Editing Downy mildew

{{Short description|Name for several species of oomycete}}
{{for|the band|Downy Mildew (band)}}
{{infobox plant disease
| name = Downy mildew
| image = Downy and Powdery mildew on grape leaf.JPG
| caption = Example of downy mildew (left) along with [[powdery mildew]] on a grape leaf
| common_names = 
| hosts = plants
| causal_agents = [[oomycete]]s
| vectors = 
| EPPO_codes = 
| distribution = 
| treatment = 
}}
'''Downy mildew''' refers to any of several types of [[oomycete]] microbes that are [[obligate parasite]]s of plants. Downy mildews exclusively belong to the [[Peronosporaceae]] family. In [[commerce|commercial]] [[agriculture]], they are a particular problem for growers of [[Cruciferae|crucifer]]s, [[grape]]s and [[vegetable]]s that grow on vines. The prime example is ''[[Peronospora farinosa]]'' featured in NCBI-Taxonomy<ref>[https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=134698 NCBI-Taxonomy] – ncbi.nlm.nih.gov</ref> and HYP3.<ref>[http://www.inra.fr/Internet/Produits/HYP3/pathogene/6perfar.htm HYP3] – ncbi.nlm.nih.gov</ref> This pathogen does not produce survival structures in the northern states of the United States, and overwinters as live mildew colonies in Gulf Coast states. It progresses northward with [[Cucurbitaceae|cucurbit]] production each spring. Yield loss associated with downy mildew is most likely related to soft rots that occur after plant canopies collapse and sunburn occurs on fruit. Cucurbit downy mildew only affects leaves of cucurbit plants.

== 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>

== Treatment and management ==

=== Cultural options ===
Because the downy mildew pathogen does not overwinter in midwestern fields, crop rotations and tillage practices do not affect disease development. The pathogen tends to become established in late summer. Therefore, planting early season varieties may further reduce the already minor threat posed by downy mildew.<ref name="purdue1"/> When downy mildew does pose a threat, the removal and destruction of plants displaying symptoms is good practice. 

=== Chemical control ===
Fungicides applied specifically for downy mildew control may be unnecessary. Broad spectrum protectant fungicides such as [[chlorothalonil]], [[mancozeb]], and fixed [[copper]] are at least somewhat effective in protecting against downy mildew infection. Systemic fungicides are labeled for use against [[Cucurbitaceae|cucurbit]] downy mildew, but are recommended only after diagnosis of this disease has been confirmed.<ref name="purdue1"/> In the United States, the [[United States Environmental Protection Agency|Environmental Protection Agency]] has approved [[oxathiapiprolin]] for use against downy mildew.<ref>{{cite web | url = http://www.mda.state.mn.us/chemicals/pesticides/regs/~/media/Files/chemicals/reviews/nair-oxathiapiprolin.pdf | title = Oxathiapiprolin | website = New Active Ingredient Review | date = October 2015 | publisher = Minnesota Department of Agriculture | access-date = 2017-11-09 | archive-date = 2017-11-07 | archive-url = https://web.archive.org/web/20171107025015/http://www.mda.state.mn.us/chemicals/pesticides/regs/~/media/Files/chemicals/reviews/nair-oxathiapiprolin.pdf | url-status = dead }}</ref> In Canada, a mixture of [[zoxamide]] and mancozeb was registered for control of the mildew under the trademark [[Gavel (fungicide)]] as early as 2008.<ref>[https://www.grainews.ca/daily/gowan-buys-dows-gavel-potato-fungicide grainews.ca: "Gowan buys Dow’s Gavel potato fungicide"], 18 Jul 2008</ref>

=== Organic control ===
One way to control downy mildew is to eliminate moisture and humidity around the impacted plants. Watering from below, such as with a drip system, and improve air circulation through selective pruning. In enclosed environments, like in the house or in a greenhouse, reducing the humidity will help as well.

=== Resistant cultivars ===
Recent breeding efforts have produced resistant basil cultivars. Choosing resistant cultivars, in combination with other prevention and control measures, can help growers with high downy mildew pressure still produce a viable crop.<ref>https://www.johnnyseeds.com/on/demandware.static/-/Library-Sites-JSSSharedLibrary/default/dwc8916fa0/assets/information/7648-basil-downy-mildew-prevention.pdf {{Bare URL PDF|date=March 2022}}</ref>

== Plant-specific mildews ==

===Basil===
Downy mildew of [[basil]] caused by ''[[Peronospora belbahrii]]'' has been a huge problem for both commercial producers and home growers. The disease was first reported in Italy in 2004,<ref>Garibaldi, A., Minuto, A., Minuto, G., Gullino, M.L., 2004. First Report of Downy Mildew on Basil (''Ocimum basilicum'') in Italy. Plant Disease 88, 312-312.</ref> was reported in the U.S. in 2007 and 2008<ref>Roberts, P.D., Raid, R.N., Harmon, P.F., Jordan, S.A., Palmateer, A.J., 2009. First Report of Downy Mildew Caused by a Peronospora sp. on Basil in Florida and the United States. Plant Disease 93, 199-199.</ref><ref>Wick, R.L., Brazee, N.J., 2009. First Report of Downy Mildew Caused by a Peronospora Species on Sweet Basil (Ocimum basilicum) in Massachusetts. Plant Disease 93, 318-318.</ref> and has been steadily increasing in prevalence, distribution, and economic importance since then. The highest disease occurrence in basil leaves occurs at temperatures around 68°F.

===Cucurbitaceae===
[[Cucurbitaceae]] downy mildew (caused by ''[[Pseudoperonospora cubensis]]'') is specific to cucurbits (e.g., cantaloupe (''[[Cucumis melo]]''), cucumber (''[[Cucumis sativus]]''), [[pumpkin]], [[Squash (plant)|squash]], watermelon (''[[Citrullus lanatus]]'') and other members of the gourd family). The disease is one of the most significant diseases of cucurbits worldwide.

===Grapes===
''[[Plasmopara viticola]]'' is the causal agent of grapevine downy mildew.

===Hops===
Hop Downy Mildew (caused by ''[[Pseudoperonospora humuli]]'') is specific to [[hops]] (''Humulus lupulus''). The disease is the single most devastating disease in Western United States hopyards, since the microbe thrives in moist climates. Infected young hop [[bine (botany)|bines]] become stunted with thickened clusters of pale curled leaves. These spikes have a silvery upper surface, while the undersides of leaves become blackened with spores. These dwarfed spikes are called "basal spikes". 'Lateral' or 'terminal' spikes occur further up the vine. An entire hop crop could be devastated in only a few days.

===Ornamentals===
A new and particularly aggressive form of [[impatiens downy mildew]] has recently emerged as a major threat to the cultivation of ornamental [[impatiens]] in the United States, where they are one of the most popular ornamental plants.

''[[Peronospora sparsa]]'' attacks rose bushes (including ''[[Rosa canina]]'') and [[Rubus]] species.<ref>{{cite journal |last1=Breese |first1=Wendy A |last2=Shattock |first2=R C |last3=Williamson |first3=B |last4=Hackett |first4=Christine |title=In vitro spore germination and infection of cultivars of Rubus and Rosa by downy mildews from both hosts |journal=Annals of Applied Biology |date=August 1994 |volume=125 |issue=1 |pages=73–85 |doi=10.1111/j.1744-7348.1994.tb04948.x }}</ref>

===Soybeans===
''[[Peronospora manshurica]]'' infects soybeans, reducing photosynthetic activity, yield, and quality.<ref name="SoyResearch">{{cite book |last1=Shanmugasundaram |first1=S. |last2=Masuda |first2=Ryoichi|last3=Tsou |first3=S.C.S. |last4=Hong|first4=T.L. |title=Vegetable Soybean Research Needs for Production and Quality Improvement |date=1991 |publisher=Asian Vegetable Research and Development Center |location=Taipei|isbn=9789290580478 |pages=86–87 |url=http://pdf.usaid.gov/pdf_docs/PNABK804.pdf|archive-url=https://web.archive.org/web/20160206192009/http://pdf.usaid.gov/pdf_docs/PNABK804.pdf|url-status=dead|archive-date=February 6, 2016|access-date=6 February 2016}}</ref> The fungus spreads by [[oospore]]s on diseased leaves and/or on infected seed. The disease spreads in environments with high humidity and favors temperatures between 20-22&nbsp;°C. Tufts of grayish to pale-colored [[Sporangium|sporangiophore]]s on the underside of leaves easily distinguish the infection from other foliar diseases.<ref>{{cite book |last1=Sinclair |first1=James Burton |last2=Backman|first2=P. A. |title=Compendium of Soybean Diseases |date=1989 |publisher=APS Press |location=St Paul, MN |isbn=9780890540930|edition=3rd |url=https://books.google.com/books?id=rDhJAQAAIAAJ}}</ref> The disease is often controlled using the fungicides [[mancozeb]], [[maneb]], or [[zineb]].<ref name="SoyResearch" />

===Spinach===
Downy mildew on [[spinach]] is caused by ''Peronospora effusa,'' an oomycete pathogen that poses a challenge to spinach production worldwide, especially in organic production.<ref>{{cite journal |title=Spinach Downy Mildew: Advances in Our Understanding of the Disease Cycle and Prospects for Disease Management|journal=Plant Disease |volume=103|issue=5|pages=791–803|publisher=American Phytopathological Society |doi=10.1094/PDIS-10-18-1720-FE|pmid=30939071|year=2019|last1=Kandel|first1=Shyam L.|last2=Mou|first2=Beiquan|last3=Shishkoff|first3=Nina|last4=Shi|first4=Ainong|last5=Subbarao|first5=Krishna V.|last6=Klosterman|first6=Steven J.|doi-access=free|bibcode=2019PlDis.103..791K }}</ref>

===Sunflowers===
''[[Plasmopara halstedii]]'' infects [[sunflower]]s, producing [[oospore]]s which can remain dormant in the soil for many years.<ref>{{cite book |last1=Friskop |first1=Andrew |last2=Markell| first2=Sam |last3=Gulya |first3=Tom |date=2009 |title=Sunflower Publications |chapter=Downy Mildew of Sunflower |chapter-format=PDF |chapter-url=https://www.ag.ndsu.edu/ndipm/publications/sunflower/documents/downy-mildew-of-sunflower |website=IPM Publications |location=Fargo, North Dakota |publisher=North Dakota State University Extension Service |url=https://www.ag.ndsu.edu/ndipm/publications |access-date=4 March 2016}}</ref>

== See also ==
* [[Blue mold]] (of tobacco plants)
* [[Peronosporaceae]] (with a list of the downy mildew genera)

== References ==
{{reflist}}
{{Viticulture}}

{{Authority control}}

[[Category:Water mould plant pathogens and diseases]]
[[Category:Fungus common names]]