Downy mildew

For the band, see Downy Mildew (band).

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.

Downy mildew
Example of downy mildew (left) along with powdery mildew on a grape leaf
Causal agents	oomycetes
Hosts	plants

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 (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]

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.^[5] In Canada, a mixture of zoxamide and mancozeb was registered for control of the mildew under the trademark Gavel (fungicide) as early as 2008.^[6]

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.^[7]

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,^[8] was reported in the U.S. in 2007 and 2008^[9][10] 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.^[11]

Soybeans

Peronospora manshurica infects soybeans, reducing photosynthetic activity, yield, and quality.^[12] 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.^[13] The disease is often controlled using the fungicides mancozeb, maneb, or zineb.^[12]

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.^[14]

Sunflowers

Plasmopara halstedii infects sunflowers, producing oospores which can remain dormant in the soil for many years.^[15]

See also

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

References

1. NCBI-Taxonomy – ncbi.nlm.nih.gov
2. HYP3 – ncbi.nlm.nih.gov
3. Schilder, Annemiek. Downy mildew - Plasmopara viticola. June 11, 2010, at the Wayback Machine MSU Plant Pathology.
4. Richard Latin, Karen Rane, "Pumpkin Diseases" (PDF), Department of Botany and Plant Pathology, purdue.edu
5. (PDF). New Active Ingredient Review. Minnesota Department of Agriculture. October 2015. Archived from the original (PDF) on 2017-11-07. Retrieved 2017-11-09.
6. grainews.ca: "Gowan buys Dow’s Gavel potato fungicide", 18 Jul 2008
7. https://www.johnnyseeds.com/on/demandware.static/-/Library-Sites-JSSSharedLibrary/default/dwc8916fa0/assets/information/7648-basil-downy-mildew-prevention.pdf ^{[bare URL PDF]}
8. 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.
9. 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.
10. 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.
11. 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.
12. 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.
13. Sinclair, James Burton; Backman, P. A. (1989). Compendium of Soybean Diseases (3rd ed.). St Paul, MN: APS Press. ISBN 9780890540930.
14. 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.
15. 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)