Potato Blight

I'm not Irish, and I don't like potatoes except as "chips," but St. Patrick's Day causes the gardener in me to pause and reflect on a terrible crop failure: the 1845-46 potato blight that caused famine in Ireland. Lest we forget that food and social systems are inextricably intertwined, recall that the arrival of late blight (Phytophthora infestans) pushed half a million Irish out of their nation.

Half a million food refugees; that's equal to about 5/6 of the population of the state of Alaska on the move, trying to find a way to feed themselves.

Our food supply may not be threatened by disease, but the high cost of oil is driving up food production and transportation. Prices are rising. A reasonable response is to grow food locally.

But back to potatoes. Like the Irish, we can grow lots of potatoes. According to the Alaska Potato Profile, Revised in 2007, the commercial potato crop averaged 8000 tons per year over the last ten years, and added over $3 million annually to Alaska's economy.

So what about this blight problem? We have had late potato blight in Alaska. There is an unofficial report of blight in Southeast Alaska in the 1950s, but the presence of blight in Alaska was not formally documented until 1995. Here is the brief history of the Alaskan experience with blight as recorded in Late Blight Disease of Potato and Tomato in Alaska:

Late blight was seen in the Matanuska Valley in 1995, 1998, and 2005. The risk of late blight is similar each summer, but many years have had no outbreaks. In September 1995, late blight was found in one field of potatoes. This discovery was made late in the season.... In 1998 and 2005 seasons, late blight appears in mid-August, and cool wet weather allowed the disease to develop quickly. By September, late blight was seen throughout the Matanuska Valley. In 2005, late blight was seen in some gardens in Anchorage. Late blight was not found in Interior Alaska or on the Kenai Peninsula.

Blight is incredibly infectious. That's why it wrecked such havoc first in Europe and then in Ireland. Winds spread the airborne disease spores. But so do people. In an article published today in the New York Times (The Fungus That Conquered Europe), the author reports that the fungus actually comes from a valley in the highlands of Central Mexico; and remarkably, that the first recorded instance of the disease was not in Europe, not in Ireland, but in the United States - near Philadelphia and New York in 1843. By 1845, late blight had destroyed potato crops from Illinois to Nova Scotia, from Virginia to Ontario. But then, fatally for Europe, it cross the Atlantic with a shipment of seed potatoes ordered by Belgium farmers, hoping to invigorate their crops. Opps. Instead, by mid-October 1945, the fungus spread throughout Europe and the British Isles, wrecking havoc.

The European experience with blight as well as the more recent Alaskan experience is the reason why there are restrictions on the seed potatoes that you can order. All seed potatoes or potato plants sold in or imported into Alaska must be certified in the state or country in which they are grown, must be inspected in storage and found to be blight free, and must be inspected at the point of shipment and found to be blight free. The National Potato Council lists 16 seed potato certification programs, one of which is located at the Alaska Plant Materials Center.

Unfortunately, late blight likes temperatures between 50 and 80 degrees F, and humidity levels above 95 percent. Sounds like summer in SE Alaska to me. I refer you to the Fact Sheet for the Home Gardener for excellent photos of infected plants. Apparently, late blight cannot develop if the leaves are dry and the humidity is low. Since it's a little hard to control the weather, experts recommend that we space our potato plants further apart in order to increase air circulation around the plants - thus minimizing periods of leaf wetness.

Happy St. Patrick's Day and may your garden potatoes always be blight free.