Powdery mildew

Powdery mildew

Example of powdery mildew (right) along with Downy mildew on a grape leaf
Causal agents Species of fungi in the orders Erysiphales
Hosts plants

Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales, with Podosphaera xanthii (a.k.a. Sphaerotheca fuliginea) being the most commonly reported cause.[1] Erysiphe cichoracearum was formerly reported to be the primary causal organism throughout most of the world.[1][2] Powdery mildew is one of the easier plant diseases to identify, as its symptoms are quite distinctive. Infected plants display white powdery spots on the leaves and stems. The lower leaves are the most affected, but the mildew can appear on any above-ground part of the plant. As the disease progresses, the spots get larger and denser as large numbers of asexual spores are formed, and the mildew may spread up and down the length of the plant.

Powdery mildew grows well in environments with high humidity and moderate temperatures.[3] In an agricultural setting, the pathogen can be controlled using chemical methods, genetic resistance, and careful farming methods. It is important to be aware of powdery mildew and its management as the resulting disease can significantly reduce crop yields.[4] Greenhouses provide an ideal moist, temperate environment for the spread of the disease.

Powdery mildew growing on a leaf (magnified).
Powdery mildew growing on a leaf in high magnification.

Reproduction

Powdery mildew fungi reproduce both sexually and asexually. Sexual reproduction is via chasmothecia (formerly cleistothecium), a type of ascocarp. Within each ascocarp are several asci. Over time, ascospores mature and are released to initiate new infections. Conditions necessary for spore maturation differ among species.

Vectors of transmission

Wooly aphids (Eriosomatinae) and other sucking insects are often vectors of transmission for powdery mildew, and other infectious diseases. Typically wooly aphids in sub temperate climates precede and are an indicator of various infections, including Powdery mildew. Aphids penetrate plant surfaces where they often reside and provide a host of potential inoculants through physical, digestive or fecal secretions. Aphids are often an indicator of other potential plant problems.

Management

In an agricultural setting, the pathogen can be controlled using chemical methods, genetic resistance, and careful farming methods.

Chemical control is possible with fungicides such as triadimefon and propiconazole.

Another chemical treatment involves treating with a silicon solution or calcium silicate slag. Silicon helps the plant cells defend against fungal attack by degrading haustoria and by producing callose and papilla. With silicon treatment, epidermal cells are less susceptible to powdery mildew of wheat.[5]

Potassium bicarbonate is an effective fungicide against powdery mildew and apple scab, allowed for use in organic farming.[6][7][8][9]

Milk has long been popular with home gardeners and small-scale organic growers as a treatment for powdery mildew. Milk is diluted with water (typically 1:10) and sprayed on susceptible plants at the first sign of infection, or as a preventative measure, with repeated weekly application often controlling or eliminating the disease. Studies have shown milk's effectiveness as comparable to some conventional fungicides,[10] and better than benomyl and fenarimol at higher concentrations.[11] Milk has proven effective in treating powdery mildew of summer squash,[11] pumpkins,[10] grapes,[12] and roses.[12] The exact mechanism of action is unknown, but one known effect is that ferroglobulin, a protein in whey, produces oxygen radicals when exposed to sunlight, and contact with these radicals is damaging to the fungus.[12]

Powdery mildews of various plants

Wheat, barley and other cereals

Blumeria graminis f. sp. tritici, causes powdery mildew of wheat, whereas f. sp. hordei causes powdery mildew of barley.

Legumes

Legumes, such as soybeans, are affected by Microsphaera diffusa.[13]

Powdery mildew in Soyabean leaves.

Grape

Powdery mildew of grape

Erysiphe necator (or Uncinula necator) causes powdery mildew of grapes.

Onions

The fungus causing powdery mildew of onions is Leveillula taurica (also known by its anamorph name, Oidiopsis taurica). It also infects the artichoke.

Apples and pears

Podosphaera leucotricha is a fungus that can cause powdery mildew of apples and pears.

Gourds and melons

Powdery mildew of cucurbits

Multiple species of fungus can cause powdery mildew of cucurbits: cucumbers, squashes (including pumpkins), luffas, melons, and watermelons.

Since 1925, commercial Cucumis melo (cantaloup and muskmelon) production has been engaged in a biological "arms race" against cucurbit powdery mildew (CPM) caused by the fungus Podosphaera xanthii, with new cultivars of melons being developed for resistance to successively arising races of the fungus, identified simply as race 1, race 2, etc. (seven in total by 2004), for races found around the world, and race N1 through N4 for some divergent races native to Japan.[14] Various subraces have been identified, and given names such as race 2U.S., race 3.5, and race 4.5.[15] A new race S was discovered in 2003, and a specific melon cultivar (C. melo var. acidulus 'PI 313970') found resistant to it, then used for backcrossing to increase resistance in other cultivars.[15] Such modern selective breeding of plants for phytopathological resistance to particular fungal races involves a great deal of genetic research; this PI 313970 versus race S case involved multi-stage hybridization to propagate a recessive gene, pm-S in successive generations, and how this may affect other recessive and codominant genes for resistance to other races of P. xanthii "remains to be determined".[15]

A 2004 literature review regarding powdery mildew races that parasitize various cucurbit plants concluded that "race identification is important for basic research and is especially important for the commercial seed industry, which requires accuracy in declaring the type and level of resistance ... in its products". However, identifying specific races was seen as having little utility in horticulture for choosing specific cultivars, because of the rapidity with which the local pathogen population can change geographically, seasonally, and by host plant.[14]

At least three other Erysiphaceae fungi can cause powdery mildew in cucurbits: The most frequent, after P. xanthii, is Erysiphe cichoracearum, the former primary causal organism throughout most of the world.[1][2] Podosphaera fusca is another, sometimes considered synonymous with P. xanthii.[16] Cucumbers in greenhouse environments have also been reported to be susceptible to Leveillula taurica.[17]

Lilacs

Microsphaera syringae is a fungus that can cause powdery mildew in lilac.[18]

Strawberries

Podosphaera aphanis is the cause of powdery mildew in strawberries and other Rosaceae like Geum rivale (the water avens)

Tree leaves

Sawadaea tulasnei is a fungus that causes powdery mildew on tree leaves. This fungus attacks the leaves of the Acer platanoides (Norway maple) in North America, Great Britain, and Ireland, Acer palmatum (also known as the Japanese maple or smooth Japanese maple).[19]

Oregon grape

Erysiphe berberidis is a fungus that causes powdery mildew on Oregon grape leaves. [20]

Arabidopsis

Golovinomyces orontii causes powdery mildew on Arabidopsis (rockcress) leaves.

Hyperparasites of powdery mildew

In the family Sphaeropsidaceae of Sphaeropsidales fungi, species of the genus Cicinnobolus are hyperparasites of powdery mildew.[21]

Ampelomyces quisqualis is an anamorphic fungus that is a hyperparasite of powdery mildews. This parasitism reduces growth and may eventually kill the mildew, so investigations on biological control of powdery mildews (especially in high-value crops such as grapes) have taken place for over 50 years, resulting in the development of products such as 'AQ10'.[22][23]

See also

Wikibooks' A Wikimanual of Gardening has more about this subject:

References

  1. 1 2 3 McGrath, M.T., 1997. Powdery Mildew of Cucurbits. http://vegetablemdonline.ppath.cornell.edu/factsheets/Cucurbits_PM.htm
  2. 1 2 Tetteh, A, et al. Watermelon Crop Information. http://cuke.hort.ncsu.edu/cucurbit/wmelon/wmhndbk/wmpm.html
  3. ↑ Huang, X.Q. et al. (2000). Molecular mapping of the wheat powdery mildew resistance gene Pm24 and marker validation for molecular breeding. Theoretical and Applied Genetics, 101. Retrieved from http://www.springerlink.com/content/engc84epbg6feqvk/fulltext.pdf.
  4. ↑ Maloy, Otis and Debra Inglis (1993) Powdery Mildew, Washington University extension, Diseases of Washington Crops. Retrieved from http://pnw-ag.wsu.edu/smallgrains/Powdery%20Mildew.html
  5. ↑ Belanger, R. r. et al. (April 2003). Cytological Evidence of an Active Role of Silicon in Wheat Resistance to Powdery Mildew (Blumeria graminis f. sp. tritici). Phytopathology, 93. Retrieved from http://www.siliforce.com/pdf/7c/Belanger-%20%20evedence%20silicon%20powdery%20mildew%20on%20wheat.pdf.
  6. ↑ http://attra.ncat.org/attra-pub/bakingsoda.html
  7. ↑ Powdery Mildew - Sustainable Gardening Australia
  8. ↑ Organic Fruit Production in Michigan
  9. ↑ Tamm, Lucius; Amsler, Thomas; Schaerer, Hansjakob; Refardt, Mathias (2006). "Efficacy of Armicarb (potassium bicarbonate) against scab and sooty blotch on apples". In Boos, Markus. Ecofruit: 12th International Conference on Cultivation Technique and Phytopathological Problems in Organic Fruit-growing (PDF). pp. 87–92 http://orgprints.org/8075/1/LT_HJS_Armicarb_7.4.06.pdf. Retrieved 10 August 2015. Missing or empty |title= (help)
  10. 1 2 DeBacco, Matthew. "Compost Tea and Milk to Suppress Powdery Mildew (Podosphaera xanthii) on Pumpkins and Evaluation of Horticultural Pots Made from Recyclable Fibers Under Field Conditions". University of Connecticut. Retrieved 5 May 2013.
  11. 1 2 Bettiol, Wagner (September 1999). "Effectiveness of cow's milk against zucchini squash powdery mildew (Sphaerotheca fuliginea) in greenhouse conditions". Crop Protection. 18 (8): 489–492. doi:10.1016/s0261-2194(99)00046-0.
  12. 1 2 3 Raloff, Janet. "A Dairy Solution to Mildew Woes". Science News Magazine. Retrieved 5 May 2013.
  13. ↑ Bennett, J. Michael; Rhetoric, Emeritus; Hicks, Dale R.; Naeve, Seth L.; Bennett, Nancy Bush (2014). The Minnesota Soybean Field Book (PDF). St Paul, MN: University of Minnesota Extension. p. 85. Retrieved 21 February 2016.
  14. 1 2 Cohen, R.; Burger, Y.; Katzir, N. (2004). "Monitoring Physiological races of Podosphaera xanthii (syn. Sphaerotheca fuliginea), the Causal Agent of Powdery Mildew in Curcubits: Factors Affecting Race Identification and the Importance for Research and Commerce". Phythoparasitica. 32 (2): 174–183. doi:10.1007/BF02979784. Retrieved 10 August 2015.
  15. 1 2 3 McCreight, James D.; Coffey, Michael D. (June 2011). "Inheritance of Resistance in Melon PI 313970 to Cucurbit Powdery Mildew Incited by Podosphaera xanthii Race S". HortScience. 46 (6): 838–840. Retrieved 10 August 2015.
  16. ↑ Pérez-García, A.; Romero, D.; Fernández-Ortuño, D.; López-Ruiz, F.; De Vicente, A.; Torés Montosa, Juan Antonio (March 2009). "The powdery mildew fungus Podosphaera fusca (synonym Podosphaera xanthii), a constant threat to cucurbits". Molecular Plant Pathology. 10 (2): 153–160. doi:10.1111/j.1364-3703.2008.00527.x. First published online 9 December 2008, doi:10.1111/j.1364-3703.2008.00527.x.
  17. ↑ Velkov, Nikolay; Masheva, Stoika (2002). "Species and Races Composition of Powerdy Mildew on Cucurbits in Bulgaria" (PDF). Cucurbit Genetics Cooperative Report. 25: 7–10. Retrieved 10 August 2015.
  18. ↑ http://www.hfrr.ksu.edu/DesktopModules/ViewDocument.aspx?DocumentID=1662
  19. ↑ Sawadaea tulasnei (Fuckel) Homma 1937 - Encyclopedia of Life
  20. ↑ Pacific Northwest Plant Disease Management Handbook
  21. ↑ faculty.ucr.edu (retrieved December 2015)
  22. ↑ http://www.rebeca-net.de/downloads/Ampelomyces%20AQ10%20Sergio%20Franceschini.pdf
  23. ↑ Powdery mildew in the Pesticide Properties DataBase (PPDB)
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