Approximately 150 seed lots of Allium cepa and more than 30 different Allium species were tested in the greenhouse under strictly controlled conditions against Sclerotium cepivorum, the causal fungus of Allium white rot disease. The quantity and quality of the inoculum was determined using a simple and rapid test to achieve a high degree of standardization. Only the A. cepa cultivar “Sweet Sandwich’ and A. porrum, A. rotundum, A. sphaerocephalon and two further unidentified Allium species showed significantly lower disease incidence than the cultivar ‘Golden Bear’. The stimulatory effect of the root exudates on sclerotial germination was compared with that of diallyl sulphide. With the exception of A. species‘PI 249549′, extracts of all species that showed low levels of disease incidence in the greenhouse test stimulated sclerotial germination only weakly.
"This allows the use of pesticides on some minor crops when manufacturers have sought approval only on major crops such as cereals. Despite efforts to find resistance to S. cepivorum within existing cultivars and other Allium species (Utkhede et al. , 1982; Brix & Zinkernagel, 1992), there are no commercial Allium cultivars with sufficient resistance that currently can be exploited for AWR control. Other potential control strategies for AWR have therefore been investigated, including soil solarization (Melero-Vara et al. , 2000; McLean et al. , 2001) and application of diallyl disulphide (DADS) to stimulate sclerotial germination in the absence of a host (Crowe et al. , 1994; Hovius & McDonald, 2002), but these approaches are generally unsuitable for UK conditions. "
[Show abstract][Hide abstract] ABSTRACT: 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.
"Sclerotia were harvested by flotation in water, retrieved on a sieve (212 µm mesh diameter), dried in an airflow cabinet for 12 h and stored at 5°C. Sclerotia used in onion seedling bioassays were first buried in the field in mesh bags with an equal volume of sand for at least 12 weeks to overcome dormancy effects (Coley-Smith et al., 1987; Brix & Zinkernagel, 1992b). After this 'conditioning', sclerotia were removed from the bags, washed, sieved and dried as before. "
[Show abstract][Hide abstract] ABSTRACT: 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.
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