Downy mildew

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Downy mildew
Downy and Powdery mildew on grape leaf.JPG
Example of downy mildew (left) along with powdery mildew on a grape leaf
Causal agents oomycetes
Hostsplants

Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to the Peronosporaceae family. In commercial agriculture, they are a particular problem for growers of crucifers, grapes and vegetables that grow on vines. The prime example is Peronospora farinosa featured in NCBI-Taxonomy [1] and HYP3. [2] 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 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.

Contents

Symptoms

Initial symptoms include large, angular or blocky, yellow areas visible on the upper surface. [3] 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; see arrow) 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. [4]

Floricolous species

Flowerheads of Succisa pratensis, infected (right) and uninfected (left) with the floricolous downy mildew Peronospora violacea. Peronospora violacea comparison.jpg
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. [5] Infection is systemic, [5] with hyphae produced at low densities in the host xylem. [6] This clade is known from hostplants in the Asteraceae, Campanulaceae, Dipsacaceae, Lamiaceae, and Orobanchaceae. [5]

One floricolous species, Peronospora violacea , causes its host Succisa pratensis to produce longer, pinker petals, [7] and abort its anthers. [6] Other authors report that on the same host it causes the host corollas to be brownish and dead-looking. [8]

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. [4] 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 cucurbit downy mildew, but are recommended only after diagnosis of this disease has been confirmed. [4] In the United States, the Environmental Protection Agency has approved oxathiapiprolin for use against downy mildew. [9] In Canada, a mixture of zoxamide and mancozeb was registered for control of the mildew under the trademark Gavel (fungicide) as early as 2008. [10]

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. [11]

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, [12] was reported in the U.S. in 2007 and 2008 [13] [14] 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, 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 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. [15]

Soybeans

Peronospora manshurica infects soybeans, reducing photosynthetic activity, yield, and quality. [16] 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. [17] The disease is often controlled using the fungicides mancozeb, maneb, or zineb. [16]

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. [18]

Sunflowers

Plasmopara halstedii infects sunflowers, producing oospores which can remain dormant in the soil for many years. [19]

See also

Related Research Articles

<span class="mw-page-title-main">White onion</span> Onion cultivar

White onion or Allium cepa are a cultivar of dry onion which have a distinct light and mild flavour profile. Much like red onions, they have a high sugar and low sulphur content, and thus have a relatively short shelf life. White onions are used in a variety of dishes, such as those of Mexican and European origin. Their uses in dishes often relate to their mild nature, they are often included in dishes to provide a light, fresh and sour taste to dishes and are often added uncooked to dishes such as salads.

<span class="mw-page-title-main">Peronosporaceae</span> Family of water moulds

Peronosporaceae are a family of water moulds that contains 21 genera, comprising more than 600 species. Most of them are called downy mildews.

<i>Hyaloperonospora brassicae</i> Species of single-celled organism

Hyaloperonospora brassicae, in the family Peronosporaceae, is a plant pathogen. It causes downy mildew of species of Brassica, Raphanus, Sinapis and probably other genera within the Brassicaceae. In the past, the cause of downy mildew in any plant in the family Brassicaceae was considered to be a single species Peronospora parasitica. However, this has recently been shown to be a complex of species with narrower host ranges, now classified in the genus Hyaloperonospora, for example Hyaloperonospora parasitica on the weed Capsella bursa-pastoris. From the perspective of plant pathology, Hyaloperonospora brassicae is now the name of the most important pathogen in this complex, attacking the major agricultural and horticultural Brassica species. Other significant Brassicaceous hosts are attacked by different species in the complex, e.g. horseradish by Hyaloperonospora cochleariae, wallflower by Hyaloperonospora cheiranthi.

<i>Hyaloperonospora parasitica</i> Species of plant pathogen

Hyaloperonospora parasitica is an oomycete from the family Peronosporaceae. It has been considered for a long time to cause downy mildew of a variety of species within the Brassicaceae, on which the disease can cause economically important damage by killing seedlings or affecting the quality of produce intended for freezing. Hyaloperonospora parasitica causes downy mildew on a wide range of many different plants. It belongs to the Kingdom Chromista, the phylum Oomycota, and the family Peronosporaceae. The former name for H. parasitica was Peronospora parasitica until it was reclassified and put in the genus Hyaloperonospora. It is an especially vicious disease on crops of the family Brassicaceae. It is most famous for being a model pathogen of Arabidopsis thaliana which is a model organism used for experimental purposes. Accordingly, the former Hyaloperonospora parasitica has been split into a large number of species. For instance, the taxonomically correct name of the parasite of the well-known model organism Arabidopsis thaliana is Hyaloperonospora arabidopsidis, not H. parasitica, whereas the pathogen of Brassica has to be called Hyaloperonospora brassicae.

<i>Peronospora sparsa</i> Species of single-celled organism

Peronospora sparsa is an oomycete plant pathogen that causes downy mildew in berry producing plants; especially in the genus's Rubus and Rosa. Downy mildew plant pathogens are often host specific and cause problems in cloudberries, blackberries, boysenberries, strawberries, and arctic bramble. Since they are host specific, Peronospora sparsa will not cause downy mildew in grapes because a different plant pathogen causes downy mildew in grapes; Plasmopara viticola. Although it depends on the cultivar, symptoms do not normally start until later stages of disease and can look different on different plants. The most common symptoms include red lesions in the veins of leaves, with dry and deformed berries.

Sclerophthora macrospora is a protist plant pathogen of the class Oomycota. It causes downy mildew on a vast number of cereal crops including oats, rice, maize, and wheat as well as varieties of turf grass. The common names of the diseases associated with Sclerophthora macrospora include "crazy top disease" on maize and yellow tuft disease on turf grass. The disease is present all over the world, but it is especially persistent in Europe.

<span class="mw-page-title-main">Peronospora farinosa</span> Species of single-celled organism

Peronospora farinosa is a species name that has been widely applied to downy mildew on leaves of wild and cultivated Amaranthaceae: Amaranthus, Atriplex, Bassia, Beta, Chenopodium, Halimione, Salsola, Spinacia, etc. However, the species name has been taxonomically rejected as the original description contained reference to multiple species and could not unequivocally be attributed to a species of Peronospora. In the past, some of the species on important crop plants have been given names as formae speciales, notably f.sp. betae on sugar beet and f.sp. spinaciae on spinach. However, phylogentic reconstructions have revealed that these "forms" of Peronospora on different genera and their subdivisions, are distinct species, most of which already have previously published scientific names. Such host specialization possibly also exists with respect to the various wild amaranthaceous species given as hosts of P. farinosa.

<i>Peronospora manshurica</i> Species of single-celled organism

Peronospora manshurica is a plant pathogen. It is a widespread disease on the leaves of soybeans and other crop plants. The fungi is commonly referred to as downy mildew, "leafspot", or "leaf-spot".

<i>Plasmopara halstedii</i> Species of single-celled organism

Plasmopara halstedii is a plant pathogen infecting sunflowers. The species is one of many pathogens commonly referred to as downy mildew. P. halstedii originated in North America.

Plasmopara obducens is a species of oomycete that causes Impatiens downy mildew. It was first described on Impatiens noli-tangere in Germany in 1877. Plasmopara obducens is known from native species of Impatiens since the 1800s, but outbreaks on cultivated varieties of Impatiens walleriana started in 2003 in the United Kingdom and in 2004 in the United States. The outbreak spread worldwide by 2016.

<i>Plasmopara viticola</i> Species of single-celled organism

Plasmopara viticola, the causal agent of grapevine downy mildew, is a heterothallic oomycete that overwinters as oospores in leaf litter and soil. In the spring, oospores germinate to produce macrosporangia, which under wet condition release zoospores. Zoospores are splashed by rain into the canopy, where they swim to and infect through stomata. After 7–10 days, yellow lesions appear on foliage. During favorable weather the lesions sporulate and new secondary infections occur.

Pseudoperonospora cannabina is a plant pathogen that causes downy mildew, which is a fungal-like disease caused by an oomycete.

<i>Pseudoperonospora humuli</i> Species of single-celled organism

Pseudoperonospora humuli is a plant pathogen that causes downy mildew on hops.

Peronosclerospora sorghi is a plant pathogen. It is the causal agent of sorghum downy mildew. The pathogen is a fungal-like protist in the oomycota, or water mold, class. Peronosclerospora sorghi infects susceptible plants though sexual oospores, which survive in the soil, and asexual sporangia which are disseminated by wind. Symptoms of sorghum downy mildew include chlorosis, shredding of leaves, and death. Peronosclerospora sorghi infects maize and sorghum around the world, but causes the most severe yield reductions in Africa. The disease is controlled mainly through genetic resistance, chemical control, crop rotation, and strategic timing of planting.

<i>Melothria scabra</i> Species of flowering plant

Melothria scabra, commonly known as the cucamelon, Mexican miniature watermelon, Mexican sour cucumber, Mexican sour gherkin, mouse melon, or pepquinos, is a species of flowering plant in the cucurbit family grown for its edible fruit. Its native range spans Mexico to Venezuela. Fruits are about the size of grapes and taste like cucumbers with a tinge of sourness. It may have been eaten by indigenous peoples before the European colonization of the Americas began.

<i>Pseudoperonospora cubensis</i> Species of single-celled organism

Pseudoperonospora cubensis is a species of water mould known for causing downy mildew on cucurbits such as cantaloupe, cucumber, pumpkin, squash and watermelon. This water mould is an important pathogen of all these crops, especially in areas with high humidity and rainfall, such as the eastern United States. In most years the disease is an annual, late-season problem on squash and pumpkin in the eastern and central United States, however, since 2004 it has become one of the most important diseases in cucumber production. Considered a highly destructive foliar disease of cucurbits, successful breeding in the mid-twentieth century provided adequate control of downy mildew in cucumber without the use of fungicides. The resurgence in virulence has caused growers great concern and substantial economic losses, while downy mildew in other cucurbit crops continues to be a yearly hindrance.

<i>Peronospora</i> Genus of plant pathogen

Peronospora is a genus of oomycetes that are obligate plant pathogens of many eudicots. Most species in this group produce a downy mildew disease, which can cause severe damage to many different cultivated crops, as well as wild and ornamental plants. There are 19 genera that produce downy mildew, and Peronospora has been placed alongside Pseudoperonospora in the group of downy mildews with coloured conidia. Peronospora has far more species than any other genus of the downy mildews. However, many species have been moved from this genus to be reclassified to other or new genera. Among these was the most famous Peronospora species, formerly known as Peronospora parasitica, and now known as Hyaloperonospora parasitica. Now, the Peronospora species of most importance is likely the Peronospora tabacina. Peronospora tabacina causes blue mold on tobacco plants and can severely reduce yields of this economically important crop to the point where it has been classified as a bioweapon.

This article summarizes different crops, what common fungal problems they have, and how fungicide should be used in order to mitigate damage and crop loss. This page also covers how specific fungal infections affect crops present in the United States.

<i>Peronospora destructor</i> Species of single-celled organism

Peronospora destructor is a plant pathogen. It causes downy mildew on leaves of cultivated and wild Allium. Allium cepa is most often affected, while Allium schoenoprasum (chives) and Allium porrum (leek) are only occasionally affected.

<i>Peronospora violacea</i> Downy mildew

Peronospora violacea is a floricolous downy mildew which infects plants in the Caprifoliaceae. It has been reported from hosts in the genera Dipsacus, Knautia, Lomelosia, Scabiosa, and Succisa.

References

  1. ↑ NCBI-Taxonomy – ncbi.nlm.nih.gov
  2. ↑ HYP3 – ncbi.nlm.nih.gov
  3. ↑ Schilder, Annemiek. Downy mildew - Plasmopara viticola. Archived June 11, 2010, at the Wayback Machine MSU Plant Pathology.
  4. 1 2 3 Richard Latin, Karen Rane, "Pumpkin Diseases" (PDF), Department of Botany and Plant Pathology, purdue.edu
  5. 1 2 3 Thines, M., Kummer, V. (1 May 2013). "Diversity and species boundaries in floricolous downy mildews". Mycological Progress. 12 (2): 321–329. doi:10.1007/s11557-012-0837-7. ISSN   1861-8952.
  6. 1 2 Horáková, J., Skalický, V. (1989). "Contribution to the ecology of Peronospora violacea Berk". Česká Mykologie. 43 (1): 13–29.
  7. ↑ Dalzell, J. (2024). "Pathogens in Grassland". Field Guide to Plant Pathogens. Retrieved 19 December 2024.
  8. ↑ Chater, A. O., Woods, R. G., Stringer, R. N., Evans, D. A., Smith, P. A. (2020). Downy Mildews (Peronosporaceae) and White Blister-Rusts (Albuginaceae) of Wales = Llwydni Gwlannog (Peronosporaceae) a Rhydau-Pothelli Gwynion (Albuginaceae) Cymru. A.O. Chater. ISBN   978-0-9565750-4-3.
  9. ↑ "Oxathiapiprolin" (PDF). New Active Ingredient Review. Minnesota Department of Agriculture. October 2015. Archived from the original (PDF) on 2017-11-07. Retrieved 2017-11-09.
  10. ↑ grainews.ca: "Gowan buys Dow’s Gavel potato fungicide", 18 Jul 2008
  11. ↑ https://www.johnnyseeds.com/on/demandware.static/-/Library-Sites-JSSSharedLibrary/default/dwc8916fa0/assets/information/7648-basil-downy-mildew-prevention.pdf [ bare URL PDF ]
  12. ↑ 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.
  13. ↑ 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.
  14. ↑ 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.
  15. ↑ Breese, Wendy A; Shattock, R C; Williamson, B; Hackett, Christine (August 1994). "In vitro spore germination and infection of cultivars of Rubus and Rosa by downy mildews from both hosts". Annals of Applied Biology. 125 (1): 73–85. doi:10.1111/j.1744-7348.1994.tb04948.x.
  16. 1 2 Shanmugasundaram, S.; Masuda, Ryoichi; Tsou, S.C.S.; Hong, T.L. (1991). Vegetable Soybean Research Needs for Production and Quality Improvement (PDF). Taipei: Asian Vegetable Research and Development Center. pp. 86–87. ISBN   9789290580478 . Retrieved 6 February 2016.
  17. ↑ Sinclair, James Burton; Backman, P. A. (1989). Compendium of Soybean Diseases (3rd ed.). St Paul, MN: APS Press. ISBN   9780890540930.
  18. ↑ Kandel, Shyam L.; Mou, Beiquan; Shishkoff, Nina; Shi, Ainong; Subbarao, Krishna V.; Klosterman, Steven J. (2019). "Spinach Downy Mildew: Advances in Our Understanding of the Disease Cycle and Prospects for Disease Management". Plant Disease. 103 (5). American Phytopathological Society: 791–803. doi: 10.1094/PDIS-10-18-1720-FE . PMID   30939071.
  19. ↑ Friskop, Andrew; Markell, Sam; Gulya, Tom (2009). "Downy Mildew of Sunflower" (PDF). Sunflower Publications. Fargo, North Dakota: North Dakota State University Extension Service. Retrieved 4 March 2016.{{cite book}}: |website= ignored (help)
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