OK, after doing a little running around, this is all I could come up with.... It is from
http://en.wikipedia.org/wiki/Powdery_mildew#Disease_cycle , and mostly it pertains to outdoor conditions, however, having a little more understanding might help us combat this issue more effectively.
Triticum sp. (wheat) is the only host of
Blumeria graminis f. sp. tritici.[SUP]
[2][/SUP] Signs on the foliage of wheat are white, powdery mycelium and conidia.[SUP]
[4][/SUP] As the disease progresses, the patches turn gray and small dark black or brown cleistothecia form in the mycelium mass.[SUP]
[5][/SUP] Symptoms progress from lower to upper leaves. Symptoms of powdery mildew are chlorotic areas surrounding the infected areas.[SUP]
[4][/SUP] The lower leaf surface corresponding to the mycelial mat will also show chlorosis.[SUP]
[5][/SUP] Lower leaves are commonly the most infected because of higher humidity around them.[SUP]
[2][/SUP]
[h=4][
edit] Disease cycle[/h]
Blumeria graminis f. sp. tritici has a polycyclic life cycle typical of its phylum, Ascomycota. Powdery mildew of wheat overwinters as cleistothecia dormant in plant debris. Under warmer conditions, however, the fungus can overwinter as asexual conidia or mycelium on living host plants. It can persist between seasons most likely as
ascospores in wheat debris left in the field. Ascospores are sexual spores produced from the cleistothecia. These spores, as well as conidia, serve as the primary inoculum and are dispersed by wind. Neither spore requires free water to germinate, only high relative humidity.[SUP]
[5][/SUP] Wheat powdery mildew thrives in cool humid conditions and cloudy weather increases chances of disease. When conidia land on a wheat leaf’s hydrophobic surface cuticle, they release proteins which facilitate active transport of lightweight anions between leaf and fungus even before germination. This process helps Blumeria recognize that it is on the correct host and directs growth of the germ tube.[SUP]
[6][/SUP] Both ascospores and conidia germinate directly with a germ tube. Conidia can recognize the host plant and within one minute of initial contact, the direction of germ tube growth is determined. The development of appressoria then begins infection following the growth of a germ tube.[SUP]
[7][/SUP] After initial infection, the fungus produces haustoria inside of the wheat cells and mycelium grows on the plant’s outer surface.[SUP]
[5][/SUP] Powdery mildew of wheat produces conidia during the growing season as often as every 7 to 10 days.[SUP]
[8][/SUP] These conidia function as secondary inoculum as growth and reproduction repeat throughout the growing season.
[h=4][
edit] Environment[/h]Powdery mildew of wheat thrives in cool, humid climates and proliferates in cloudy weather conditions.[SUP]
[1][/SUP] The pathogen can also be an issue in drier climates if wheat fields are irrigated.[SUP]
[9][/SUP] Ideal temperatures for growth and reproduction of the pathogen are between 60 and 70 degrees Fahrenheit with growth ceasing above 77 degrees Fahrenheit. Dense, genetically similar plantings provide opportune conditions for growth of powdery mildew.[SUP]
[5][/SUP]
[h=4][
edit] Management[/h]Controlling the disease involves eliminating conducive conditions as much as possible by altering planting density and carefully timing applications and rates of
nitrogen. Since nitrogen
fertilizers encourage dense leafy growth, nitrogen should be applied at precise rates, less than 70 pounds per acre, to control decrease severity. Crop rotation with non-host plants is another way to keep mildew infection to a minimum, however the aerial nature of
conidia and
ascospore dispersal makes it of limited use. Wheat powdery mildew can also be controlled by eliminating the presence of volunteer wheat in agricultural fields as well as tilling under crop residues.[SUP]
[8][/SUP]
Chemical control is possible with fungicides such as
triademefon and
propiconazole. Another chemical treatment involves treating wheat 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.[SUP]
[10][/SUP]
The most effective way to control wheat powdery mildew is through genetic resistance, using
R genes (resistance genes) to prevent infection. There are at least 25 loci on the wheat genome that encode resistance to powdery mildew. If the particular variety of wheat has only one loci for resistance, the pathogen may be controlled only for a couple years. If, however, the variety of wheat has multiple loci for resistance, the crop may be protected for around 15 years. Because finding these loci can be difficult and time consuming, molecular markers are used to facilitate combining resistant genomes.[SUP]
[1][/SUP] One organization working towards identifying these molecular markers is the Coordinated Agricultural Project for Wheat. With these markers established, researchers will then be able to determine the most effective combination of resistance genes.[SUP]
[11][/SUP]
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