Editor’s note: The University of Alaska Fairbanks Cooperative Extension Office on Kalifornsky Beach Road in Soldotna prepared the following article on potato late blight disease.
As spring returns to the Kenai Peninsula, gardening thoughts turn to what to plant for this upcoming season. While many gardeners are happy with colorful flower gardens, there are those who insist no garden is complete without vegetables.
Until now, Alaska has been fortunate in being relatively disease free, but gardeners need to be concerned with late blight or phytophthora infestans in their potato patch.
This disease, which caused the Irish potato famine, has been found in potato fields throughout the world, but was first identified in Alaska in 1953 in Wrangell.
It was first reported in the Matanuska Valley in 1995, again in 1998 and finally in multiple locations in 2005.
This fungus-like pathogen can be found on several members of the nightshade family, including potatoes and tomatoes. Last fall it was identified on tomatoes from two separate locations.
Due to the ability of this disease to be rapidly blown from plant to plant and field to field, gardeners on the Kenai Peninsula need to review some information about late blight before the planting and growing season starts for 2006.
· Avoid introduction. This can be accomplished by either growing your own tomato plants from seeds or purchasing tomato transplants that were grown from seed in state.
Imported tomato transplants grown in areas where blight is a problem could transfer the disease here to tomatoes or potatoes. Ask where the transplants come from and don’t purchase imports.
When planting seed potatoes, do not plant potatoes that were grown and sold for eating.
Potatoes bought in the grocery store or from a local farmers market or wherever, are supposed to be eaten and not planted. Eat and enjoy them.
It doesn’t matter where they were grown, that you have done this in the past or that they are full of sprouts.
Late blight can over-winter on live plant tissue; this includes the tubers people eat.
Plant only certified Alaska-grown seed potatoes. Purchase seed from a reliable local source, such as nurseries, greenhouses and garden centers, not from out- of-state mail-order catalogs.
If gardeners had disease problems in their gardens last year, or if potatoes showed signs of rot while in storage, saved seed should not be planted.
Importing potatoes from outside the state is not only risky for introducing disease, it is also against state seed laws.
The easiest way to avoid introducing late blight is not to unwittingly plant it.
· Increase plant space. Plan your garden to include more space between plants to increase air circulation.
Late blight thrives in the cool, wet conditions often found when potatoes have grown up and form a dense canopy.
Water early in the day to increase evaporation on the leaf and, if possible, use drip irrigation.
In the greenhouse, the same applies for tomatoes.
Allow more space for the full-grown plants for air circulation and water at the ground level, not overhead.
If space permits, allow a minimum 14 inches between plants and 2 feet between rows.
· Destroy volunteer plants. Because phytophthora infestans requires live tissue to grow and reproduce, its spread can be stopped by getting rid of any volunteer potato plants that appear in the garden from last year’s crop. These need to be removed and either burned using safe burn practices or double bagged and taken to the landfill.
Volunteer and diseased plants should not be composted.
· Become familiar with late blight. Many good resources are available. Some excellent Web sources are listed at the end of this article.
The UAF Cooperative Extension Service has a free publication, “Late Blight Disease of Potato and Tomato in Alaska.” It can be ordered online or from the nearest cooperative extension office. On the peninsula, contact UAF-CES, 43961 Kalifornsky Beach Road, Suite A, Soldotna, Alaska 99669. The phone number is 262-5824 or (800) 478-5824.
People who think they have late blight on tomato or potato plants can arrange to have a site visit. Call the office for information on how to safely ship a sample.
Late blight lesions are not restricted to leaf veins. Spores develop on the underside of the leaves, and stems turn brown quickly and have a distinctive bad odor.
Tubers show symptoms with brownish-black discoloration under the skin, while tomato fruits turn brown, then to smelly mush.
Because this fungal-like disease spreads rapidly, it is important not to wait. The earlier it is removed, the less time there is for spread.
· Finally, what to do if you find it. Hopefully all the above steps will be followed, and late blight will not appear on the peninsula in either the home garden or in a larger commercial field.
If we work together and educate ourselves, we can help avoid its arrival or speed its removal. If it does appear and is identified as phytophthora infestans, then there are controls that can be used.
The first is to remove and destroy all the infected plants. They can be double bagged and taken to the landfill or burned.
It is important not to spread the spores during plant removal. Use caution when bagging diseased plants. A list of fungicides is available for effectively preventing late blight. Some are acceptable for organic growing, however it is time consuming and an additional cost.
Call the local Cooperative Extension Service regarding fungicides and schedules of application. On the central peninsula, call 262-5824; on the southern peninsula or in Seward, call (800) 478-5824.
The following is a list of online resources on late blight: Cooperative extension at www.uaf.edu/coop-ext.
Dr. Roseann Leiner’s site has good photos of Mat-Su blight, at www.matsu.alaska.edu/pfrml.
Michigan Potato Diseases, www.lateblight.org.
North Dakota State University, www.ndsu.nodak.edu/instruct/gudmesta/lateblight.
Keeping eyes open can keep late blight off the Kenai Peninsula this season. Happy gardening!
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