Deoxynivalenol (DON) and nivalenol (NIV) are trichothecene mycotoxins produced principally by Fusarium graminearum Schwabe (telemorph Gibberella zeae (Schw.)Petch.), one of the major pathogenic fungi of head blight diseases in wheat and barley and rot in corn.They occur worldwide in these plant products intended for human and animal consumption, causing the economical loss and the health risk in humans and animals.In Japan, DON and NIV are recognized as two major trichothecenes coincidentally occurring in domestic wheat and barley grains.As for the mechanism of the epidemic of head blight disease and mycotoxin occurrence in wheat and barley, the following sequential steps are involved : i) the production of G. zeae perithecia on plant debris, like overwintered rice-stubbles, ii) the aerial ascospores dispersal from perithecia to infection sites (spikes), iii) the infection at anthesis and the clonization in spikes under the suitable moisture and temperature conditions, and iv) the fungal growth and the accumulation of mycotoxins in the process of head maturation.Although several papers have suggested the geographic difference in relation to the occurrence of trichothecenes in Japanese wheat and barley, there are only few reports studying the above steps systematically.Recently, we have attempted comprehensive chemical and mycological studies on the features of the occurrence of trichothecenes in Japan.This paper reviews the characteristics of natural occurrence of DON and NIV in Japanese wheat and barley.Dynamics of the incidence of the dispersal ascospores of toxigenic G. zeae A 3-year field study was conducted in a fixed field point with wheat-rice rotation in Miki, Kagawa.The monthly changes in the incidence of aerial dispersal ascospores were as follows : the highest (21.9 CFU/plate) in April (spiking and flowering season), 2.9-4.9CFU/plate in July to October (rice-cultivating season), and 0.2-0.6CFU/plate in November to February.Among 177 isolates of G. zeae examined, 172 isolates (97%) showed the ability to produce trichothecenes and were classified into NIV-(148 isolates, 86%) and DON-(24 isolates, 14%) chemotypes.Even though the incidence of dispersal ascospores varied greatly with season, the ratio of NIV-chemotype to DON-chemotype remained mostly constant ranging from 80 : 20 to 90: 10, suggesting constant ratios for individual regions regardless of crop years, and changes in *
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Yoshizawa et al. (1998) studied this question.