Flavor and odour characteristics of species of Allium in relation to their capacity to stimulate germination of sclerotia of Sclerotium cepivorum

Of many species or cultivars of Allium tested only six ornamental species showed little or no capacity to stimulate germination of sclerotia of Sclerotium cepivorum. All six species had S-methyl-L-cysteine sulphoxide as their principal flavour and odour precursor and their overall flavour and odour levels were low. All other species and cultivars were highly stimulatory, contained considerahle amounts of S-1 or S-2-propenyl-L-cysteine sulphoxide as flavour and odour precursors and, with a few exceptions, they possessed high overall flavour and odour levels. These included several species which are thought to be related to cultivated edible forms. With the possible exception of A. caeruleum and A. cyaneum no evidence of resistance to infection by S. cepivorum was detected.     

Selection of fungal biological control agents of Sclerotium cepivorum for control of white rot by sclerotial degradation in a UK soil

A range of fungal isolates was tested in a three-stage screening system for their ability to degrade sclerotia of Sclerotium cepivorum on agar and in soil, and to reduce white rot disease on onion seedlings. Biological control agents (BCAs) were identified that could degrade up to 60% of sclerotia in soil and significantly reduce white rot disease on onion seedlings. The efficacy of the BCAs was enhanced when applied as wheat bran cultures compared with spore suspensions, and two of the best BCAs from the screening procedures were both identified as Trichoderma viride (L4, S17A). When L4 and S17A were fluid-drilled in guar gum with bulb onion seed in the field white rot symptoms were significantly reduced, but stem base applications applied mid-season had little effect. The strategy of selecting and using BCAs that degrade sclerotia of S. cepivorum and integration with other control methods is discussed.     

Susceptibility of cultivars of garlic to Sclerotium cepivorum

Five garlic cultivars, including one claimed to be resistant to Allium white rot, were similar in their effects on the germination of sclerotia of Sclerotium cepivorum. There was no evidence of resistance to the pathogen in any of the cultivars when tested under field conditions.     

Allium White Rot Suppression with Composts and Trichoderma viride in Relation to Sclerotia Viability

ABSTRACT Allium white rot (AWR) is a serious disease of Allium spp. caused by the sclerotium-forming fungus Sclerotium cepivorum. This work has examined the effects of onion waste compost (OWC) and spent mushroom compost (SMC), with and without Trichoderma viride S17A, on sclerotia viability and AWR in glasshouse and field experiments. Incorporation of OWC into soil reduced the viability of sclerotia and the incidence of AWR on onion plants in glasshouse pot bioassays, whereas SMC or T. viride S17A only reduced incidence of AWR. In two field trials, OWC reduced sclerotia viability and was as effective in reducing AWR as a fungicide (Folicur, a.i. tebuconazole). Field application of SMC had no effect on sclerotia viability and did not control AWR. However, the addition of T. viride S17A to SMC facilitated proliferation of T. viride S17A in the soil and increased the healthy onion bulb yield. The results indicate two mechanisms for the suppression of AWR: (i) reduction in the soil population of viable sclerotia, which may be due to volatile sulfur compounds detected in OWC but absent in SMC, and (ii) prevention of infection of onion plants from sclerotia following amendment of soil with OWC, SMC, or T. viride S17A.     

Long-term survival of sclerotia of Sclerotium cepivorum and Stromatinia gladioli

In an experiment with pure-culture sclerotia of Sclerotium cepivorum and Stromatinia gladioli a large proportion of the sclerotia survived burial for 20 years in the field in the absence of host plants. Some evidence for metabolic activity or leakage leading to the gradual erosion of the sclerotial contents was obtained with S. cepivorum . The results of other experiments suggested that, for unknown reasons, a variable proportion of sclerotia formed naturally on infected plants may decay shortly after their formation, but those which survive beyond this limited period are likely to remain viable for many years. The viability of sclerotia was also reduced by prolonged flooding, but the period for which land would have to be under water excludes this as a method of eliminating sclerotia from infested land in the UK. Treatment with the mycoparasite Teratosperma oligocladum did not affect the survival or infectivity of sclerotia of S. cepivorum or S. gladioli .     

Interactions between Sclerotium cepivorum and cultivars of onion, leek, garlic and Allium fistulosum

Laboratory and field experiments showed that germination of sclerotia of Sclerotium cepivorum was slower in the presence of cultivars of leek than in the presence of those of onion, garlic and Allium fistulosum . Spread of infection amongst closely spaced plants was also less in leeks than in the other three species. No evidence was found for any differences in the resistance of root tissues of the four Allium species. It is considered that differential stimulation of sclerotium germination is likely to have been the principal reason for the low incidence of white rot infection in leeks which was reported previously.     

Studies of dormancy in sclerotia of Sclerotium cepivorum

Sclerotia of Sclerotium cepivorum formed on onion bulbs in soil were shown to undergo a period of constitutive dormancy lasting 1–3 months during which they failed to respond maximally to the germination stimulant diallyl disulphide. Even after the breakdown of constitutive dormancy sclerotia were subject to exogenous dormancy from which they were only released by the presence of species of Allium or of germination stimulants known to be produced by them.     

Effect of environmental factors and Sclerotium cepivorum isolate on sclerotial degradation and biological control of white rot by Trichoderma

Laboratory assays demonstrated that two isolates of Trichoderma viride and one isolate of Trichoderma pseudokoningii degraded up to 80% of sclerotia of four isolates of Sclerotium cepivorum in a silty clay soil, and also degraded up to 60% of sclerotia in three other soil types. Relationships were defined between the degree of sclerotial degradation by the two T. viride isolates in the silty clay soil and both temperature and soil water potential. Sclerotia were degraded between 10 and 25°C at −0·00012 MPa, but there was little activity of T. viride at 5°C or at −4 MPa. Degradation of S. cepivorum sclerotia also occurred in the absence of Trichoderma at soil water potentials approaching saturation . Experiments using onion seedling bioassays showed that the efficacy of Trichoderma isolates for the control of white rot using the same selection of soils and S. cepivorum isolates was variable, but that there was significant disease control overall. The importance of environmental factors and pathogen isolate in relation to effective biological control of white rot is discussed.     

A comparison of flavour and odour compounds of onion, leek, garlic and Allium fistulosum in relation to germination of sclerotia of Sclerotium cepivorum

The low ability of cullivars of leek to stimulate germination of sclerotia of Sclerotium cepivorum compared with those of onion, garlic and Allium fistulosum appears to be associated with their relative iv small seeds, the lower levels of flavour and odour precursors in their roots and the relatively high proportion of the inactive methyl radical in their flavour and odour profiles.     

Effects of cultivation, conditioning and isolate on sclerotium germination in Sclerotium cepivorum

Sclerotium germination in various isolates of S. cepivorum was studied following different cultivation and conditioning treatments. A simple and rapid laboratory test was developed to trigger sclerotial germination under unsterile conditions. In most isolates, sclerotia produced under sterile and unsterile conditions showed a constitutive dormancy immediately after maturing. In contrast to this, the sclerotia of some isolates germinated quite well as soon as they were mature. The dormancy of sclerotia from axenic cultures broke down after storage in soil for 12 weeks. In a few cases sclerotia produced under unsterile conditions germinated remarkably well. Freezing and thawing of sclerotia decreased the ability of most isolates to germinate.     

‘Instant’ Bordeaux and Burgundy Mixtures

Four annual experiments with garlic (Allium sativum) were established in fields naturally infested with Sclerotium cepivorum, causal agent of garlic white rot, to investigate the effect of time of planting on yield. Initial stands were unaffected by time of planting. Final stands and yields were highest when garlic was planted approximately 1 month earlier (28 January-15 February) when soils were warmer than at the traditional time of planting (15 March-15 April). When garlic was planted at the traditional time, final stands and yields were reduced 56-92% primarily from the attack of S. cepivorum. A 100% loss in yield occurred in some crops planted in cool soils in late March. With an early planting (15 January), final stands were similar to the plantings of 28 January and 15 February, but yields were reduced by half because of a much lower mean weight of the harvested bulbs. In a late planting (15 May), the final stand was 75% lower with correspondingly lower yield than that obtained with the early plantings. The largest bulbs (21.6 g) were obtained from the planting on 28 February; however, the yields from this planting were reduced by half from the planting 2 weeks earlier because of the 50% reduction in final stand. Early planting of garlic is recommended as an important management strategy to avoid white rot in areas with soils infested with sclerotia of S. cepivorum.     

A new large scale soil DNA extraction procedure and real-time PCR assay for the detection of Sclerotium cepivorum in soil

A real-time PCR assay specific for Sclerotium cepivorum, the causal agent of white rot in onions, was developed for use with a new DNA extraction method capable of processing up to 1?kg of soil in weight. The assay was specific for S. cepivorum when tested against 24 isolates representative of 14 closely related species and other pathogens of onion. The assay was highly sensitive when used with soil DNA extracted using the new DNA extraction procedure. In three different field soils tested, a good relationship between cycle threshold (Ct) and number of sclerotia was observed (R2?=?0.89). Twenty-nine soil samples from onion and leek crops were obtained and the pathogen was detected in four samples. All four positive samples were associated with current or past outbreaks of white rot of onion. Additional assays were also used on the 29 field soil samples, Botrytis aclada and Rhizoctonia solani AG8 were also detected, in one soil sample each. Rhizoctonia solani AG2-1 was more widespread and was detected in eight different soil samples. The method is therefore suitable to quantify levels of S. cepivorum in soil samples, with the added advantage that the technique allows other soil pathogens of interest to be assayed from the same DNA sample. The bulk soil DNA extraction method described here has the potential to be used to detect soil-borne pests and pathogens for other crops in a wide range of soil types.     

Glasshouse trials of Coniothyrium minitans and Trichoderma species for the biological control of Sclerotinia sclerotiorum in celery and lettuce

Coniothyrium minitans, Trichoderma harzianum (HH3) and Trichoderma sp. (B1) were tested for ability to control disease caused by Sclerotinia sclerotiorum in a sequence of a celery crop and two lettuce crops in the glasshouse. In control plots, over 80% of celery and 90 and 60% of lettuce in first and second crops, respectively, were infected at harvest. Only the C. minitaris treatment in the first lettuce crop decreased disease and increased marketable yield. Nevertheless, C. minitans reduced the number of sclerotia recovered at harvest in the celery and first lettuce crops and decreased sclerotial survival over the autumn fallow periods following the celery and second lettuce crop. C. minitans survived in soil for over 1 year and spread to infect sclerotia in virtually all other plots. C. minitans infected sclerotia at all times of the year but sclerotia still failed to degrade during the summer months when the soil was dry. The Trichoderma species tested had no effect on disease and almost no effect on the survival of the sclerotia. even though they could be recovered from soil for the duration of the experiments.     

Integrated control of Allium white rot with Trichoderma viride, tebuconazole and composted onion waste

Two isolates of Trichoderma viride (L4 and S17A) were assessed for biological control of Allium white rot (AWR) with different onion accessions and cultivars, alone and in combination with a tebuconazole-based seed treatment or composted onion waste. In glasshouse tests, 23 new bulb-onion accessions from previous work to detect resistance to Sclerotium cepivorum showed no differences in susceptibility to AWR but, when combined with S17A, disease was reduced by up to two-thirds over all accessions. Trichoderma viride L4 and S17A also reduced the proportion of infected plants for five commercial bulb-onion cultivars and one advanced breeding line by at least one-third. Further glasshouse tests using a salad-onion cultivar showed that L4, S17A, tebuconazole or composted onion waste controlled AWR and at least halved the proportion of diseased plants. Combination treatments of T. viride with either tebuconazole or compost enhanced control and, in some treatments, disease was almost eliminated. In field trials, control of AWR by S17A was significant for 17 out of 18 individual or mixed bulb-onion accessions, with disease reduced overall by more than half. In another field experiment, S17A failed significantly to reduce AWR for two out of three commercial bulb-onion cultivars, while tebuconazole reduced the final proportion of AWR-infected plants over all cultivars from 0·47 to 0·09. Combining S17A and tebuconazole resulted in a similar level of AWR to using tebuconazole alone. The use of T. viride in an integrated strategy with other treatments to enhance control of S. cepivorum is discussed.     

影响盾壳霉寄生核盘菌菌核的几个生态因子的分析

 在室内测定了盾壳霉(Coniothyrium minitans)对不同寄主上的核盘菌(Sclerotinia sclerotiorum)菌核的寄生致腐作用,研究了温度、含水量和土壤类型等生态因子对核盘菌菌核的寄生致腐作用的影响,通过检测土壤的呼吸速率探讨了它在土壤中定殖与核盘菌菌核的关系。结果表明:盾壳霉能寄生致腐核盘菌属所有供试菌株的菌核;寄生致腐菌核的最适温度是20℃,最适相对含水量为50%~60%;盾壳霉在供试的8种土壤中均能寄生致腐菌核,对它们的pH值要求不严格,但土壤类型影响其寄生致腐速度;在土壤中添加菌核和菌核提取液都可不同程度地刺激它的生长。     

The process of antagonism of Sclerotium cepivorum in white rot affected onion roots by Trichoderma koningii

Trichoderma koningii (strain Tr5) grew in the epidermal mucilage of onion roots without entering healthy epidermal tissue. When placed on the epidermis of Sclerotium cepivorum -infected roots, T. koningii colonized epidermal passage cells, with little colonization of other epidermal tissues, then branched and spread throughout the root cortical tissues damaged by enzymes and toxins which diffused ahead of S. cepivorum hyphae, and impeded the path of the infection. When T. koningii colonized infected tissue, many S. cepivorum hyphae became detached at septa, cell walls dissolved and many hyphal apices burst. Contact between hyphae was not necessary for lysis to occur. T. koningii produced two endochitinases ( R _f 0·15 and 0·24) and two exo-acting chitinolytic enzymes ( R _f 0·46 and 0·62) during degradation of crabshell chitin and S. cepivorum cell walls. The R f 0·24 and 0·46 proteins were detected when T. koningii colonized S. cepivorum -infected roots and are likely to be a component of the antagonism process.     

Differential stimulation of germination of sclerotia of Sclerotium cepivorum by cultivars of onion and its effect on white rot disease

Differences were found in the ability of onion cultivars to stimulate the germination of sclerotia of Sclerotium cepivorum under UK conditions. The differences between European and Egyptian cultivars were particularly marked. Differences also occurred in the development of white rot disease in European and Egyptian cultivars. These differences were related to the pattern of growth of the cultivars and not to levels or types of flavour and odour compounds. The effect of the pattern of growth of plants on germination of sclerotia was confirmed in experiments with different types of planting material. Germination beneath plants grown from sets was particularly rapid, but very slow beneath plants produced by direct seeding. An experiment with plants grown under different levels of sulphate nutrition showed that low levels of flavour and odour compounds were associated with a weak capacity to stimulate the germination of sclerotia. No evidence was found to suggest that control of white rot disease by selection for growth features or for differences in flavour and odour characteristics would be a practicable proposition for the foreseeable future.     

Methods for the production and use of sclerotia of Sclerotium cepivorum in field germination studies

Methods are described for maintaining isolates of Sclerotium cepivorum and for producing large numbers of sclerotia of this organism for field studies. Techniques for testing the viability of sclerotia and for measuring their germination in response to host plants are also described together with other aspects of field studies.     

Infection of cultivars of onion, leek, garlic and A llium fistulosum by Sclerotium cepivorum

In two field experiments on land severely infested with Sclerotium cepivorum largo differences in the incidence of white rot in four Allium species were recorded. There was inuch less disease ° Cultivars of leek than in those of onion, garlic or A. fistulosum. Differences between cultivars within species were either small or absent.     

Germination of Sclerotinia minor and S. sclerotiorum Sclerotia Under Various Soil Moisture and Temperature Combinations

ABSTRACT Sclerotial germination of three isolates each of Sclerotinia minor and S. sclerotiorum was compared under various soil moisture and temperature combinations in soils from Huron and Salinas, CA. Sclerotia from each isolate in soil disks equilibrated at 0, -0.03, -0.07, -0.1, -0.15, and -0.3 MPa were transferred into petri plates and incubated at 5, 10, 15, 20, 25, and 30 degrees C. Types and levels of germination in the two species were recorded. Petri plates in which apothecia were observed were transferred into a growth chamber at 15 degrees C with a 12-h light-dark regime. All retrievable sclerotia were recovered 3 months later and tested for viability. Soil type did not affect either the type or level of germination of sclerotia. Mycelial germination was the predominant mode in sclerotia of S. minor, and it occurred between -0.03 and -0.3 MPa and 5 and 25 degrees C, with an optimum at -0.1 MPa and 15 degrees C. No germination occurred at 30 degrees C or 0 MPa. Soil temperature, moisture, or soil type did not affect the viability of sclerotia of either species. Carpogenic germination of S. sclerotiorum sclerotia, measured as the number of sclerotia producing stipes and apothecia, was the predominant mode that was affected significantly by soil moisture and temperature. Myceliogenic germination in this species under the experimental conditions was infrequent. The optimum conditions for carpogenic germination were 15 degrees C and -0.03 or -0.07 MPa. To study the effect of sclerotial size on carpogenic germination in both S. minor and S. sclerotiorum, sclerotia of three distinct size classes for each species were placed in soil disks equilibrated at -0.03 MPa and incubated at 15 degrees C. After 6 weeks, number of stipes and apothecia produced by sclerotia were counted. Solitary S. minor sclerotia did not form apothecia, but aggregates of attached sclerotia readily formed apothecia. The number of stipes produced by both S. minor and S. sclerotiorum was highly correlated with sclerotial size. These results suggest there is a threshold of sclerotial size below which apothecia are not produced, and explains, in part, why production of apothecia in S. minor seldom occurs in nature.