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Downy mildew
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== Symptoms == [[File:Peronospora variabilis on Lamb's Quarters - Chenopodium album (45372554682).jpg|thumb|[[Conidiophores]] of ''[[Peronospora variabilis]]'' on the underside of a leaf of ''[[Chenopodium album]]''. ]] Initial symptoms include large, angular or blocky, yellow areas visible on the upper surface.<ref>Schilder, Annemiek. [http://grapes.msu.edu/downymildew.htm Downy mildew - Plasmopara viticola.] {{webarchive |url=https://web.archive.org/web/20100611230101/http://grapes.msu.edu/downymildew.htm |date=June 11, 2010 }} ''MSU Plant Pathology.''</ref> They can also be distinguished by their sporadic yellow patch appearance. As lesions mature, they expand rapidly and turn brown. The under surface of infected leaves appears watersoaked. Upon closer inspection, a purple-brown mold ([[conidiophores]]) becomes apparent. Small spores shaped like footballs can be observed among the mold with a 10x hand lens. As a result of numerous infectious sites, leaves might show a blighted appearance if the disease continues to spread. In disease-favorable conditions (cool nights with long dew periods), downy mildew will spread rapidly, destroying leaf tissue without affecting stems or petioles.<ref name="purdue1">{{citation|author=Richard Latin, Karen Rane |title=Pumpkin Diseases |work=Department of Botany and Plant Pathology |publisher=purdue.edu |url=http://www.extension.purdue.edu/extmedia/BP/BP-17/BP-17.pdf}}</ref> === Floricolous species === [[File:Peronospora violacea comparison.jpg|thumb|Flowerheads of ''[[Succisa pratensis]]'', infected (right) and uninfected (left) with the floricolous downy mildew ''[[Peronospora violacea]]''.]] One monophyletic clade of ''[[Peronospora]]'', known as the ''floricolous downy mildews'', only cause symptoms in the flowers of their hosts.<ref name="Thines">{{cite journal | vauthors=((Thines, M.)), ((Kummer, V.)) | journal=Mycological Progress | title=Diversity and species boundaries in floricolous downy mildews | volume=12 | issue=2 | pages=321–329 | date=1 May 2013 | issn=1861-8952 | doi=10.1007/s11557-012-0837-7| bibcode=2013MycPr..12..321T }}</ref> Infection is systemic,<ref name="Thines"></ref> with [[hyphae]] produced at low densities in the host xylem.<ref name="Contribution">{{cite journal | vauthors=((Horáková, J.)), ((Skalický, V.)) | journal=Česká Mykologie | title=Contribution to the ecology of Peronospora violacea Berk. | volume=43 | issue=1 | pages=13–29 | date= 1989}}</ref> This clade is known from hostplants in the [[Asteraceae]], [[Campanulaceae]], [[Dipsacaceae]], [[Lamiaceae]], and [[Orobanchaceae]].<ref name="Thines"></ref> One floricolous species, ''[[Peronospora violacea]]'', causes its host ''[[Succisa pratensis]]'' to produce longer, pinker petals,<ref name="Field Guide">{{cite web | vauthors=((Dalzell, J.)) | year=2024 | title=Pathogens in Grassland | website=Field Guide to Plant Pathogens | url=https://plantpathogens.neocities.org/habitats/grassland | access-date=19 December 2024}}</ref> and abort its [[anthers]].<ref name="Contribution"></ref> Other authors report that on the same host it causes the host [[petals|corollas]] to be brownish and dead-looking.<ref>{{cite book | vauthors=((Chater, A. O.)), ((Woods, R. G.)), ((Stringer, R. N.)), ((Evans, D. A.)), ((Smith, P. A.)) | date= 2020 | title=Downy Mildews (Peronosporaceae) and White Blister-Rusts (Albuginaceae) of Wales = Llwydni Gwlannog (Peronosporaceae) a Rhydau-Pothelli Gwynion (Albuginaceae) Cymru | publisher=A.O. Chater | isbn=978-0-9565750-4-3}}</ref>
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