Striga spp. are noxious parasitic weeds that attack important crop species such as rice, maize and sorghum. Estimated yield losses can reach 1 billion US dollars annually. Striga belongs to the Orobanchaceae family, which contains approximately 90 genera of parasitic species. The degree of parasitism varied among species, from facultative parasites that can set seeds without host plants, to obligate holoparasites that cannot photosynthesize and thus completely depend on their hosts. The obligate parasites (e.g., Striga and Orobanche) germinate only when host plants are close by, via recognition of host-secreted strigolactones. The germinated parasitic plants develop an invasive haustorium on their roots. The haustoria penetrate host roots and establish vascular connections between hosts and parasites to absorb water and nutrients. Induction of haustorium is provoked by host-derived small compounds, named haustorium-inducing factors, including quinones and flavonoids. Diversity of germination stimulants and haustorium-inducing factors is likely to contribute to host recognition by parasitic plants. On the other hand, host plants respond to parasitic plants by activating plant immunity. This review focuses on the interaction between the parasites and their hosts.
We collected 1017 and 1931 isolates of wheat eyespot pathogen from 555 fields in various regions of Hokkaido in 1989 and 1992, respectively. Of these, 52.3% in 1989 and 43.7% in 1992 were identified as Oculimacula acuformis based on mycelial growth and culture morphology. The others (47.7% in 1989 and 56.7% in 1992) were O. yallundae. The 2-year data showed that O. acuformis was widely distributed in Hokkaido and was the dominant species in the northern and eastern regions. The susceptibility of O. yallundae and O. acuformis to demethylation inhibitor (DMI) fungicides was tested. The EC50 values for propiconazole ranged from 0.16 to 0.32 ppm and 1.0 to 4.5 ppm, and those for prochloraz ranged from 0.03 to 0.09 ppm and 0.04 to 0.18 ppm in O. yallundae and O. acuformis, respectively. The 2-year results indicate that the susceptibility of O. acuformis to propiconazole was relatively low. Efficacy of propiconazole and prochloraz on wheat eyespot was investigated in the fields where the proportion of O. acuformis was different from 5.0 to 100%. Propiconazole was effective in the fields dominated by O. yallundae, whereas the effect was not observed in those dominated by O. acuformis. Prochloraz had a stable effect in all the fields tested.
A severe postharvest rot of acerola fruits (cv. Kanmikei) was found in Okinawa Prefecture, Japan in 2017. A fungus frequently isolated from the diseased fruits was identified as Colletotrichum tropicale based on morphological, cultural characteristics and sequence similarity of ApMat and GS gene regions. The isolates reproduced the symptoms on acerola fruits after inoculation and were reisolated from the inoculated fruits. This is the first report of anthracnose on acerola fruit in Japan.