium extracts which were too dilute to cause antibiotic responses in laboratory tests using a number of bacteria. Synthetic methyl-methanethiolsulphinate had little effect on germination of sclerotia. The specific reversal, by Allium species and their extracts, of the mycostatic effect of unsterile soil on sclerotia of S. cepivorum does not appear to be due to antibiotic effects.     

The production by species of Allium of alkyl sulphides and their effect on germination of sclerotia of Sclerotium cepivorum Berk

A number of n -propyl and allyl sulphides were shown to stimulate germination of sclerotia of Sclerotium cepivorum in soil. Several of these sulphides were evolved by chopped garlic cloves and onion bulbs, and by their extracts, as well as by distillates and condensates prepared from extracts. Sulphides produced by garlic were methyl and allyl compounds, whereas those of onion were methyl and n -propyl derivatives. Few or no volatile sulphides were detected by gas chromatography of head spaces of intact Allium plants although it is considered that small quantities of sulphides may be produced from these plants in soil. The stimulatory activity of distillates and condensates appears to be entirely due to their alkyl sulphide content and that of extracts at least partially so.     

Effects of volatile products of Allium species and their extracts on germination of sclerotia of Sclerotiutn cepivorum Berk.

It was shown that sclerotia of Sclerotium cepivorum, normally subject to the mycostatic influence of unsterile soil, germinated in response to a volatile principle evolved from growing onion and leek seedlings. Air, which was bubbled through diluted onion and garlic juices, contained vapours which stimulated sclerotia! germination. The vapours evolved at room temperature by garlic juice were collected as a condensate which also induced germination. Stimulatory activity was found in both distillates and distillands of garlic as well as in air bubbled through such a distillate.     

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.     

Production of volatile alkyl sulphides by microbial degradation of synthetic alliin and alliin-like compounds, in relation to germination of sclerotia of Sclerotium cepivorum Berk

Onion distillands proved more stimulatory than distillates towards germination of sclerotia of Sclerotium cepivorum. Synthetic allylcysteine and n-propylcysteine and their sulphoxides stimulated germination of sclerotia when added to soil. Solutions of these compounds in the presence of unsterile soil produced gaseous alkyl mercaptans, sulphides and disulphides which were detected by gas-liquid chromatography and which stimulated sclerotial germination when drawn through soil containing sclerotia. No volatile sulphides were detected above autoclaved solutions but the atmosphere over propyl and allylcysteines was slightly stimulatory although the corresponding autoclaved sulphoxides showed no such activity. The gases above all four solutions sterilized by membrane-filtration were not stimulatory. Six common soil bacteria were shown to be capable of degrading the synthetic precursors, causing the evolution of appropriate volatile alkyl sulphides.     

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.     

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.     

The effect of iprodione on sclerotium germination, root infection and mycelial spread of Sclerotium cepivorum in salad onions

Iprodione seed treatment at 125 g a.i./kg seed and stem base sprays at 0–3 and 0.125 g a.i./m row delayed the appearance and reduced the incidence of stem base symptoms and root infection with Sclerotium cepivorum in autumn- and spring-sown salad onions. Viability of sclerotia was unaffected by the treatments but their germination was delayed. The spread of mycelium between plants was suppressed but growth of hyphae within infected roots was much less affected.     

Sclerotium cepivorum Berk. in onion (Allium cepa L.) crops: isolation and characterization of bacteria antagonistic to the fungus in Queensland

Bacteria antagonistic to the fungal pathogen Sclerotium cepivorum Berk., which causes onion white rot disease, were isolated and characterized. Eighty percent of the bacterial antagonists isolated from soil samples and sclerotia of S. cepivorum recovered from soil were identified as belonging to the genus Bacillus. The other antagonists were coryneforms and Pseudomonas spp. In contrast, 90% of the antagonists isolated from onion roots were mostly Pseudomonas spp. and the remainder were Erwinia spp. The significance of these antagonistic bacteria is discussed in relation to the control of white rot disease of onion.     

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.     

Effect of some antibiotics on Sclerotium cepivorum Berk., the cause of white rot of onion

The effect of some antibiotics onSclerotium cepivorum, the cause of white rot of onion was studied in agar culture and soil. The growth ofS. cepivorum was inhibited in Czapek Dox yeast agar containing 50µg of gliotoxin, viridin, actidione and 100µg of patulin per ml of the medium. Lower concentrations of the antibiotics retarded the growth of the fungus. In soil, patulin had no effect in the control ofS. cepivorum infection of onion seedlings. Concentration of actidione of 5µg/g of soil completely controlled white rot infection but severely stunted the growth of onion seedlings; 40µg/g of actidione killed the seedlings. Despite the importance of actidione as a fungistatic agent its use on onion is limited by its phytotoxicity.     

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.     

The use of arbuscular mycorrhizae to control onion white rot (Sclerotium cepivorum Berk.) under field conditions

 A field experiment was carried out to determine the effects of the inoculation of onion (Allium cepa L.) with Glomus sp. Zac-19 on the development of onion white rot (Sclerotium cepivorum Berk.) and on onion production. Mycorrhization delayed onion white rot epidemics by 2 weeks and provided a significant protection against the disease for 11 weeks after onion transplanting, as compared with nonmycorrhizal controls. Mycorrhizal plants showed an increase of 22% in yield, regardless of the presence of the white rot pathogen. Accepted: 8 January 1996     

The respiration of sclerotia of Sclerotium rolfsii

Summary The respiration of sclerotia ofS. rolfsii was investigated using the Warburg constant-volume respirometer to measure oxygen uptake. The effects of age of sclerotia, pH, and temperature were studied. Sclerotia produced on prune agar were ideal for respirometric studies, being uniformly round and of approximately equal size. On a dry weight basis, the respiration rates of sclerotia were considerably less than those of vegetative mycelium. Sclerotia showed a decrease in respiration with increasing age. This was accompanied by morphological changes in the outer hyphal rind of the sclerotium during maturation. The respiration rate of sclerotia was approximately the same at 30° and 40° C, but was significantly lower at 45° C. Respiration of sclerotia was not markedly affected by normally encountered hydrogen-ion concentrations. However, a pH of 8.0 markedly repressed oxygen uptake. Sclerotia produced in rye grain cultures were chemically analyzed. The nitrogen content was 4.7 %, the petroleum-ether-soluble lipid content was 0.7 %, and the crude glycogen content was 14.2 % of the oven dry weight of the sclerotia.Contribution No. 345 from The Department of Botany. Portion of a thesis presented by the senior author in partial fullfillment for the M.S. degree.     

Further experiments on the control of white rot (Sclerotium cepivorum Berk;) in onions, shallots and leeks

Trials in 1945 confirmed earlier work on the efficacy of mercurous chloride (calomel) for the control of Sclerotium cepivorum in spring-sown onions. The application of 1 lb. 4% calomel dust to 50 yd. of seed drill at sowing time gave sufficiently good control of the disease in salad onions. Broadcast application of 1 oz. 4% calomel dust per sq.yd. before sowing was less effective but promising. Post-emergence application of the dust was useless. In autumn sowings, seed-drill treatment at 1 lb./so yd. controlled the disease in the following spring, but caused severe retardation and loss of crop.
Onion seedlings raised in heat and dipped in an aqueous paste of calomel dust before transplanting showed a high degree of resistance to a severe attack. Similar treatment of shallot bulbs was also effective. In both cases, the treatment substantially increased the yield and somewhat reduced storage losses.
Leek seedlings transplanted into dibber holes did not develop the disease.