Home gardeners fight, avoid late potato blight

Posted: Monday, May 28, 2007

As spring returns to the Kenai Peninsula, our gardening thoughts turn to what we are going to plant for the upcoming season. While many gardeners are happy with colorful flower gardens there are those of us that insist no garden is complete without vegetables. As a happy vegetable gardener I would like to help us all avoid introducing a devastating disease into our gardens. In Alaska we have been fortunate in being relatively disease free but we now need to be aware of Late Blight or Phytophthora infestans in our potato patch. This disease has been found in potato fields throughout the world (think Irish potato famine) but was first identified in Alaska in 1953 in Wrangell. The first report in the Matanuska Valley was in 1995, again in 1998, in multiple locations in 2005 and in just one location in 2006. This fungus-like pathogen can be found on many members of the nightshade family, including potatoes, tomatoes, peppers and eggplants. Last fall it was identified on tomatoes in one home greenhouse.

Due to the ability of this disease to be rapidly blown from plant to plant and field to field we need to review some information about Late Blight before the planting and growing season starts for 2007.

1. 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 either tomatoes or potatoes. Ask where the transplants come from and don’t purchase imports.

On seed potatoes — do not plant potatoes that were grown and sold for eating! If you bought them in the grocery store or from a local farmers market or wherever, these potatoes are supposed to be eaten and not planted. Eat and enjoy them. It doesn’t matter where they were grown or 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 we eat. Plant 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 you had disease problems in your garden last year do not plant saved seed, or if your potatoes showed signs of rot while in storage. Importing potatoes from outside the state is not only risky for introducing disease it is also against our seed laws. The easiest way to avoid introducing Late Blight is to not unwittingly plant it.

2. 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 you have the room allow 14 inches minimum between plants and 2 feet between rows.

3. Destroy Volunteer Plants. Because Phytophthora infestans requires live tissue to grow and reproduce you can stop its spread by getting rid of any volunteer potato plants that appear in your garden from last years crop. These need to be removed and either burned (use safe burn practices) or double bagged and taken to the landfill. Volunteer and diseased plants should not be composted.

4. Become familiar with Late Blight. There are many good resources available to us. Some Web sources are listed at the end. 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 that’s: UAF-CES 43961 K-Beach Road, Suite A, Soldotna 99669, or call 262-5824 or (800) 478-5824.

If you think you have Late Blight on your tomato or potato plants, you can arrange to have a site visit or 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 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.

5. If you find it, follow the above steps so it does 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. But 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. There is a list of fungicides available that effectively prevent Late Blight and some are acceptable for organic growing, however, it is time consuming and an additional cost.

Contact your local Cooperative Extension Service regarding fungicides and schedules of application. On the central peninsula that’s 262-5824, on the southern peninsula or Seward call (800) 478-5824.

On the Web:

Alaska Cooperative Extension home page: www.uaf.edu/coop-ext

Dr. Roseann Leiner - photos of Mat-Su blight www.matsu.alaska.edu/pfrml

Michigan Potato Diseases: www.lateblight.org

North Dakota State University: www.ndsu.nodak.edu/instruct/gudmesta/lateblight/

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