Bridging the Gulf: Phytophthora and Downy Mildews Are Connected by Rare Grass Parasites

Abstract

Downy mildews and root and foliar rots caused by Phytophthora are among the most destructive plant pathogens and therefore have attracted considerable attention during the past two decades. Although it has been realized that a close phylogenetic relationship exists, so far sharp distinction has been made between the obligate biotrophic downy mildews and the hemibiotrophic Phytophthora. In the study presented here, it is shown that a continuum of character states from hemibiotrophic Phytophthora species to obligate biotrophic downy mildews is present. Intermediate character states between downy mildews and Phytophthora species exist in several rare parasites of grasses, which are not embedded within the major clades of the downy mildews but are placed sister to these, with unresolved affinities to both these clades and to Phytophthora. They still have retained traits hitherto thought to be exclusive for Phytophthora. A careful review of previous research is presented and it is highlighted that uniquely for downy mildews, Poakatesthia may form an intracellular mycelium, growing through several host cells. In addition, scanning electron microscopy reveals that sporangiophore growth is not determinate in Viennotia and that outgrowth from sporangiophores is very similar to Phytophthora infestans. It is concluded that the sharp morphological distinction between downy mildews and Phytophthora species (that are often placed in separate families and even different orders), is rather artificial, since all features thought to be exclusive to Phytophthora or the downy mildews are united in the rare grass-parasitizing down mildew genera Viennotia and Poakatesthia and the enigmatic genus Sclerophthora. Therefore, several paradigms regarding the distinction between Phytophthora and the downy mildews need to be reconsidered.

The cellulose synthase 3 (CesA3) gene of oomycetes: structure, phylogeny and influence on sensitivity to carboxylic acid amide (CAA) fungicides

“…However, the CesA3 gene tree resulting from the ML analysis, resolves the genus Phytophthora as monophyletic, which is in agreement with the findings of Kroon et al (2004) but contradictory to other studies that resolved Phytophthora as clearly paraphyletic (Cooke et al 2000;Voglmayr 2003;G€ oker et al 2007;Runge et al 2011). Yet only shown with significant support by G€ oker et al (2007), the present analyses on the CesA3 gene sequences support the monophyly of the downy mildews (Fig 2), which was mentioned to be uncertain by Thines et al (2009).…”

<I>Salisapiliaceae</I> – a new family of oomycetes from marsh grass litter of southeastern north America

“…This phylogenetic placement not only favours the broad circumscription of the Peronosporaceae to include the closely related groups of Phytophthora and the downy mildews (Göker et al 2007, Thines 2009) as supposed in Thines et al (2009b), but also the inclusion of the genera Halophytophthora s.str. and Phytopythium , which are morphologically similar to Phytophthora , for avoiding an inflationary introduction of family names within the Peronosporales .…”

The molecular phylogeny of the white blister rust genus Pustula reveals a case of underestimated biodiversity with several undescribed species on ornamentals and crop plants

“…Molecular phylogenetic reconstructions (Riethm€ uller et al 2002;Hudspeth et al 2003;Thines et al 2008) have revealed that obligate biotrophy has evolved independently in the WBR and the downy mildews (DM). The DM appear to be relatively late branching within the Peronosporomycetidae, apparently arising from within Phytophthora, with some genera exhibiting Phytophthora-like traits (Thines 2009). In contrast, the WBR have recently been shown to form an early branch of the Peronosporomycetidae (sensu Dick 2001), diverging before both the Pythiaceae and Peronosporaceae (Peronosporales sensu lato).…”