One possibly unexpected group of plant pathogens is the Trypanosomatida—the same group that includes Leishmania and Trypanosoma, the causative agents of kala-azar, African sleeping sickness, etc. It also includes the genus Phytomonas, which live in phloem cells and latex vessels of various plants. These were first discovered about 110 years ago in some Euphorbiaceae species, and are now known from 24 plant families.

Most Phytomonas species don't seem to cause much harm, but Phytomonas staheli causes wilting diseases of oil and coconut palms, and Phytomonas leptovasorum causes phloem necrosis in coffee. Both are an economic problem in South America. (Source: Jaskowska et al.,Phytomonas: Trypanosomatids Adapted to Plant Environments ) There may be trypanosomatid species in other genera that infect plants—the taxonomy of this group doesn't seem to be well resolved. Most trypanosomatids are spread among vertebrate hosts by the bites of bloodsucking insects—but evidently, in this case, some of them came to be spread among plants by the bites of sapsucking insects. Interesting bit of host switching, that.

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Https://doi.org/10.1016/0169-4758(90)90388- K.

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Over 100 years after trypanosomatids were first discovered in plant tissues, Phytomonas parasites have now been isolated across the globe from members of 24 different plant families. Most identified species have not been associated with any plant pathology and to date only two species are definitively known to cause plant disease. These diseases (wilt of palm and coffee phloem necrosis) are problematic in areas of South America where they threaten the economies of developing countries. In contrast to their mammalian infective relatives, our knowledge of the biology of Phytomonas parasites and how they interact with their plant hosts is limited. This review draws together a century of research into plant trypanosomatids, from the first isolations and experimental infections to the recent publication of the first Phytomonas genomes. The availability of genomic data for these plant parasites opens a new avenue for comparative investigations into trypanosomatid biology and provides fresh insight into how this important group of parasites have adapted to survive in a spectrum of hosts from crocodiles to coconuts.