- Phytophthora infestans
name = "Phytophthora infestans"
classis = Oomycetes
genus = "
species = "P. infestans"
binomial = "Phytophthora infestans"
binomial_authority = (Mont.) de Bary
"Phytophthora infestans" is an
oomycetethat causes the serious potatodisease known as late blight or potato blight. ( Early blight, caused by " Alternaria solani", is also often called 'potato blight'). It was a major culprit in the 1845 Irish and 1846 Highland potato famines. The organism can also infect tomatoes and some other members of the Solanaceae. It is currently being remedied by genetic engineering, taking a resistance gene from the plant " Solanum bulbocastanum" and introducing it into the genome of cultivated varieties of the potato.
spores of this water moldoverwinter on infected tubers, particularly those that are left in the ground after the previous year's harvest, in cull piles, soil or infected volunteer plants and are spread rapidly in warm and wet conditions.Paul A. Koepsell and Jay W. Pscheidt. "1994 Pacific Northwest Plant Disease Control Handbook," Oregon State University Press (1994): p 165.] This can have devastating effects by destroying entire crops.
Spores develop on the leaves, spreading through the crop when temperatures are above 10C (50°F) and humidity is over 75% for 2 days or more.
Raincan wash spores into the soilwhere they infect young tubers, or else spores can be blown in from miles away by the wind.
The early stages of blight are easily missed, and not all plants are affected at once. Symptoms include the appearance of dark blotches on leaf tips and plant stems. White mould will appear under the leaves in humid conditions and the whole plant may quickly collapse. Infected tubers develop grey or dark patches that are reddish brown beneath the skin, and quickly decay to a foul-smelling mush caused by the infestation of secondary soft bacterial rots. Seemingly healthy tubers may rot later when in store.
Up until the 1970s, there was only one type of blight (A1) in the UK, and this was unable to produce resistant spores that could survive the winter. There are now two types (A1 and A2) which can mate and after that produce resistant spores, although the indications so far are that this rarely, if ever, happens in the UK. Mating can occur only between moulds of different mating-types and is required for the production of resistant spores.
"P. infestans" is still a difficult disease to control today by ordinary methods. There are many options in
agriculturefor the control of both damage to the foliageand infections of the tuber. Potatoes fill throughout the season, but it is estimated the assimilates stop going to the tubers (they stop growing) when 75% of the canopyis destroyed. [Assessment of Plant Diseases and Losses by W C James. Annual Review of Phytopathology Vol. 12 p 27-48 (1974)] This must also be taken into account when growing potatoes, as it means that plants grown do not have to be 100% resistant to blight.
In recent years, a resistance gene effective against all known strains of blight has been identified and successfully copied from a wild relative of the potato, "Solanum bulbocastanum", and introduced into the genome of cultivated varieties of the potato. [ [http://www.pnas.org/cgi/content/full/100/16/9128 Gene RB cloned from Solanum bulbocastanum confers broad spectrum resistance to potato late blight, Junqi Song et al, PNAS 2003] ]
ources of inoculum
Blight can be controlled by limiting the source of
inoculum. Only good quality seeds obtained from certifiedsuppliers should be planted. Often discarded potatoes from the previous seasonand self-sown tuberscan act as sources of inoculum. [ MJ Zwankhuizen, F Govers, JC Zadoks "Development of potato late blight epidemics: Disease foci, disease gradients, and infection sources" Phytopathology, Volume 88, 1998]
There are several environmental conditions that are
conduciveto "P. infestans". By using weather forecastingsystems, such as BLITECAST, if the following conditions occur as the canopyof the crop closes, then the use of fungicides is recommended to prevent an epidemic. [Scheduling fungicide applications for potato late blight with Blitecast by DR MacKenzie. Plant Disease Volume 65 pages 394-399 (1981]
*A Beaumont Period is a period of 48 consecutive hours, in at least 46 of which the hourly readings of
temperatureand relative humidityat a given place have not been less than 20°C (68°F) and 75%, respectively. [ [http://www.eumetcal.org/euromet/glossary/beaumont.htm Beaumont period ] ]
*A Smith Period is at least two
consecutivedays where min temperature is 10°C (50°F) or above and on each day at least 11 hours when the relative humidity is greater than 90%. [http://www.potatocrop.com/blight/blightaboutsmith.asp]
Potato varieties vary in their susceptibility to blight. Most early varieties are very vulnerable; so that the crop matures before blight starts (usually in July) plant them early. Many old crop varieties, such as
King Edward potatoare also very suceptible but are grown because they are wanted commercially. Maincrop varieties which are very slow to develop blight include Cara, Stirling, Teena, Torridon, Remarka and Romano. Some so-called resistant varieties can resist some strains of the blight and not others, so their performance may vary depending on which are around. These crops tend to have had polygenic resistancebred into them, and are known as field resistant. New varieties such as Sarpo Mira and Sarpo Axona show great resistance to blight even in areas of heavy infestation. These varieties are likely to gain great popularity as consumers increasingly embrace organically produced crops and reject food items that have been grown using fungicides and other chemicals.
Use of fungicides
Fungicides for the control of potato blight are normally only used in a preventative manner, perhaps in conjunctionwith disease forecasting. In susceptible varieties, sometimes fungicide applications may be needed weekly. An early spray is most effective. Metalaxylwas a fungicide that was marketed for use against "P. infestans", but suffered serious resistance issues when used on its own.
Control of tuber blight
Ridging is often used to reduce tuber contamination by blight. This normally involves piling
soilor mulcharound the stems of the potato blight meaning the pathogenhas farther to travel to get to the tuber. [J. R. Glass, K. B. Johnson, and M. L. Powelson, "Assessment of Barriers to Prevent the Development of Potato Tuber Blight". 2001. Plant Disease Volume 85.]
The canopy can also be destroyed around 2 weeks before
harvest. This can be done via a contact herbicideor using sulfuric acidto burn off the foliage.
The effects of "Phytophthora infestans" in Ireland in 1845-57 were one of the factors which caused over one million to starve to death and forced another two million to emigrate from affected countries. Most commonly referenced is the
Great Irish Famine, during the late 1840s.
The origin of "Phytophthora infestans" can be traced to a valley in the highlands of central Mexico. The first recorded instances of the disease were in the United States, in Philadelphia and New York City in early 1843. Winds then spread the spores, and in 1845 it was found from Illinois to Nova Scotia, and from Virginia to Ontario. The fungus crossed the
Atlantic Oceanwith a shipment of seed potatoes destined for Belgian farmers in 1845. [cite news |first=Reader |last=John |authorlink= |coauthors= |title=The Fungus That Conquered Europe |url=http://www.nytimes.com/2008/03/17/opinion/17reader.html |quote= |publisher= New York Times|date= March 17, 2008|accessdate=2008-03-18 ]
*Erwin, Donald C. and Olaf K. Ribeiro. "Phytophthora Diseases Worldwide", American Phytopathological Society (1996).
*Lucas, J.A. (editor), R. C. Shattock (editor), D. S. Shaw (editor), Louise Cooke (editor). "Phytophthora (British Mycological Society Symposia)", Cambridge University Press (1991)
*Govers, Francine and Mark Gijzens. Phytophthora Genomics: The Plant-Destroyer's Genome Decoded, "Molecular Plant-Microbe Interactions" 19(12):1295-1301 (December 2006).
* [http://www.btny.purdue.edu/USDA-ARS/Goodwin_lab/results/Phytoph_biblio.html Online Phytophtora bibliography]
* [http://www.barc.usda.gov/psi/vl/lateblight.htm USDA-BARC Phytophthora infestans page]
* [http://www.tigr.org/tdb/e2k1/pima1/index.shtml Phytophthora infestans Mitochondrial Genome Project]
* [http://www.cipotato.org International Potato Center]
* [http://www.pfgd.org/ Phytophthora Functional Genomics Database]
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