Relationship between sorghum plant and grain characteristics, colonisation by mycotoxigenic Fusarium spp. and mycotoxin levels

[摘要] Sorghum grain mold is an economically destructive disease that can influence both the yield and quality of the grain. Causal fungi include Fusarium spp. that produce mycotoxins that affect the wellbeing of humans and animals. Ways to improve disease resistance have been explored and include exploitation of physical and chemical traits, and cultivation practices.Field trials were conducted at Cedara and Alma to evaluate sorghum grain mold resistance in 22 sorghum lines representing the range of plant and grain characteristics in commercially planted cultivars. Grain mold visual ratings, fungal biomass through ergosterol analysis, FGSC (F. graminearum species complex) quantification by means of qPCR and subsequent mycotoxin concentrations were analysed. Only purple plant colour was associated with higher grain mold ratings. No other grain characteristic were associated with resistance to colonisation or mycotoxins produced, which led to the assumption that resistance appears to be the result of complex reactions. The wetter Cedara had higher grain visual ratings, FGSC colonisation and subsequent mycotoxin production. AMMI analysis revealed genetic stability to grain mold through changing environments when visual assessments were applied, while ergosterol levels indicated genotypic adaptions to different environments. A multi-variable approach is needed for evaluation of resistance.Grain of the 22 sorghum lines planted in Cedara and Alma were evaluated for chemical traits contributing to grain mold resistance. The highest total phenolic acid levels were recorded in red pericarp grain compared to the lowest found in white pericarp grain. Total phenolic acids for half of the lines remained stable levels across environmental changes as indicated through AMMI analysis. HPLC analysis indicated inconsistent levels of specific phenolic compounds across genotypes, with the exception of gallic acid that correlated with resistance to grain mold in genotypes. Higher total protein levels were associated with coloured pericarp grain on a tan plant colour, but no relationship with disease severity was recorded. Similarly no relationship was observed with glucanase and chitinase levels. Further in-depth studies are needed to understand the complex mechanisms involved in resistance.Rotation systems were introduced into a field trial in Alma to evaluate the effect on grain mold and root rot severity in a high tannin and white tan genotype. Soil nutrient status, root and grain visual ratings, root mass and yield, ergosterol levels, FGSC and subsequent mycotoxin levels were measured. Throughout the trial, an increase in nutrients, root mass and yield were recorded in legume systems compared to fallow and monoculture sorghum systems. The effect of rotation systems on other parameters were variable. Cultivars also differed in their response with NS 5511, the high tannin cultivar, being more resistant to grain mold compared to PAN 8706W, the white tan cultivar, that was more resistant to root rot. Throughout the trial, beneficial effects of legume systems on root rot were observed, while responses to grain pathogens were variable.Two fungicides, azoxystrobin/difenoconazole and epoxyconazole/pyraclostrobin were applied to four cultivars at different time periods (6, 8, 10, 6 + 8, 8 + 10 weeks after planting) in Potchefstroom, Greytown and Standerton. Only superficial discolourations on the roots and grain, general fungal colonisation and root growth were affected by fungicide treatments. Both fungicides reduced root rot, with only azoxystrobin/difenoconazole being effective against grain mold. Only root growth was stimulated with fungicide applications. Yield, FGSC colonisation and subsequent mycotoxins were influenced more by environment, i.e. locality and seasons, and choice of cultivars and not as a result of fungicide treatments.

[发布机构] University of the Free State