sphaerotheca Fuliginea/Sandbox
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| Podosphaera xanthii | |
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| Species: | P. xanthii |
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| Podosphaera xanthii (Castagne) U. Braun & Shishkoff, (2000) | |
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Podosphaera xanthii (formerly Sphaerotheca fuliginea, among other names) is fungus that acts as a plant pathogen. P. xanthii is the leading cause of cucurbit powdery mildew, an infection of cucurbit plants including cucumbers and melons. It can also infect some other plants, including verbena.[citation needed]
Taxonomy
The taxonomy of this species in biological nomenclature has frequently changed from 1819 to the late 20th century. As recently as the 1980s P. xanthii had been considered a synonymous with, variously Sphaerotheca phaseoli (or S. astragali var. phaseoli), S. cucurbitae, or S. fuliginea (as well as Euoidium erysiphoides), with Podosphaera and Sphaerotheca principally distinguished "by having appendages which are branched and simple, respectively". However, based on rDNA evidence, these genera were merged in 2000–2001, and the P. xanthii and S. fuliginea species, along with P. phaseoli, were consequently merged, in full combinatio nova form as Podosphaera (sect. Sphaerotheca) xanthii.
Since 2008, some have suggested merging P. xanthii into P. fusca, and they have been labelled synonymous, with P. fusca being the senior name. Other sources, relying on the 2000–2001 rDNA work, consider them separate species in the Magnicellulatae subsection of the Sphaerotheca section of Podosphaera, as of 2011[update], based on the size of chasmothecia, and on the thin-walled portion of the asci (oculus).
Races
There are multiple races of the fungus, identified simply as race 1, race 2, etc. (seven in total by 2004), for races found around the world, and an additional race N1 through N4 for some divergent races native to Japan. Various subraces have been identified, and given names such as race 2U.S., race 3.5, and race 4.5. A new race S was discovered in 2003, and melon cultivars are being backcrossed to increase resistance to it.
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.
Classification and taxonomy of these fungal races can be difficult, because host plants' responses to particular populations of fungi can be affected by humidity, light, temperature, and other environmental factors; different host plants may not all respond to particular fungal populations or vice versa; and genetic differences between populations thought to form distinct fungal races can be elusive.
Signs and symptoms
Powdery mildew is manifest on the plant by white powdery fungal growth on the surface of the leaf; usually both sides of the leaf show fungal growth. The host tissue is frequently stunted, distorted, discolored, and scarred. The fruit of infected plants are usually smaller and the flavor is affected negatively, as fewer sugars and solids are stored in the fruit.
Disease cycle
P. xanthii uses haustoria to gain access to the leaf epidermal cells. The fungus is usually spread during the spring through mycelium from infected plant, or through ascocarps. Signs appear after three to seven days of infection, if conditions are favorable for the fungus. The mycelium grows rapidly during the warm summer months, with an optimum temperature of about 50–90 degrees F. The leaves are most susceptible 16–23 days after unfolding. High humidity favors the development of disease, but infection can occur at relative humidity as low a 50%. The conidia of the fungus are spread through the air and thus can travel over great distances. The mycelium can also overwinter in the buds of infected plants.
Control
The most common way to control the spread of P. xanthii is with the use of fungicides. Usually sulfur-based or demethylation-inhibitor fungicides are applied. Fungicides are usually applied once a week. Plants are also kept physically separated to control spread because older plants can be a source of conidia.
Cucurbit powdery mildew
Since 1925, commercial Cucumis melo (cantaloup and muskmelon) production has been engaged in a biological "arms race" against the disease cucurbit powdery mildew (CPM) caused by P. xanthii in particular, with new cultivars of melons being developed for resistance to successively arising races of the fungus.
External links
- Species Fungorum database; Index Fungorum project of Landcare Research and Kew Royal Botanic Gardens
- U.S. National Fungus Collections Fungal Database; Agricultural Research Service of the Department of Agriculture
References
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