Fungicide Application for Management of Potato Late Blight in the Columbia Basin

Published by Permission from:

Dennis A. Johnson, Philip B. Hamm, and Jeffrey S. Miller. Washington State University, Pullman, WA; Oregon State University, Hermiston, OR; and Miller Research LLC, Rupert, ID, respectively.


Management of potato late blight (LB) in the Columbia Basin of Washington and Oregon requires a combination of several strategies: strict sanitation practices, proper irrigation management, good cultural practices, and proper application of fungicides. Sanitation practices (such as not planting infected seed and using a seed treatment containing mancozeb or cymoxanil, and destroying cull piles before planting), and proper cultural practices (such as not planting within 80 – 100 ft. of the pivot center and preventing wet areas to develop in fields) will reduce disease pressure and increase the effectiveness of foliar fungicides.

Ø Fungicides are most effective when they are applied to foliage i) before infection occurs or ii) when the disease is in very early stages of development and cannot be detected yet by the human eye. Later applications are helpful in reducing the rate in which the disease spreads but are not nearly as effective as early applications. Late blight is very difficult to manage once infections become established in sprinkler-irrigated fields because the microclimate within the canopy usually favors further disease spread whenever the field is irrigated. In other words, preventing infection is far better and likely cheaper than trying to manage LB in fields where infection has occurred.

Ø Total crop and canopy coverage with fungicides is essential for late blight management. The late blight organism, Phytophthora infestans, will most likely find and infect any plants or plant surfaces skipped during application. Given the nature of the potato canopy after row closure, achieving complete coverage of leaves and stems with fungicides can be difficult if proper application methods are not followed. Application skips from air, chemigation, or ground applications have frequently resulted in large areas of late blight infected plants.


Many fungicides are labeled for use against potato late blight. Each product has specific conditions for use and is labeled with details regarding rates and application method. Fungicides work against late blight by inhibiting one or more of the following: germination of spores (and as a result, reduced infection of plants), growth within the plant, production of spores (sporulation), and formation or development of lesions.

Spore suppression. Some combinations of fungicides, such as Forum (dimethomorph) plus an EBDC, Curzate (cymoxanil) plus an EBDC, and Oronidis Opti (oxathiapiprolin + chlorothalonil) have post-infection activity that inhibits sporulation and/or restricts lesion expansion (5,6). These fungicides may also help reduce tuber infection when applied during and after tuber bulking. Their use at times can be very beneficial, but they should never be used as a predetermined “rescue” instead of using protectant fungicides when recommended because control likely will not be adequate. Again, proper use of protectant fungicides, prior to infection, will ensure good and economical protection.

Soil barrier. Mancozeb and Polyram (metiram) when worked into fungicide programs during tuber maturation are effective on the soil surface in protecting against tuber infection (10). Shallow daughter tubers and soil cracks are main avenues allowing the pathogen to access tubers (11). Plant seed tubers as deep as possible and adequately cover hills with soil to help prevent tuber infections. Cultivars with tubers moderately resistant to late blight include Umatilla, Gem, Alturas, Legend, and Defender. Tubers of Bannock and Ranger are very susceptible and require extra management.

Examples of late blight fungicides

Ø EBDC (ethylene bis-dithiocarbamate) fungicides. Examples: Metiram (Polyram), Mancozeb (Dithane M-45, Manzate Pro-Stick and Penncozeb), and Maneb (Manex). Chlorothalonil (Bravo, Echo)

Ø Cyazofamid (Ranman) plus an EBDC or chlorothalonil

Ø Cymoxanil (Curzate) plus an EBDC or chlorothalonil. Tanos contains 25% cymoxanil and 25% Famoxate

Ø Dimethomorph (Forum) plus an EBDC or chlorothalonil. Zampro contains ametoctradin + dimethomorph

Ø Propamocarb hydrochloride (Previcur), plus EBDC or chlorothalonil

Ø Mandipropamid + difenoconazole (Revus Top)

Ø Zoxamide (Gavel = zoxamide + mancozeb, Zing = zoxamide + chlorothalonil)

Ø Oxathiapiprolin (Oronidis Opti = oxathiapiprolin + chlorothalonil)

Ø Fluazinam (Omega, Omega Top MP)

Ø Phosphorus Acid (Phostrol and other salts of phosphorous acid) – Two to three foliar applications at two week intervals (beginning at initial tuber bulking, tubers 14 to 70g in weight) provide excellent tuber protection in storage, but little protection on foliage. Two applications are effective for cultivars with moderately resistant tubers such as Umatilla and three applications are needed for cultivars with very susceptible tubers such as Ranger (7). Post-harvest application of tubers is effective if late blight is found in tubers prior to harvest, or if late blight is present in the field at the end of the season. The phosphorous acid application cannot cure infected tubers, but keeps healthy tubers from becoming infected if they are exposed to spores of the late blight pathogen during the harvest operation.

Fungicides not recommended

Ø Mefenoxam (Ridomil Gold, Ultra Flourish) prepacks are not recommended for management of late blight when the pathogen population is resistant (US-8 and US-11 for example); in addition, mefenoxam can be effective for management of pink rot and Pythium leak.

Ø Super Tin by itself will not adequately control severe late blight, but it is effective when mixed with Polyram or another EBDC fungicide.

Ø Copper fungicides alone will not adequately control foliar late blight in conventional (non-organic) fields in the Columbia Basin. Copper may be an alternative for organic potatoes.

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