downy mildew

File:Peronospora variabilis on Lamb's Quarters - Chenopodium album (45372554682).jpg 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.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. 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.{{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}}

File:Peronospora violacea comparison.jpg, 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.{{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 }} Infection is systemic, with hyphae produced at low densities in the host xylem.{{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}} This clade is known from hostplants in the Asteraceae, Campanulaceae, Dipsacaceae, Lamiaceae, and Orobanchaceae.

One floricolous species, Peronospora violacea, causes its host Succisa pratensis to produce longer, pinker petals,{{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}} and abort its anthers. Other authors report that on the same host it causes the host corollas to be brownish and dead-looking.{{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}}

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. When downy mildew does pose a threat, the removal and destruction of plants displaying symptoms is good practice.

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 cucurbit downy mildew, but are recommended only after diagnosis of this disease has been confirmed. In the United States, the Environmental Protection Agency has approved oxathiapiprolin for use against downy mildew.{{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 }} In Canada, a mixture of zoxamide and mancozeb was registered for control of the mildew under the trademark Gavel (fungicide) as early as 2008.[https://www.grainews.ca/daily/gowan-buys-dows-gavel-potato-fungicide grainews.ca: "Gowan buys Dow’s Gavel potato fungicide"], 18 Jul 2008

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.

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.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}}

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,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. was reported in the U.S. in 2007 and 2008Roberts, 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.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. 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 downy mildew (caused by Pseudoperonospora cubensis) is specific to cucurbits (e.g., cantaloupe (Cucumis melo), cucumber (Cucumis sativus), pumpkin, squash, watermelon (Citrullus lanatus) and other members of the gourd family). The disease is one of the most significant diseases of cucurbits worldwide.

Plasmopara viticola is the causal agent of grapevine downy mildew.

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 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.

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.{{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 }}

Peronospora manshurica infects soybeans, reducing photosynthetic activity, yield, and quality.{{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}} The fungus spreads by oospores on diseased leaves and/or on infected seed. The disease spreads in environments with high humidity and favors temperatures between 20-22 °C. Tufts of grayish to pale-colored sporangiophores on the underside of leaves easily distinguish the infection from other foliar diseases.{{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}} The disease is often controlled using the fungicides mancozeb, maneb, or zineb.

Downy mildew on spinach is caused by Peronospora effusa, an oomycete pathogen that poses a challenge to spinach production worldwide, especially in organic production.{{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 }}

Plasmopara halstedii infects sunflowers, producing oospores which can remain dormant in the soil for many years.{{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}}

• Blue mold (of tobacco plants)
• Peronosporaceae (with a list of the downy mildew genera)

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Category:Water mould plant pathogens and diseases

Category:Fungus common names