August 31, 2005
From: Anchorage Daily News, 18 Aug 2005 [edited]
<http://www.adn.com/news/alaska/story/6829509p-6724776c.html>

Matanuska-Susitna area potato farmers are fighting an outbreak of "late blight" for the 3rd time in the past decade. Potato late blight (PLB) can cause devastation to plants including stored potatoes, tomatoes, eggplant and hot peppers. First discovered in the Mat-Su in 1995 and again in 1998, it has not been seen since until last week when it was spotted in a local farmer's field, according to state Agriculture Division Director Larry DeVilbiss.

So far, parts of 3 commercial farms in the Valley have been affected and plants from a half-dozen Palmer-area backyard gardens are also being tested for PLB, he said. One commercial grower has blight in more than half his field and 2 other growers have only small pickup-size spots of infected plants. The PLB fungus is a common problem in the lower 48 states, where farmers spray fungicide for disease control. But it's rare in Alaska, where the state's cold winters and relative isolation help prevent its spread.

The fungus thrives in cool, wet weather and is spread by wind and water. Infected potatoes produces spores that spread to other plants. It can also survive in potatoes and re-emerge if replanted. PLB can also cause stored
potatoes to rot.

The spores can spread quickly -- traveling as far as 80 miles per day, wiping out entire fields in less than a week, said Jenifer Huang McBeath, a Fairbanks- based plant pathology and biotechnology professor who inspected 2 of the 3 infected fields in Mat-Su.

Potatoes bring in about $2 million a year for Alaska growers, making them a small part of farming in the state. But they are one of the largest field crops, DeVilbiss said. Seed potatoes are also one of Alaska's few farm exports.

Fall is the peak growing season, with potatoes doubling their weight in the next 2 weeks, DeVilbiss said. That makes farmers reluctant to harvest the potatoes now, one way to keep the disease from spreading, he said. Despite that, at least one farmer plans to do just that, and other growers plan to, or already have sprayed their plants with fungicide.

While farmers can spray from the ground, DeVilbiss has asked the state Department of Environmental Conservation for permission for aerial spraying. The chemical is on a national list of approved fungicides, but farmers need the agency's approval for aerial spraying, said Kim Stricklan, DEC's pesticide program manager.

PLB is not indigenous to Alaska, so growers suspect it was imported. In 1998, McBeath traced the outbreak to seed potatoes from Canada although some dispute that claim, she said.

Besides harvesting plants immediately, McBeath recommended that growers who suspect plants are infected cut off the plant above the ground. The potatoes will still grow, but it will reduce the chances of disease spread, she said. Potatoes suspected of being infected should not be replanted, she said.

[Byline: S.J. Komarnitsky]

--
ProMED-mail
<promed@promedmail.org>

[Potato blight caused by the fungal pathogen _Phytophthora infestans_ [Pi] has been a problem for over 150 years. Many approaches have been developed to control it, but the fungus still poses a major threat. It has evolved to overcome most of the measures that were introduced over the years.

Several management strategies can be deployed

Control of Pi has traditionally relied on copper-based fungicides such as Bordeaux mixture, but copper is potentially phytotoxic, so disease forecasting was developed to enable growers to predict when the environmental conditions were highly conducive to spread of Pi. Forecasting methods for PLB differ in different countries, but in Britain they are based on the "temperature- humidity rule" devised by Beaumont. Copper is a broad-spectrum protectant fungicide which must be applied to prevent disease. It has been superseded by modern systemic fungicides, which move within the plant and can both protect and eradicate existing infections. The acylalanine fungicides such as metalaxyl and furalaxyl act specifically on the RNA polymerase of Pi and closely related fungi. However, resistance to these chemicals can develop quickly in the pathogen population -- it requires only a single gene mutation leading to a minor change in the RNA polymerase molecule. In many parts of the world, Pi is now resistant to these fungicides.

Destruction of foliage

If Pi becomes established on potato foliage, sporangia can be washed down into the soil to infect tubers, or tubers can be contaminated with sporangia during crop harvesting, thus leading to rotting of the tubers during storage, and carry-over of inoculum from one season to the next. Destruction of the foliage (the haulm) with sulphuric acid or herbicide 2-3 weeks before the tubers are lifted is a recommended strategy.

Resistance breeding

The cultivated potato (_Solanum tuberosum_) originated from the Andean region of South America, where there are several other species of the genus _Solanum_. _Solanum demissum_ proved to be an important source of resistance, and by conventional plant breeding this resistance was bred into commercial potato cultivars. 4 major resistant genes (R genes) were discovered and were introduced successively into commercial cultivars.

However, within a few years of each R gene being widely introduced into potato cultivars, resistance was overcome by new strains (termed physiologic races) of Pi that developed in response to selection pressure imposed by the specific R genes. Many plant breeders now prefer to develop cultivars that have "polygenic" or "field resistance" to the pathogen. Such plants have combinations of several "minor" genes, none of which gives absolute resistance, but together they slow the rate of development of the fungus and enable the plant to tolerate infection.

Emergence of new pathogenic strains through sexual crossing

Like many members of the _Oomycota_, Pi has 2 mating types (A1 and A2). It can undergo sexual reproduction only if both mating types are present in a population. Both mating types occur in Mexico, near the centre of origin of _Solanum_ and thus presumably of its pathogen _P. infestans_. In Europe only the A1 strain was [originally] known to occur -- presumably because it was introduced by chance on potatoes imported from elsewhere. The A2 mating type was discovered in Britain in 1978 and is now common in much of continental Europe.

Links: <http://www.kodiakdailymirror.com/?pid=18&path=A/AK_POTATO_BLIGHT>
<http://helios.bto.ed.ac.uk/bto/microbes/blight.htm>
<http://www.uidaho.edu/ag/plantdisease/lbhome.htm>
- Mod.DH]

[see also in the archive:
Potato late blight - UK (England, Wales) 20050630.1846
2004
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Phytophthora sp., root and foot rot, tomato - Belgium 20040123.0266 Potato late blight - Ireland: 1st report 2004 20040602.1499 Potato late blight - USA (Idaho) 20040729.2071 Potato late blight, tomato - USA (multistate) 20040817.2279 2003
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Late blight, potato - Papua New Guinea 20030306.0554
2002
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Potato late blight, potato - Canada (Newfoundland) 20020818.5091 2001
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Phytophthora infestans, potato late blight - Russia 20010620.1177 Phytophthora infestans, potato late blight - Russia 20010430.0840 2000
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Potato late blight, etiology: Correction 20000330.0473
Potato late blight, etiology 20000325.0432
Potato late blight: global initiative 20000516.0765
Potato late blight, global research efforts 20001031.1903
1997
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Potato late blight: integrated management 19970321.0595
Potato late blight: global initiative 19970117.0092]