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.
Citation: Thines M (2009) Bridging the Gulf: Phytophthora and Downy Mildews Are Connected by Rare Grass Parasites. PLoS ONE 4(3): e4790. https://doi.org/10.1371/journal.pone.0004790
Editor: Frederick M. Ausubel, Massachusetts General Hospital, United States of America
Received: June 1, 2008; Accepted: July 28, 2008; Published: March 10, 2009
Copyright: © 2009 Thines. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This study has been funded by grants from the German Science Foundation and the Elite Program for Post-Docs of the Landesstiftung Baden-Württemberg. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The author has declared that no competing interests exist.
Phytophthora species are among the most destructive rot-causing pathogens of plants, responsible for several catastrophic events, like the sudden oak death in North America, caused by Ph. ramorum  and the Irish Potato Famine caused by Ph. infestans , . These pathogens can be grown on synthetic media and are hemibiotrophic in nature. The downy mildews can cause severe damage in several important crops, such grape, cucurbits, sunflower, spinach, lettuce, sorghum, millet, tobacco and spinach. Downy mildews are generally believed to be obligate biotrophic and fully dependent on living host cells – although the reasons why are still obscure – so they can not be grown on media. Downy mildews are mainly host genus or even host species specific, while Phytophthora species often have a much wider host range. In addition to these characteristics, there are three main morphological characteristics that are thought to distinguish Phytophthora species from downy mildews. First, downy mildews do not form intracellular mycelium, but invade host cells only by haustoria , while Phytophthora is able to grow through both living and dead cells . Second and equally important, sporangiophore growth is terminated in downy mildews, i.e. once sporangia are formed no further growth takes place, while in Phytophthora the sporangiophore may grow further after sporangia have formed. Third, it is generally believed that in downy mildews all sporangia ripen simultaneously, while in Phytophthora sequential maturation takes place. These differences were thought to be of major importance and have been used as an argument to postulate a deep divide between downy mildews and Phytophthora, placing the downy mildews in a family of its own, the Peronosporaceae, while Phytophthora – on the basis of its thallus growth and general morphological characteristics – has usually been placed in the family Pythiaceae of the order Pythiales , . Recent molecular phylogenetic investigations –, however reveal that downy mildews are very closely related to a paraphyletic Phytophthora. Only in one study , was there significant support for a monophyly of the downy mildews, but this result could be attributed to an artefact due to a too narrow search radius, especially in Maximum Likelihood  phylogenetic reconstruction . However, in all molecular phylogenies computed so far, the clade containing both downy mildews and Phytophthora was found to be monophyletic, often with maximum support , . This appears at odds with the developmental and morphological differences between these two groups and the fact that Phytophthora infected plants usually rot and die, whilst those infected with downy mildews may remain almost or completely asymptomatic for long periods and may even recover, particularly in secondary infections of natural populations. However, the effectors secreted by these pathogens are similar –. The complex interactions involved in pathogenesis – differ mostly in respect of the final necrotic phases of infection in Phytophthora which is initiated by necrosis inducing proteins . Although necrosis can also be observed in later stages of infection in some downy mildews, such as Bremia, only seldom do plants die from infections with this pathogen. Bremia is not culturable on artificial media and therefore it is unlikely that necrotic parts of infected plants can serve as a source of nutrition for the hyphae. In general, the downy mildews have evolved into well adapted obligate biotrophic pathogens, which may be transmitted with the seeds of their host plants and many hardly cause observable symptoms. This is especially the case in Basidiophora and some species of Hyaloperonospora, which systemically infect their hosts and in effect develop endosymbiotically .
Because of the close phylogenetic relationship revealed by molecular studies and the similarity in the effector genes between the hemibiotrophic Phytophthora and the obligate biotrophic downy mildews, the question then whether some missing links between these two groups still exist. Recent molecular phylogenetic studies –, , revealed that the graminicolous downy mildews appear to be sister clades to the monophyletic major clades of the downy mildews, which contain around 95 % of all species so far described (i.e. brassicolous downy mildews – Hyaloperonospora, Perofascia; downy mildews with coloured conidia – Peronospora, Pseudoperonospora; downy mildews with pyriform haustoria – Basidiophora, Benua, Bremia, Novotelnova, Paraperonospora, Plasmopara, Plasmoverna, Protobremia). However, in none of the analyses to date did these apparent sister relationships receive high bootstrap support and therefore this molecular evidence must be treated cautiously. If the before mentioned artefact in phylogenetic reconstruction ,  is taken into account, then the affinities of the graminicolous species to the other clades of the downy mildews or to Phytophthora remains unclear. Göker et al.  concluded that the evolution of the brassicolous downy mildew genera Hyaloperonospora and Perofascia had started from hosts in the Poaceae and Thines et al.  also noted that the possibility that downy mildew evolution in general might have started in the Poaceae cannot excluded. The basic question arising from this molecular work, addressed in this study, is whether there are any characteristics in the graminicolous downy mildews that are reflecting the close relationship of downy mildews and Phytophthora and the rather basal position of the graminicolous downy mildews that was revealed by recent molecular phylogenetic investigations.
Results and Discussion
In light microscopy (LM), the apophyses, which are typical for Phytophthora, can also be observed in Viennotia (Fig. 1). In scanning electron microscopy (SEM), the similarity of the apophyses in Phytophthora and Viennotia becomes even more apparent (Fig. 1). In both species, not only one, but up to three apophyses on an individual ultimate branchlet can be observed (Fig. 2). These apophyses are formed, when, after a sporangium is dispersed, the ultimate branchlet of the sporangiophore grows out again. The process of producing a sporangium, dispersal and outgrowth may repeat several times. In both Phytophthora and Viennotia, sporangiophores with three apophyses have been observed, which means that four times, a sporangium was produced on a single ultimate branchlet. The process of outgrowth and apophyse formation in Viennotia is illustrated in Figure 2. It should be noted that in contrast to Phytophthora, outgrowth takes place almost simultaneously in Viennotia. In both species, however, the growth of the sporangiophore is not terminated by the formation of sporangia.
A: Phytophthora infestans. B: Viennotia oplismeni. Big pictures: Sporangiophores as seen in phase contrast light microscopy. Small pictures: Close-up of the apophyses as seen in scanning electron microscopy. Arrows point to apophyses on the ultimate branchlets, which are typical for Phytophthora, but in downy mildews are only present in V. oplismeni. Bar = 20 µm in the big pictures and 5 µm in the small pictures.
A: Primary sporangiophore tip after sporangium detachment. B–E: Sequence showing the outgrowth from the middle of the site where the sporangium had been detached. F: Secondary sporangiophore tip after sporangium detachment. G: Tertiary sporangiophore tip after sporangium detachment. Bar = 5 µm in all pictures.
The possibility for Phytophthora-like outgrowth in the genus Viennotia is unique for downy mildews. Outgrowth in Viennotia and Phytophthora takes place trough the middle of the scar left after sporangium dispersal and not as previously depicted by de Bary , and subsequently copied by many authors, from a swelling below the pedicel, on which the first formed sporangium may still sit , . Scars on the apophyses that should be present if development was the result of the inflation of branchlets below the original tip could not be observed, although 500 terminal branchlets were scrutinized for each genus. Also in Plasmopara halstedii, abnormal sporulation with outgrowth from a sporangiophore bearing sporangia has been observed on sunflower roots by Novotelnova . Determinate growth of sporangiophores in downy mildews versus indeterminate sporangiophore growth in Phytophthora has been considered the most important characteristic in delimitating downy mildews. Therefore the finding that in Viennotia sporangiophore growth is indeterminate provides important evidence that sharp distinction made between Phytophthora and downy mildews based on morphogenetic criteria is rather artificial.
It is generally assumed that in downy mildews hyphal growth only takes place extracellularly and only the determinate haustoria intrude into host cells, whilst in Phytophthora mycelium may grow through both living and dying cells. Intracellular mycelium is not uncommon in Phytophthora species, (e.g. Phytophthora cinnamomi; 5), although many species of this genus also produces digit-like haustoria. In Poakatesthia penniseti, a rare downy mildew parasite of Pennisetum glaucum, it has been observed that intracellular, callose-covered, hyphae form from haustoria, which may grow through several host cells before they enter the apoplast again . Interestingly, Tippett et al.  have noted constrictions at sites where the intracellular mycelium passes from one cell into another which is similar to the observations made for Poakatesthia . Intracellular mycelium apart from haustoria has so far not been reported from any other downy mildew. However, Sclerophthora, some basal Pseudoperonospora and Peronospora species, as well Peronosclerospora, which general mycelium morphology is much alike Poakatesthia , have not been investigated thoroughly in this respect. The development of an intracellular mycelium from haustoria in Poakatesthia penniseti raises questions regarding the obligate biotrophy of this downy mildew. Unfortunately, as only the type collection is available as a herbarium specimen, this conclusion can not be supported by cultivation experiments.
The finding of Tokura  that Sclerophthora is able to grow on artificial media is not unexpected, as there is a long-standing and ongoing debate –, as to whether this pathogen should actually be considered a member of the genus Phytophthora. In a recent phylogenetic reconstructions, the placement of this genus could not be unambiguously resolved ; though it could be shown unequivocally that it belongs to the Peronosporales and not to the Saprolegniomycetidae, as placed by Dick . That the placement of Sclerophthora is controversial becomes already apparent from its name: “Sclero” is referring to the thick-walled oospores, which are similar to those of the graminicolous downy mildew Sclerospora , , , while “phthora” refers to the Phytophthora-like vegetative mycelium including sporangiophores. The main reasons for the inclusion of Sclerophthora in the downy mildews have been the morphology of the oospores and the determinate sporangiophore growth. However, like in Phytophthora, sporangia do not form synchronously in this genus . The main argument for inclusion of Sclerophthora in Phytophthora has been the similarity in the shape of sporangia und the supporting sporangiophores . However, as the shape of sporangiophores and sporangia can be highly misleading characters, as has become apparent from recent molecular phylogenetic investigations in downy mildews , , , the exclusion of the genus from Phytophthora on grounds of oospore morphology might still be justified. Also the apparently fast and rather long independent evolutionary history, which can be deduced from the long branch for Sclerophthora in Thines et al. , contributes some evidence for this conclusion. Certainly, the rather isolated position of this genus on the phylogenetic trees, suggests it should not be considered to be a member of the genus Phytophthora.
The second downy mildew genus which has been reported to be culturable on artificial media is Sclerospora , which has always been accepted as a distinct genus. Recent phylogenetic reconstructions clearly place Sclerospora amongst the downy mildews ,  and not with the Saprolegniomycetidae as proposed by Dick . The report of cultivation by Tiwari & Arya  was supported by a photograph showing sporangiophores from which new sporangiophores were repeatedly arising. However, the successful cultivation of neither Sclerophthora nor Sclerospora has been reported again and so it is possible that the observations made by Tiwari & Arya  and Tokura  were fortuitous and not readily repeatable. Nevertheless, these results support the obvious assumption that obligate biotrophy has gradually evolved in the basal downy mildews until complete dependence on living host cells was established. Likewise, the synchronous development of sporangia seems to have gradually evolved, as in Sclerophthora the ripening of sporangia is asynchronous , whereas in Viennotia, sporangiogenesis is synchronous.
Therefore, in addition to the LM and SEM observations reported here, a careful examination of the literature also reveals that several characters states are in common for some genera of the graminicolous downy mildews with lasting sporangiophores (Graminivora, Poakatesthia, Viennotia), Sclerophthora and Phytophthora. Taking into account the comparison given in Table 1, it becomes apparent that not a single trait that was thought to separate the downy mildews from Phytophthora is always present in downy mildews and that most characteristics thought to be exclusive to Phytophthora can also be observed in lineages of the graminicolous downy mildews.
The three main paradigms about downy mildew distinction (obligate biotrophy, absence of intracellular mycelium, determinate sporangiophore growth) need to be rethought, as there are downy mildews species which show traits that are at variance with these basic assumptions.
The arguments listed above necessitate a critical revision of the paradigm that downy mildews are much different from Phytophthora. Phytophthora and downy mildews are both phylogenetically and morphologically connected by “bridging taxa” and share an intimate relationship. The downy mildews are therefore to be considered a more specialised sister-taxon to some advanced Phytophthora lineages, as they have evolved from a Phytophthora-like ancestor at a time, when some other basal lineages of Phytophthora s.l. had already parted from the downy mildew-Phytophthora s.str. lineage (which also includes Phytophthora infestans). Whether the “bridging taxa” of the graminicolous downy mildews take an intermediate phylogenetic position between Phytophthora and all other downy mildew clades or whether the plesiomorphic, Phytophthora-like, character states that can be observed in some downy mildews have been lost several times independently can so far not be resolved.
It seems likely that in the enigmatic genus Sclerophthora and in the rare downy mildew species Poakatesthia penniseti and Viennotia oplismeni, several pathogenicity effector genes and developmental genes intermediate to the model organisms in Phytophthora and the downy mildews could be found. Therefore, the investigation of these organisms might shed light on the shift from hemibiotrophy to obligate biotrophy and thereby help to decipher the most basal chapter in downy mildew evolution.
Materials and Methods
Of the extremely rare parasites Poakatesthia penniseti and Viennotia oplismeni only the type collections are available (IMI 137328c and IMI 103944, respectively), which were investigated in cause of this study. The Phytophthora specimen depicted in this study is deposited in the herbarium of the University of Hohenheim (HOH) as HUH 992.
For light microscopy (LM) of sporangia and sporangiophores, small pieces of the samples were transferred to 5% chloral hydrate to restore turgidity. Preparation for scanning electron microscopy was done as previously described .
Thanks are due to Annerose Heller for allowing access to the scanning electron microscope at the Institute of Botany of the University of Hohenheim and to Gordon Beakes, as well as Bilbo and Jana Lehmann for general input on the subject.
Conceived and designed the experiments: MT. Performed the experiments: MT. Analyzed the data: MT. Contributed reagents/materials/analysis tools: MT. Wrote the paper: MT.
- 1. Rizzo DM, Garbelotto M, Davidson JM, Slaughter GW, Koike ST (2002) Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densiflorus in California. Pl Dis 86: 205–214.DM RizzoM. GarbelottoJM DavidsonGW SlaughterST Koike2002Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densiflorus in California.Pl Dis86205214
- 2. Fry WE, Goodwin SB, Matuszak JM, Spielman LJ, Milgroom MG, Drenth A (1992) Population genetics and intercontinental migrations of Phytophthora infestans. Ann Rev Phytopathol 30: 107–129.WE FrySB GoodwinJM MatuszakLJ SpielmanMG MilgroomA. Drenth1992Population genetics and intercontinental migrations of Phytophthora infestans.Ann Rev Phytopathol30107129
- 3. Ristaino JB (2002) Tracking historic migrations of the Irish potato famine pathogen, Phytophthora infestans. Microbes Inf 4: 1369–1377.JB Ristaino2002Tracking historic migrations of the Irish potato famine pathogen, Phytophthora infestans.Microbes Inf413691377
- 4. Fraymouth J (1956) Haustoria of the Peronosporales. Trans Br Mycol Soc 39: 79–107.J. Fraymouth1956Haustoria of the Peronosporales.Trans Br Mycol Soc3979107
- 5. Tippett JT, Holland AA, Marks GC, O'Brien TP (1976) Penetration of Phytophthora cinnamomi into disease tolerant and susceptible eucalypts. Arch Microbiol 108: 231–242.JT TippettAA HollandGC MarksTP O'Brien1976Penetration of Phytophthora cinnamomi into disease tolerant and susceptible eucalypts.Arch Microbiol108231242
- 6. Dick MW, Wong PTW, Clark G (1984) The identity of the oomycete causing ‘Kikuyu Yellows’, with a reclassification of the downy mildews. Bot J Linn Soc 89: 171–197.MW DickPTW WongG. Clark1984The identity of the oomycete causing ‘Kikuyu Yellows’, with a reclassification of the downy mildews.Bot J Linn Soc89171197
- 7. Dick MW (2001) Straminipilous Fungi: Systematics of the Peronosporomycetes Including Accounts of the Marine Straminipilous Protists, the Plasmodiophorids and Similar Organisms. Dordrecht/Boston/London: Kluwer Academic Publishers. MW Dick2001Straminipilous Fungi: Systematics of the Peronosporomycetes Including Accounts of the Marine Straminipilous Protists, the Plasmodiophorids and Similar OrganismsDordrecht/Boston/LondonKluwer Academic Publishers
- 8. Cooke DEL, Drenth A, Duncan JM, Wagels G, Brasier CM (2000) A molecular phylogeny of Phytophthora and related oomycetes. Fungal Gen Biol 30: 17–32.DEL CookeA. DrenthJM DuncanG. WagelsCM Brasier2000A molecular phylogeny of Phytophthora and related oomycetes.Fungal Gen Biol301732
- 9. Riethmüller A, Göker M, Weiß M, Oberwinkler F, Voglmayr H (2002) Phylogenetic relationships of the downy mildews (Peronosporales) and related groups based on nuclear large subunit ribosomal DNA sequences. Mycologia 94: 834–849.A. RiethmüllerM. GökerM. WeißF. OberwinklerH. Voglmayr2002Phylogenetic relationships of the downy mildews (Peronosporales) and related groups based on nuclear large subunit ribosomal DNA sequences.Mycologia94834849
- 10. Göker M, Voglmayr H, Riethmüller A, Oberwinkler F (2007) How do obligate parasites evolve? A multi-gene phylogenetic analysis of downy mildews. Fungal Gen Biol 44: 105–122.M. GökerH. VoglmayrA. RiethmüllerF. Oberwinkler2007How do obligate parasites evolve? A multi-gene phylogenetic analysis of downy mildews.Fungal Gen Biol44105122
- 11. Thines M, Göker M, Oberwinkler F, Spring O (2007) A revision of Plasmopara penniseti, with implications for the host range of the downy mildews with pyriform haustoria (DMPH). Mycol Res 111: 1377–1385.M. ThinesM. GökerF. OberwinklerO. Spring2007A revision of Plasmopara penniseti, with implications for the host range of the downy mildews with pyriform haustoria (DMPH).Mycol Res11113771385
- 12. Thines M, Göker M, Telle S, Ryley M, Mathur K, et al. (2008) Phylogenetic relationships of graminicolous downy mildews based on cox2 sequence data. Mycol Res 112: 345–351.M. ThinesM. GökerS. TelleM. RyleyK. Mathur2008Phylogenetic relationships of graminicolous downy mildews based on cox2 sequence data.Mycol Res112345351
- 13. Felsenstein J (1981) Evolutionary trees from DNA sequences: a maximum likelihood approach. J Mol Evol 17: 368–376.J. Felsenstein1981Evolutionary trees from DNA sequences: a maximum likelihood approach.J Mol Evol17368376
- 14. Göker M, Stamatakis A (2006) Maximum likelihood phylogenetic inference: An empirical comparison on a multi-locus dataset. German Conference on Bioinformatics 2006, Tübingen, Germany.M. GökerA. Stamatakis2006Maximum likelihood phylogenetic inference: An empirical comparison on a multi-locus dataset. German Conference on Bioinformatics 2006, Tübingen, Germany.Poster available online at http://diwww.epfl.ch/~stamatak/index-Dateien/publications/GCB2006_Poster.pdf. Accessed 2008 May 20. Poster available online at http://diwww.epfl.ch/~stamatak/index-Dateien/publications/GCB2006_Poster.pdf. Accessed 2008 May 20.
- 15. Göker M, Voglmayr H, Riethmüller A, Weiß M, Oberwinkler F (2003) Taxonomic aspects of Peronosporaceae inferred from Bayesian molecular phylogenetics. Can J Bot 81: 672–683.M. GökerH. VoglmayrA. RiethmüllerM. WeißF. Oberwinkler2003Taxonomic aspects of Peronosporaceae inferred from Bayesian molecular phylogenetics.Can J Bot81672683
- 16. Armstrong MR, Whisson SC, Pritchard L, Bos JIB, Venter E, et al. (2005) An ancestral oomycete locus contains late blight avirulence gene Avr3a, encoding a protein that is recognized in the host cytoplasm. Proc Natl Acad Sci U S A 102: 7766–7771.MR ArmstrongSC WhissonL. PritchardJIB BosE. Venter2005An ancestral oomycete locus contains late blight avirulence gene Avr3a, encoding a protein that is recognized in the host cytoplasm.Proc Natl Acad Sci U S A10277667771
- 17. Birch PRJ, Rehmany AP, Pritchard L, Kamoun S, Beynon JL (2006) Trafficking arms: Oomycete effectors enter host plant cells. Trends Microbiol 14: 8–11.PRJ BirchAP RehmanyL. PritchardS. KamounJL Beynon2006Trafficking arms: Oomycete effectors enter host plant cells.Trends Microbiol14811
- 18. Morgan W, Kamoun S (2007) RXLR effectors of plant pathogenic oomycetes. Curr Op Microbiol 10: 332–338.W. MorganS. Kamoun2007RXLR effectors of plant pathogenic oomycetes.Curr Op Microbiol10332338
- 19. Whisson SC, Boevink PC, Moleleki L, Avrova AO, Morales JG, et al. (2007) A translocation signal for delivery of oomycete effector proteins into host plant cells. Nature 450: 115–118.SC WhissonPC BoevinkL. MolelekiAO AvrovaJG Morales2007A translocation signal for delivery of oomycete effector proteins into host plant cells.Nature450115118
- 20. Sasabe M, Takeuchi K, Kamoun S, Ichinose Y, Govers F, et al. (2000) Independent pathways leading to apoptotic cell death, oxidative burst and defense gene expression in response to elicitin in tobacco cell suspension culture. Europ J Biochem 267: 5005–5013.M. SasabeK. TakeuchiS. KamounY. IchinoseF. Govers2000Independent pathways leading to apoptotic cell death, oxidative burst and defense gene expression in response to elicitin in tobacco cell suspension culture.Europ J Biochem26750055013
- 21. Kamoun S (2006) A catalogue of the effector secretome of plant pathogenic oomycetes. Ann Rev Phytopathol 44: 41–60.S. Kamoun2006A catalogue of the effector secretome of plant pathogenic oomycetes.Ann Rev Phytopathol444160
- 22. Greenshields DL, Jones JDG (2008) Plant Pathogen Effectors: Getting Mixed Messages. Curr Biol 18: R128–R130.DL GreenshieldsJDG Jones2008Plant Pathogen Effectors: Getting Mixed Messages.Curr Biol18R128R130
- 23. Qutob D, Hraber PT, Sobral BWS, Gijzen M (2002) Comparative analysis of expressed sequences in Phytophthora sojae. Pl Physiol 123: 243–253.D. QutobPT HraberBWS SobralM. Gijzen2002Comparative analysis of expressed sequences in Phytophthora sojae.Pl Physiol123243253
- 24. Parniske M (2000) Intracellular accommodation of microbes by plants: A common developmental program for symbiosis and disease? Curr Op Pl Biol 3: 320–328.M. Parniske2000Intracellular accommodation of microbes by plants: A common developmental program for symbiosis and disease?Curr Op Pl Biol3320328
- 25. Thines M, Göker M, Spring O, Oberwinkler F (2006) A revision of Bremia graminicola. Mycol Res 110: 646–656.M. ThinesM. GökerO. SpringF. Oberwinkler2006A revision of Bremia graminicola.Mycol Res110646656
- 26. de Bary A (1863) Recherches sur le développement de quelques champignons parasites. Ann Sci Nat (Bot) 20: 5–148.A. de Bary1863Recherches sur le développement de quelques champignons parasites.Ann Sci Nat (Bot)205148
- 27. Tavel F (1892) Vergleichende Morphologie der Pilze. Jena: Gustav Fischer Verlag. F. Tavel1892Vergleichende Morphologie der PilzeJenaGustav Fischer Verlag
- 28. Shaw CG (1978) Peronosclerospora species and other downy mildews of the Gramineae. Mycologia 70: 594–604.CG Shaw1978Peronosclerospora species and other downy mildews of the Gramineae.Mycologia70594604
- 29. Novotelnova (1966) Muchnistaya rosa posolnechnika [Downy mildew of sinflower]. Moskwa, Russia: Nauka. Novotelnova1966Muchnistaya rosa posolnechnika [Downy mildew of sinflower]Moskwa, RussiaNauka
- 30. Ryley MJ (2001) The location and activity of hyphae of the downy mildew, Peronosclerospora noblei (Family Peronosporaceae), and its relationship to symptom expression in wild sorghum (Sorghum leiocladum). Austral J Bot 49: 487–492.MJ Ryley2001The location and activity of hyphae of the downy mildew, Peronosclerospora noblei (Family Peronosporaceae), and its relationship to symptom expression in wild sorghum (Sorghum leiocladum).Austral J Bot49487492
- 31. Tokura R (1975) Axenic or artificial culture of the downy mildew fungi of gramineous plants. Trop Agr Res Ser 8: 57–60.R. Tokura1975Axenic or artificial culture of the downy mildew fungi of gramineous plants.Trop Agr Res Ser85760
- 32. Saccardo PA (1890) Fungi aliquot australiensis. Hedwigia 29: 154–156.PA Saccardo1890Fungi aliquot australiensis.Hedwigia29154156
- 33. Tanaka I (1940) Phytophthora macrospora S. Ito & I. Tanaka on wheat plant. Ann Phytopathol Soc Jap 10: 127–138.I. Tanaka1940Phytophthora macrospora S. Ito & I. Tanaka on wheat plant.Ann Phytopathol Soc Jap10127138[in Japanese, with an English summary]. [in Japanese, with an English summary].
- 34. Thirumalachar MJ, Shaw CG, Narasimhan MJ (1953) The sporangial phase of the downy mildew on Eleusine coracana with adiscussion of the identity of Sclerospora macrospora Sacc. Bull Torrey Bot Club 80: 299–307.MJ ThirumalacharCG ShawMJ Narasimhan1953The sporangial phase of the downy mildew on Eleusine coracana with adiscussion of the identity of Sclerospora macrospora Sacc.Bull Torrey Bot Club80299307
- 35. Waterhouse GM (1963) Key to the species of Phytophthora de Bary. C.M.I. Mycological Paper, no. 92. Surrey, England, UK: C.M.I., Kew. GM Waterhouse1963Key to the species of Phytophthora de Bary. C.M.I. Mycological Paper, no. 92Surrey, England, UKC.M.I., Kew
- 36. Kenneth RG (1981) Downy mildews of graminaceous crops. In: Spencer DM, editor. The Downy Mildews. London, England, UK: Academic Press. pp. 367–394.RG Kenneth1981Downy mildews of graminaceous crops.DM SpencerThe Downy MildewsLondon, England, UKAcademic Press367394
- 37. Shaw CG (1981) Taxonomy and evolution. In: Spencer DM, editor. The Downy Mildews. London, U.K.: Academic Press. pp. 17–29.CG Shaw1981Taxonomy and evolution.DM SpencerThe Downy MildewsLondon, U.K.Academic Press1729
- 38. Erwin DC, Ribeiro OK (1996) Phytophthora Diseases Worldwide. St. Paul, MN, U.S.A.: American Phytopathological Society Press. DC ErwinOK Ribeiro1996Phytophthora Diseases WorldwideSt. Paul, MN, U.S.A.American Phytopathological Society Press
- 39. Payak MM, Renfro BL (1967) A new downy mildew disease of maize. Phytopathol 57: 394–397.MM PayakBL Renfro1967A new downy mildew disease of maize.Phytopathol57394397
- 40. Tiwari MM, Arya HC (1969) Sclerospora graminicola - axenic culture. Science 163: 291–292.MM TiwariHC Arya1969Sclerospora graminicola - axenic culture.Science163291292
- 41. Thines M (2006) Evaluation of characters available from herbarium vouchers for the phylogeny of the downy mildew genera (Chromista, Peronosporales), with focus on scanning electron microscopy. Mycotaxon 97: 195–218.M. Thines2006Evaluation of characters available from herbarium vouchers for the phylogeny of the downy mildew genera (Chromista, Peronosporales), with focus on scanning electron microscopy.Mycotaxon97195218