Grain mould of sorghum with specific reference to grain quality in South Africa

[摘要] English: The major sorghum grain mould fungi in South Africa were determined by assaying five sorghum cultivars from three localities in 2002 and eight localities in 2003. The predominant fungal isolates from all the cultivars and localities were Alternaria, Curvularia, Fusarium and Phoma spp. The relative frequency of grain mould fungal isolates differed with locality. Grain mould fungi were least frequently encountered at Bethlehem followed by Heilbron. The incidence of grain mould fungi negatively affects the milling and malting quality of sorghum grains. The effect of fungal isolates on various grain quality parameters was examined by inoculating sorghum panicles under glasshouse conditions. All inoculated fungi reduced seed germination with C. lunata showing the highest reduction, followed by a mixed population of fungi. F. proliferatum, A. alternata, F. graminearum, P. sorghina and a fungal mixture were important pathogens in terms of reducing the 1000 kernel mass. F. thapsinum, F. proliferatum and the mixed fungal population followed by P. sorghina, C. lunata and A. alternata resulted in higher levels of grain discolouration in glasshouse studies. Various levels of mycotoxin were produced in grains inoculated in the glasshouse and sorghum grains from experimental plots.The role of weather on the grain mould fungal incidence was determined by planting five cultivars on different planting dates at three localities. Fungal frequency varied across localities and flowering dates. In all cultivars, highest grain mould incidence was recorded at Potchefstroom. A. alternata was the most dominant fungus at all localities and at all flowering dates. An increase in moisture and temperature was positively correlated with fungal invasion. Results indicated significant correlations between grain mould incidence and certain grain quality parameters, such as the milling and malting quality. Wetness duration significantly affected grain mould development and subsequent grain quality under controlled conditions. Increased wetness duration resulted in an increase in grain infection by the respective grain mould fungi.Sorghum grain infection and subsequent grain quality deterioration as affected by insect damage was assessed in experimental plots at Bethlehem, Cedara and Potchefstroom. Insects were collected from sorghum heads and grain mould fungi were isolated from all specimens collected. A. alternata were most frequently isolated from specimens collected from Cedara while the incidence of Fusarium spp. were highest in insect samples from Bethlehem. Insecticides significantly increased 100 kernel mass compared with unsprayed grains. Mycotoxin production of grains from the insecticide treated and untreated plots were assessed and some of them produced significant levels of mycotoxins, but the effect was not consistent. Puncture marks created by insect damage were positively correlated with the incidence of Fusarium spp. It was evident that insects are involved either in transporting the fungal propagules or damaging the sorghum kernels resulting in higher fungal incidence, which consequently reduced the quality of the grains.Mycotoxins in samples from different stages of malting were assayed. The highest concentration of aflatoxin was found in samples from the steeping stage. Different concentration levels zearalenone and fumonisin were also isolated from the malting samples. Mycotoxin contamination of commercial sorghum products and other cereal products were assayed. Higher concentrations of aflatoxin were found in sorghum products followed by maize samples while aflatoxin was only recorded in two of six wheat samples. No fumonisin was found in all the samples tested. NaOCl in water used for steeping resulted in reduced isolation frequency of grain mould fungi with significant improvement in the percentage germination and root length. It was concluded that grain mould fungi are potential mycotoxin producers in sorghum products and may reduce its quality.The results of the current study confirmed the involvement of various fungi in the disease complex known as grain mould and elucidate their possible role in both qualitative and quantitative losses in sorghum. The results also demonstrated the role of favourable weather conditions and insects in the development of grain mould and the effect that the pathogens have on quality and quantity of sorghum grain. The involvement of insects in the development of grain mould warrants further attention. It is hoped that the findings of this study may serve as motivation for the development of a holistic strategy to manage grain moulds and to maintain the quality of sorghum product.

[发布机构] University of the Free State