Stenocarpella maydis and its toxic metabolites: a South African perspective on diplodiosis

Abstract

Stenocarpella maydis is one of the most prevalent ear and stalk rot pathogens of maize globally, causing reductions of grain quality and yield. Various molecular methods, including polymerase chain reaction (PCR)-based techniques and nucleotide microarrays, have been developed for the identification of S. maydis infestation in maize grain. In addition to diplodiatoxin, new metabolites, namely dipmatol, diplonine and chaetoglobosins K and L, have been isolated from S. maydis infected cultures. S. maydis infected maize is also associated with intoxication in ruminants. Diplodiosis, a nervous disorder of cattle and sheep, results from ingestion of mouldy ears, kernels and maize stubble infected by S. maydis. Although this disease is most common in southern Africa, it has also been reported in Australia, Argentina and Brazil. Diplodiosis is characterised by reluctance of the animals to move, a wide-based stance, incoordination, paralysis and death. Myelin degeneration (status spongiosis) is the only histopathological change observed in affected animals, especially in cases of perinatal mortality. To date, none of the purified S. maydis metabolites has been administered to ruminants in order to reproduce diplodiosis. However, recent studies have focused on investigating the toxicity of the metabolites on cell cultures. Cytotoxicity studies where cultured cells were exposed to the S. maydis metabolites indicated that diplodiatoxin and dipmatol affected the activity of the mitochondrial succinate dehydrogenase enzyme and the overall viability of the cells. More detailed in vitro toxicity studies are still required to elucidate how the currently available S. maydis metabolites influence parameters such as the mechanism of cell death. Development of analytical test methods to quantify and establish the presence and distribution of these mycotoxins in infected maize commodities also needs investigation. It is also critical that the role of S. maydis stalk rot be evaluated as a potential source and cause of diplodiosis.