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Learn to recognize late blight symptoms
Early detection and treatment are important for controlling late blight. Familiarize yourself with plant and tuber symptoms of late blight so you can recognize this disease.
Understand conditions favoring development
Late blight is more likely to develop during periods of high humidity in combination with temperatures between 55° to 80° F. Even in the absence of rainfall, sprinkler irrigation provides ideal conditions for late blight development.
Eliminate sources of inoculum
The initial sources of late blight inoculum are likely to be infected plants in cull piles, volunteer potato plants infected the previous year that have survived the winter, and infected seed tubers. In addition, infected potato and tomato plants in home gardens, greenhouses and nurseries can serve as sources for late blight. Regulations from the Idaho Department of Agriculture mandate daily treatment of waste potatoes and other non-usable material from all potato operations, including seed cutting operations, in a manner that renders tubers unable to support late blight spore production. These regulations are in effect from April 15 to September 20 in regions west of Raft River, and from May 15 to September 20 in areas east of Raft River. Methods for disposing of cull and non-usable material include freezing, chopping, feeding to livestock, composting and burial. In regions where soil temperatures at 4 to 6 inches deep have not reached 20 to 25° F during the winter, volunteer potato plants may be very common. Use cultivation and labeled broadleaf herbicides, where possible, to suppress growth of volunteer potatoes in rotation crops. Check with your local extension office for herbicides available to use for controlling volunteer potatoes. Planting potatoes back to back will make control of volunteers impossible. Potato fields directly downwind from fields that had late blight the previous season may be at a higher risk for late blight because of the potential for spores to move from infected volunteers. Planting "eliminator" or "year-out" seed is risky because of the possibility of having late blight-infected tubers. Purchase only certified seed from a seed operation with which you are familiar. The occurrence of late blight in a region does not mean seed tubers are infected, but there is an increased risk. Examine field inspection records and shipping point inspection reports for information about the seed lots that you are considering for purchase.
Avoid conditions that favor late blight
Weather conditions strongly influence the incidence and severity of late blight. Cool (55 to 80° F), rainy weather, high relative humidity (near 100 percent), and heavy dew formation favor infection, disease progress and spore production. Although weather conditions are beyond our control, field selection and carefully managing irrigation practices can help reduce the extent of periods favorable for disease development.
Grow less susceptible varieties
All commercial potato varieties grown in Idaho are considered susceptible to late blight. However, early-maturing varieties, such as Russet Norkotah, Shepody and Frontier Russet, seem more prone to yield losses because defoliation tends to progress rapidly, and diseased leaves are not replaced by new growth. Shepody and Ranger Russet also seem to be very susceptible to tuber infection. The table below gives a relative ranking of the susceptibility of potato varieties to the foliar stage of late blight. It is important to note that susceptibility to foliar infection does not seem to be directly related to the level of tuber infection.
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Do not mix seed during cutting and planting
Keep seed lots separate to avoid mixing uninfected lots with seed lots potentially-infected with late blight. Mixing healthy seed with infected seed will increase the chances of spreading the disease over a larger area. It may also be a good practice to keep record of where each seed lot is planted. During the cutting operation, eliminate and save suspected seed pieces that show a rust brown, firm decay typical of late blight. Send suspicious tubers to a laboratory for positive identification.
Do not plant problem areas
Do not plant areas that you suspect may remain wet for extended periods or will be difficult to spray such as near trees or power lines. Consider not planting the area under the first few nozzles of a center pivot system. Depending on the size of the span, the area under one span will be about 1 to 2.3 acres.
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Do not let cull potatoes accumulate
Eliminating cull potatoes early in the season is critical because these potatoes could potentially carry the late blight fungus. Potato pieces resulting from seed cutting operations or cull potatoes left after loading or unloading at storage facilities may support the production of late blight spores whether or not the pieces are sprouting and should be disposed of properly. Because freezing is generally not an option in the spring, daily burial or feeding non-treated potatoes to livestock may be appropriate methods for disposing of cull and waste material. Do not allow cull piles to build up at feeding areas. Check with the Idaho Department of Agriculture or local county officials for questions on how to properly dispose of cull potatoes in your area. Potatoes may sprout and produce plants even when buried to depths greater than two feet, so it is important to continually monitor disposal sites to prevent volunteer plants from developing.
Cultivate and properly hill fields
Cultivating and hilling early may promote better water infiltration and reduce weed populations. Form high and wide hills to help minimize exposure of tubers to late blight spores that may be washed from infected plants. On sloping fields, use basin tillage to increase water infiltration and minimize water ponding in low areas.
Control alternate hosts
Hairy nightshade can be infected with late blight and may contribute to disease spread under some conditions. Apply effective pre or post emergence herbicides, such as Eptam or Matrix, during early season to reduce hairy nightshade populations. Although other weed species are not hosts of late blight, they can contribute to conditions that favor the disease by restricting air movement within the canopy. Heavy weed infestations also prevent adequate coverage of potato foliage with fungicides. Use information available from the University of Idaho to plan an effective weed control program for each field.
Scout fields regularly
Closely monitor the growing potato crop and submit any suspected late blight samples to the University of Idaho for identification. Concentrate scouting in areas of fields that tend to stay wet for long periods, such as center pivot wheel tracks and low areas where water collects. Especially scout plants under the innermost tower of center pivots because this area is almost constantly wet, which is conducive for late blight development. The windward sides of fields are usually infected from wind-borne spores so check these areas first. Also, concentrate scouting in areas that may have escaped a fungicide application because of power lines, trees or other obstacles. Look for early indications of late blight on volunteer potatoes, especially in fields that had late blight the previous year.
Forecasting models have been, and are being, tested in Idaho, but none have been found to reliably and regularly predict late blight. We will continue testing new models and refining existing models to develop a system that will accurately forecast late blight.
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A leaf or stem needs to remain wet for about 8 to 10 hours for late blight spores to germinate and infect a plant. The longer leaves or stems remain wet, the greater the risk of late blight infection. Avoid irrigating during or immediately after periods of cool, rainy weather. However, delaying irrigation too long may increase chances for developing disorders such as sugar-ends. When possible, allow the plant foliage to completely dry between irrigations. Heavy, less frequent water applications may be better than light, frequent ones during midseason. For irrigations applied during the evening, consider beginning after midnight when dew would normally wet the leaves anyway. For solid-set, or set-and-move systems, consider watering for periods of less than 8 hours to prevent plants from remaining wet for this length of time, which is necessary for late blight infection. If large wet spots form in localized areas of a field, make sure the system is not leaking, and if necessary, make repairs or turn off nozzles to allow these spots to dry.
Continue scouting fields, especially low lying areas, field borders, weedy patches, and any place where lack of air movement or shading allows plants to remain wet for prolonged periods. Submit suspect samples for positive identification. Continue monitoring cull disposal sites and volunteer potatoes so that they can be treated with herbicide before the foliage can act as a source of late blight inoculum.
Although late blight is more likely to spread from field to field by wind than by contact with people, it is unwise for any industry personnel to take risks. Therefore, people entering fields may want to wear high boots that can be disinfected between fields with products such as diluted household bleach mixed 1 part bleach to 9 parts water. An alternative is to wear disposable boots and pants that can be changed between fields, and reused after washing and drying.
Destroy hot spots
Current knowledge indicates that when late blight infestations are found early in small patches, it may be beneficial to disk, burn with a propane burner, or spray these patches with a desiccant to remove these local sources of inoculum. The area to be killed needs to extend at least 25 feet beyond the visible symptoms. While this can be effective, remember that visible late blight lesions show up 3 to 5 days after leaves become infected. If conditions were favorable for disease spread during these 3 to 5 days, killing an infected area after symptoms appeared may not have been done soon enough to prevent further spread of the disease. These areas should also be marked and inspected prior to harvest for presence of late blight in the tubers.
The University of Idaho has been utilizing a weather forecasting system called "Blitecast" that uses precipitation, temperature and relative humidity to predict when the risk of late blight is high. This forecasting model was originally developed for potato production regions in the eastern U.S., but recent experience indicates that it may be useful under the irrigated conditions in Idaho. Other forecasting methods are also being evaluated. Recommendations to intensify scouting efforts will be made when conditions become favorable for late blight. This model also provides recommendations for fungicide spray intervals based on weather and stage of crop growth. This information will be available through your county Extension office, on the internet, (http://www.uidaho.edu/ag/plantdisease/) and through the late blight hotline(1-800-791-7195).
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Avoid excessive irrigation
Potato tubers become infected with late blight when spores wash down from infected leaves and stems through cracks in the soil surface. Spores may directly contact tubers exposed through soil cracks, or may swim short distances through the soil to infect shallow tubers. In either case, excess moisture from irrigation may increase tuber infection.
Monitor and regulate fertility
Late season fertilizer applications help maintain green vines and promote tuber bulking. However, remember that green and vigorous vines can also be difficult to kill with desiccants, and immature tubers are more prone to skinning at harvest. Green vines may also harbor late blight spores that can infect tubers during harvesting, and skinned tubers may be more susceptible to infection because damaged areas remain moist for a longer period. At the end of the season, petiole nitrate levels should drop below 10,000 ppm to encourage vine senescence.
Scout fields regularly
Continue scouting on a weekly schedule to identify late blight infection. Infected areas should be marked and harvested last so infected tubers have time to decompose, or consider not harvesting these areas. If the areas are not harvested, then lay the tubers on top of the ground so they are more likely to freeze during winter. Tubers from these areas should be placed near the door in a storage so the potatoes can be marketed immediately if tuber decay becomes evident. Even small amounts of foliage infection may lead to significant tuber infection if green vines are present during harvest.
Prevent next year's volunteers
Volunteer potatoes are difficult to control so you may want to consider using a preharvest sprout suppressant. Preharvest sprout suppressants must be applied while vines are green and actively growing, and must be made at least two weeks before vine kill. Maleic hydrazide (Royal MH-30, Super Sprout Stop) may provide 70 to 80 percent control of next year's volunteers when applied at the proper time under good environmental conditions at the full labeled rate. Maleic hydrazide should not be applied to seed potatoes, and has been reported to cause some foliar and tuber injury when applied to stressed crops or if overlap occurs during application. Because of this potential, maleic hydrazide should not be applied to potatoes that have been stressed at any time during the growing season. Check the label for proper use and specific recommendations.
Kill vines completely
Late blight cannot survive and produce spores without green foliage or stem tissue. During harvest, infected vines mixed with tubers may lead to tuber infections that are not visible until later in storage. Kill vines at least 2 to 3 weeks prior to the anticipated harvest date. This interval minimizes the chance of tubers getting contaminated with late blight spores during harvesting, and allows previously infected tubers to decompose in the field. Mechanical, chemical or natural (frost) methods may be used to desiccate vines. No data are available that suggests that one method is better than another as long as vines are completely killed. Vine rolling or flailing may be helpful to expose the soil and lower canopy to drying in fields with heavy vine growth. Some vine killing methods are very sensitive to weather conditions. Watch weather forecasts, and if necessary, kill vines early if wet conditions are forecast.
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Harvest and Storage
Identify level of tuber infection
Sort tubers going into storage during harvesting, removing as many decayed tubers as possible. Identifying tubers infected only with late blight can be difficult at this time of the year, especially if tubers are covered with soil. Wash tuber samples and have them evaluated by University of Idaho personnel. Potato lots with more than 5 percent late blight infection are very difficult to store and even lower infection percentages in marginal storage facilities can cause serious problems, so it is important to know what level of infection is present.
Avoid harvesting during wet conditions, before skins are mature, and minimize skinning, cuts, and shatter bruises that provide ideal places for late blight, and other diseases, to gain entry into the tubers. Although late blight does not need a wound to infect tubers, cut, skinned, and shatter-bruised tubers are more likely to become infected because damaged areas remain wet for an extended time period, giving the late blight spores time to infect the tuber. Remember late blight is a water mold needing about 8 to 10 hours of continuous moisture to infect a plant or tuber.
Carefully monitor and regulate storage conditions
The ability to provide high volumes of air throughout the pile for drying tubers is critical during the early storage period. Remove vines, loose soil, and anything else that may interfere with air distribution in the pile. If foliar late blight was present in the field prior to harvest, it is important to dry the tubers as quickly as possible in storage. It may be necessary to continuously run the fans with reduced or no humidity until tubers are completely dry. This drying time should usually require no more than 72 hours. Expect increased pressure bruise and shrinkage losses in potatoes subjected to these storage conditions, especially if they are not marketed early. Begin observing potatoes in storage immediately for developing rotting areas (hot spots). If hot spots develop, supply additional air to those areas of the cellar, and plan on removing the potatoes as soon as possible.
Minimize volunteer potatoes
Small tubers left in the field are potential volunteers the next year, and, if infected, may produce late blight-infected plants. The number of tubers left in the field may be reduced by using narrower pitch chain on the harvester. However, this may also increase the soil load in the harvester, and the soil must then be removed before placing potatoes in storage so as not to hamper air circulation. Use shallow tillage practices that leave tubers on the surface or within the top two inches of soil to encourage freezing during the winter.
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