Studies were undertaken to investigate the factors affecting mycotoxin production in Fusarium phytopatliogens. The conclusions of this work are divided into the following categories.
Substantive findings:Contamination of cereal grains with Fusarium mycotoxins is a global and continuing issue and is likely to remain so until a systematic elucidation of controlling factors has been accomplished.
An important observation at the commencement of the research programme examined the validity of using direct TLC analysis of agar culture plugs to establish mycotoxin profiles in Fusarium species.
The results in this thesis indicate that this method only detected a limited number of mycotoxins and this was not consistent between surfaces of the plugs tested or between experiments in the preliminary series.
A noteworthy observation was that HT-2 toxin production from T-2 toxin was not time- related as claimed by other workers. There may be a real species differences in this respect between F. sporotrichioides and F. poae.
Mixtures of carbendazim and propiconazole or carbendazim plus maneb or carbendazim plus maneb plus tridemorph all enhanced T-2 toxin formation.
The fungicide-induced enhancement of mycotoxin production has now been extended, for the first time, to HT-2 toxin and NEO.
A substantive finding, not previously noted, is that difenoconazole failed to stimulate T-2 toxin formation at 25°C but was capable of transforming it to HT-2 toxin and then stimulating the production of the latter product.
Difenoconazole appears to be a fungicide in a class of its own in that DAS and NEO production are consistently higher than for a large majority of other fungicides tested.
NEO production was not substantially affected by fungal exposure to Bavistin, whereas carbendazim acted in a stimulatory manner. It is suggested that this discrepancy is due to fungicide form, an effect not previously reported.A novel finding has been a consistent indication of reduced efficacy of pesticides for the control of ZEN in the 25-11°C regime compared to the constant incubation at 25°C.
The effects of incubation temperature on T-2 toxin production is dependent upon fungicide type whereas in the case of HT-2 toxin, DAS and NEO, there is a clear effect of temperature. Contrasting effects were observed in fungicide efficacy for the control of HT-2 toxin in the two temperature regimes. At 25°C, stimulation occurred at the 100 pig/inl dose but in the 25- 11°C regime, HT-2 toxin production was almost eliminated at the same concentration.
It is suggested that the complex interactions involving fungicides, temperature and duration of exposure to these factors may be critical in the timing of fungicide applications in the field.
A new classification for fungicide efficacy is proposed, based on the capacity to completely control FHB/grain infection/fungal growth and mycotoxin production on grain or in culture. Three classes have been identified. Most fungicides appear in the Class III group, incorporating fungicides that are either ineffective or capable of stimulating trichothecene production.
Although maneb has been identified as a Class I fungicide from evidence in the literature, its high efficacy is not conferred to mixtures incorporating this fungicide.
A novel finding in the present study with F. culmorum was the demonstration of 3-ADON enhancement induced by the herbicide combination of bromoxynil, ioxynil and mecoprop added as Swipe.
A radical re-design of pesticides is proposed to incorporate efficacy for regulation of mycotoxin biosynthesis.
Secondary findings:A rule-of-thumb based on colour differences of Fusarium colonies is proposed. The present results suggest that the striking differences in pigmentation might be associated with the pattern of mycotoxin production. Thus yellow is possibly indicative of the presence of T-2 toxin, HT-2 toxin, DON, 15-ADON and 3-ADON while pink/red coloration might suggest the production of ZEN. This rule-of-thumb, if sufficiently developed, might find application in field assessments of toxigenic potential in cases of cereal diseases such as FHB.
The lack of efficacy of propiconazole to regulate trichothecene production has been extended to instances where it is combined with other fungicides.
Confirmatory findings:Present results with F. sporotrichioides confirm for only the second time in recent years that this phytopathogen is a consistent producer of ZEN.
Biosynthesis of ZEN is clearly temperature-sensitive but there may be species differences regarding this effect.
The enhancement effect of fungicides on T-2 toxin production has now been extended to carbendazim.
New hypothesis:The current studies on gene expression indicate future potential for elucidating the biochemical mechanisms underlying trichothecene biosynthesis as affected by factors such as incubation temperature and fungicide applications. Such an approach might lead to more effective pesticides.