Sir
In your News report on the re-emergence of late blight in Russia1, the causal agent is referred to as “the fungus Phytopthora infestans”. The genus name literally means 'plant destroyer' and is derived from the Greek words phyton and phthora, so the correct spelling is Phytophthora infestans.
Phytophthora is grouped in the class of Oomycetes, which behave like fungi in that their main body consists of a network of hyphae that grow at the tip, and they propagate via spores. Yet they are no longer classified in the kingdom Fungi.
Molecular phylogenetic analysis has clearly demonstrated that Oomycetes evolved completely independently from the true fungi — the Ascomycetes and Basidiomycetes — and instead are more closely related to golden-brown algae and heterokont algae in the eukaryotic crown group of Stramenophiles. Lynn Margulis2 has classified them in the kingdom Protoctista.
Hence, it is debatable whether access to fungicides would help small farmers in Russia to fend off a potato famine. The misconception that P. infestans is a fungus has put us on the wrong track for decades. Many fungicide targets are absent in oomycetes, and our knowledge of their biology is limited. New approaches are needed to find novel drug targets and to develop what I suggest should be called 'oomicides'.
Understanding the biology of oomycetes should reveal how these notorious pathogens interact with plants and why host resistance is lost so quickly, providing new leads for durable resistance-breeding strategies.
The aggressive strains currently prevailing in Russia, Western Europe and the United States are the result of a worldwide population displacement of P. infestans that started in the mid-1970s. Unlike the earlier infestation discussed in your News report that caused the Irish famine in the 1840s (see this issue, pages 644 and 695), the 1970s infestation included strains with the two different mating types3,4, so P. infestans can now propagate sexually in almost every area in the world where potatoes are grown. The egg-shaped sexual spores, oospores, can survive in soil independent of the host plant and can act as an extra inoculum source early in the growing season. Moreover, sexual recombination allows the pathogen to adapt even more easily to adverse conditions.
Fending off a potato famine in Russia requires more than access to suitable pesticides for small farmers. What is really needed is a rational design of control strategies, based on knowledge of the behaviour of the late-blight pathogen and on implementation of this knowledge by pathologists, breeders, biotechnologists and agronomists.
References
Nature 410, 1011 (2001).
Margulis, L. & Schwartz, K.V. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth (3rd edn) 109–202 (W. H. Freeman, New York, 1998).
Drenth, A., Tas, I. C. Q. & Govers, F. Eur. J. Plant Pathol. 100, 97–107 (1994).
Fry, W. E. et al. Ann. Rev. Phytopathol. 30, 107–130 (1992).
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Govers, F. Misclassification of pest as 'fungus' puts vital research on wrong track. Nature 411, 633 (2001). https://doi.org/10.1038/35079764
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DOI: https://doi.org/10.1038/35079764
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