Chemical Control of Potato Late Blight

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Seed piece treatments Fungicide types and selection Fungicide resistance
Application methods When to apply fungicide Terminology

1999 Late Blight Spray Recommendations

Seed Piece Treatments

Several experiments conducted over the last 30 to 40 years have indicate that most of the seed tubers or seed pieces infected with late blight will readily decay from soft rot after planting and be self-eliminating. Yet, seed transmission is an important means of initiating the disease in a field. Recent information indicates that late blight spores may also spread from infected tubers to healthy tubers during handling, cutting and planting operations.

Seed piece fungicide treatments that have activity against late blight have been shown to be effective in minimizing this type of disease spread. There are several products available to Idaho growers that can be used as seed piece treatments against late blight. Gustafson has added late blight suppression to their label for TOPS-MZ and has obtained a 24C label for MZ-Curzate (mancozeb + cymoxanil) that is specifically labeled for management of seed-borne late blight.

Keep in mind that no seed piece treatment should ever be used in an effort to "rescue" a badly infested seed lot. Growers who suspect or know they will be receiving seed that contains some late blight are strongly encouraged to use seed piece treatments - like those described above - that have late blight activity. Growers using seed lots with known or strongly suspected late blight infestation should also place the affected fields on a regular fungicide spray program for the entire season.


Fungicide Types and Selection

The fungicide selected is not as important as application coverage and timing as discussed below. However, the use of copper or tin fungicides alone is not recommended for controlling late blight. These products provide excellent control when used in combination with other fungicides.

Chemical list for materials labeled for use on potatoes to control late blight is provided on another web page.


Fungicide Resistance

University of Idaho Extension personnel will be keeping track of late blight strains that occur in Idaho. Most late blight strains, including US-8 and US-11, found in Idaho since 1995 are resistant to Ridomil, therefore, Ridomil is not recommended for use in controlling late blight at this time. Ridomil is very effective for controlling pink rot and Pythium leak when applied according to label directions. The companion fungicide in Ridomil prepacks, such as Bravo or mancozeb, is a protectant fungicide effective for controlling late blight. When applying Ridomil for control of pink rot and Pythium leak, consider the companion fungicide in the prepack to be part of your protectant spray program for late blight.

Remember that chlorothalonil, triphenyltin hydroxide and the EBDC fungicides have limits on the amount of active ingredient that can be applied per acre each season. Carefully follow label directions to be certain that you are not applying more per season than is allowed. Alternating fungicide classes is a good practice, and may be necessary if these use limits are being approached.

Consider These Factors When Selecting and Using an Application Method

To ensure thorough and complete fungicide coverage, a necessity for controlling late blight, a fungicide may need to be applied up to two weeks prior to potatoes being exposed to late blight. Application method and frequency, and amount of water applied all influence fungicide redistribution in the canopy. More redistribution is required for air application, less for ground, and least is required for chemigation.

  • Fungicide label directions - Fungicides may be applied by ground rig, aircraft or chemigation. Check the label for application restrictions and how the fungicide may be applied.

  • Field size, shape, tillage practices, and obstacles - Choosing an application method depends on many factors, including field size and shape, location of obstacles, irrigation system, and tillage practices. For example, fields with houses or tall trees along the edges may not be good candidates for aerial applications. Likewise, fields with basin tillage pose some problems for ground application. It is also important to realize that an application method that works best for a disease affecting mainly the lower stems, such as white mold, may not provide the best control of a foliar disease such as late blight.

  • Availability of equipment - Product choice also influences application method because some products have limitations on how they may be applied. Another factor to seriously consider is whether you can apply a fungicide in a timely manner. A successful late blight management program, and this cannot be overemphasized, must include having a fungicide applied to the potato crop before late blight is seen in a field. If you plan on using air or ground application methods, be certain the equipment will be available when needed.

  • Equipment must provide complete coverage and be timely - Regardless whether you use ground rig, air, or chemigation, planning and management are required to ensure the best results. No matter which application method is used, be sure to completely cover the crop leaving no skips or areas untreated, and apply the fungicide at the appropriate interval for the disease pressure in your area.



  • Ground application - Applications using a ground sprayer tend to be very effective in controlling late blight because the water volumes and pressures used provide good leaf coverage and penetration of fungicide into the canopy. Research at the University of Wisconsin found that hollow cone and extended range flat fan nozzles were superior to floodjet nozzles. Recalibrate the sprayer often, and replace nozzles that are under or over applying by more than 10 percent. Raise the boom height as the crop grows to maintain the proper overlap in the spray pattern.

    The main disadvantages of ground applications are the time required to cover large acreage, and incompatibility with certain irrigation systems and basin tillage. Multiple trips through the field with a ground sprayer will also increase soil compaction, especially on heavy soils. During 1996, the University of Idaho documented a 1 to 3 percent reduction in yield due to sprayer traffic in two fields in the Treasure Valley. This yield effect should be factored into the cost of ground application when comparing with air and sprinkler application methods.

  • Air application - Air applications deposit most of the fungicide on top of the crop, and the material is then redistributed into the lower canopy by irrigation water or precipitation. A minimum of 5 gallons water per acre is required for adequate coverage. Research has shown that 5 gallons water per acre is as effective as using 7 gallons. Remember, nozzle calibration is just as important for air applications as it is for ground applications.

    One of the biggest problems with aerial application is untreated strips caused by inadequate overlap between passes. Skips can be avoided by marking spray passes with permanent flags and alternating spray passes on the flags and between flags on subsequent applications. Availability of planes for timely applications has occasionally been a problem in parts of Idaho. Arrange for an applicator for the season as early as possible. Work with the aerial applicator to identify field areas that cannot be treated due to obstacles (trees, power lines, houses, etc.). Treat these areas with a ground applicator to ensure proper coverage.

  • Sprinkler application - An advantage of using a sprinkler system for applying a fungicide is the system is already in place so you do not wait for equipment. A 1998 survey in Idaho has shown that potato fields where late blight was controlled with chemigation had disease severity ratings slightly lower than fields where a ground applicator or aerial application methods were used. The survey does not, however, consider the number of fungicide applications or products used. An important point to note, though, is that all methods adequately controlled late blight.

    For any type of irrigation system, make sure the field is the same size as the system (i.e. no potatoes outside the water coverage area). The uniformity of fungicide application is very dependent on the uniformity of water distribution. Corners of fields irrigated with center pivot systems do not usually receive adequate fungicide coverage and should be sprayed with a ground applicator, or not planted to potatoes. For solid-set or set-and-move systems, inject the fungicide during the last portion of the irrigation set, or make a separate application between irrigations. Make sure the fungicide has flushed out of the end nozzles before shutting off the system.

    One of the main disadvantages of using a center pivot to apply fungicides is the huge volume of water applied with the chemical. Therefore, it is important to adjust the revolution time to the fastest setting so as to apply the least amount of water to the foliage. Always use the highest labeled rate when applying a fungicide through a sprinkler system.



(Always read and follow label directions.)

  • Initial applications - All potato-producing areas of Idaho have been exposed to late blight, therefore, all fields should be sprayed with a protectant fungicide before row closure (plants touching between rows) followed by a second application in 7 to 10 days. The need for additional applications will depend on weather conditions and occurrence of late blight. University of Idaho will make recommendations on the number of fungicide applications needed. These early applications treat the stems and lower canopy where late blight often begins once the vines close the rows and conditions within the canopy become more favorable for late blight. Early fungicide applications are recommended because of the chances of infections starting from local sources such as volunteers, cull piles or seed. These applications are especially important since there was widespread occurrence of late blight the previous season in many areas of Idaho.

  • Applications after initial applications and up to late season - Rigorously scout for late blight and pay careful attention to weather conditions favoring late blight development, i.e. weather that is cooler and wetter than normal. After the first two fungicide applications, if late blight is found in the area or weather conditions are conducive for disease development, then we recommend continuing spraying protectant fungicides. A preventative attitude is very important when dealing with late blight. Attempting to "rescue" a field after late blight occurs is very likely to fail. There are no "curative" fungicides (including the limited systemic products such as Curzate and Acrobat) that will effectively stop a late blight epidemic. It is vital to stay ahead of the late blight fungus with a protectant fungicide spray program.

    Recommendations on spray application frequency will be issued regularly throughout the growing season. Protectant fungicides must be applied at intervals that maintain coverage on new leaves, especially when environmental conditions are conducive for late blight development.

  • Late season - Continue fungicide applications at intervals based on weather conditions and recommendations. Protectant fungicides may need to be applied even after vine desiccation until all green vines are completely dead if late blight was present in the region or field. We do not recommend applying a fungicide as a tank mix with a vine killing agent. Early vine killing may be the best option for growers with out-of-control late blight infestations. Remember if you kill the potato plant, you kill the fungus growing on the plant.

  • Recommendations to reduce late blight tuber rot - Season-long control of the late blight fungus on the vines (leaves and stems) is extremely important for reducing infection of tubers. Serious tuber blight infections have occurred even when only small percentages of foliar blight were observed in the field. Follow recommendations above to reduce chances of tuber infection during harvest.



Aggressive - Used to describe the potential of a late blight genotype (strain) to cause damage to potato. Aggressive genotypes produce more sporangia, cause larger leaf and stem lesions, and/or have a shorter latent period.

Asexual - Reproducing or producing spores (sporangia and zoospores) without mating (no sex).

Fungus - (plural, fungi) A microscopic plant lacking chlorophyll that may cause disease in higher plants, such as potatoes and tomatoes.

Genotype - A population of late blight fungus with similar traits. Late blight fungal genotypes are designated using the letters "U.S." followed by a number, such as U.S.-8.

Inoculum - Structures of late blight fungus capable of infecting plants.

Latent period - The period from the time a late blight spore(s) infects a plant, but symptoms are not visible, to when the plant shows symptoms of the disease.

Lesion - Area on a leaf or stem showing symptoms of late blight.

Mating type - Term used to distinguish the "sexes" of late blight. Mating type is used rather than male and female because the two mating types are indistinguishable outside of the laboratory, but both types, referred to as A1 and A2, must be present for late blight fungus to sexually reproduce.

Oospore - A thick walled structure, capable of surviving unfavorable environmental conditions outside a living host, produced from the sexual mating of A1 and A2 mating types.

Spore - A fungal structure containing one or more cells capable of reproducing, germinating, and causing infection.

Sporangia - Asexual reproductive structures of the late blight fungus that can germinate directly, or indirectly by producing zoospores. Sporangia can be carried in moist air, spreading the disease (see also zoospores).

Strain - Genotype.

Zoospores - Spores produced inside a sporangium (plural, sporangia) that have limited locomotive (swimming) ability in water, and capable of infecting a susceptible host with late blight. Zoospores can be carried in water for great distances.




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