Late blight (caused by Phytophthora infestans) has occurred sporadically in Idaho since 1995. Late blight was first identified this year in a field in eastern Idaho. The extent to which the pathogen spreads is affected by several factors, including weather and fungicide use patterns of growers.
The purpose of this article is to provide an overview of fungicides as they relate to late blight management. Foliar fungicides can be highly effective if used properly. Three factors to consider when designing a foliar fungicide program include: 1) fungicide choice, 2) application method, and 3) timing and frequency of applications.
Which Fungicide Should I Use?
Selecting the right product for disease control can save you money. On the other hand, using the wrong fungicide or a less effective fungicide can cost you money. Some of the more expensive products don’t necessarily provide better disease control. Two groups of standard fungicides that have a broad spectrum of activity against potato diseases are chlorothalonil (e.g. Bravo or Echo) and ethylene bisdithiocarbamate (EBDC, e.g. Dithane or Penncozeb) based products. Both are effective against early and late blight and their efficacy is similar under the semiarid conditions of the Pacific Northwest.
Add-in products such as Agri Tin, Super Tin, Curzate, Previcur Flex, and Forum can be mixed with the standard protectants for added disease control. Agri Tin and Super Tin are also effective against early blight and brown spot. However, these two products can also cause phytotoxicity when applied at low spray volumes or when temperatures are high.
It is recommended that Quinone outside inhibitor (QoI) fungicides be tank-mixed with a standard protectant to manage fungicide resistance in the early blight population. This resistance has already been documented in southern Idaho. Historically QoI fungicides such as Evito, Headline, Quadris, Tanos, and Reason have shown high activity against early blight. In a trial conducted by Miller Research in 2010 Quadris and Headline did not perform as well as Tanos and Gem. Most strobilurin fungicides were similar in performance in 2011 and did not perform as well as chlorothalonil. Headline and Quadris are effective against late blight at double the early blight rate (12 fluid ounces per acre for both products). Tanos is a mixture of famoxate (good against early blight) and Curzate (good against late blight). The active ingredient in Reason is fenamidone. Reason is effective against both early blight (5.5 fluid ounce rate) and late blight (8.2 ounces).
QoI fungicides should never be applied consecutively unless they are applied with a tank mix partner, such as a standard protectant (chlorothalonil or EBDC). Read the label carefully to follow manufacturer recommendations with respect to resistance management.
Endura has been very effective for controlling early blight and white mold when used at the appropriate rate (5.5 ounces). However, resistance to Endura has also been documented in the early blight pathogen in southern Idaho. Endura is not effective against late blight.
Gavel, Omega, and Ranman are extremely effective against late blight. These products can be used in place of the standard protectants when late blight pressure is high. University research shows that foliar applications of these products can be more effective in reducing the tuber blight phase of late blight than standard protectants. Omega has the added benefit of effective white mold control. Ranman has shown some activity against pink rot, but the timing of application for pink rot control is much earlier in the season that when it would be used to manage late blight. (Foliar applications of Ranman have not shown any protection against pink rot.) Omega Top is a relatively new fungicide which combines Omega with difenoconazole (active against early blight and brown spot). While this mixture should theoretically be effective against white mold, tests at Miller Research have not shown good efficacy in this area. The product is effective against early blight.
Revus Top is a newer fungicide that is strong against late blight and early blight/brown spot. Revus Top contains two active ingredients. Mandipropamid (the “Revus” portion) is strong against late blight; difenoconazole (the “Top” portion) is strong against early blight and brown spot.
Fosphite, Phostrol, Prophyt, Resist 57, and Topaz are just a few of the fungicide formulations of phosphorous acid labeled for use on potatoes in Idaho. Other phosphorous acid products are available, but efficacy data are scarce for control of potato diseases in the Pacific Northwest. Applications of phosphorous acid have resulted in increased tuber protection to both late blight and pink rot. However, research shows that phosphorous acid applied at high rates and multiple times (eight parts per acre twice for late blight and 10 parts per acre three times for pink rot) through the season provides significant tuber protection. Additionally, phosphorous acid is not effective in protecting the foliage from late blight. These fungicides are not cost effective as foliar applications for controlling late blight on potato vines.
Storage rots (pink rot and Pythium leak) can be suppressed by applying foliar fungicides containing mefenoxam (Ridomil Gold products, Metastar, Ultra Flourish). Some of the Ridomil Gold products are pre-packed with a standard protectant such as chlorothalonil or mancozeb. In recent years, most isolates of the late blight pathogen have been from strains that are resistant to mefenoxam. However, some of the newer strains are sensitive to mefenoxam. This is one reason it is important to submit late blight samples to university or extension personnel. They can evaluate for mefenoxam sensitivity. Regardless of what strain of the late blight pathogen is present, the pre-pack partner will provide protection against late blight. Mefenoxam is still effective against some populations of the pink rot pathogen.
University research around the country has shown that spray programs based on copper fungicides do not perform as well as programs based on standard protectants for controlling late blight.
What Application Method is the Best?
Ground, air and chemigation applications of fungicides can all be effective if time is taken to ensure proper application. Chemigation has been criticized for having low levels of fungicide residue in the potato canopy following application. Research performed in Idaho with Dithane DF Rainshield has shown that fungicide residues on stems are just as high with chemigation as stem residues from aerial application. Research and field observations have shown that chemigation is an effective method for late blight control. It is the least expensive application method but requires more grower management. If an airplane is used, care must be taken to ensure fungicide is applied to areas which are difficult to fly (i.e. near power lines and buildings). Aerial applications have the benefits of quick application time and less management time by the grower. Irrigation is needed to redistribute residues down in the potato canopy.
How Often and How Frequently Should Fungicides be Applied?
Timing and frequency of application are perhaps the most critical components of a good spray program and this is where taking shortcuts can be dangerous. The first fungicide application for late blight should be made just prior to row closure. Once a canopy has formed, conditions are often favorable for late blight. When growers experience frequent rain events, weekly applications are needed to cover and protect the crop. As the interval between applications increases, the risk of late blight increases.
Whenever late blight is confirmed in an area, protectant programs should be maintained in areas near affected fields until the end of the growing season. Thundershowers can quickly spread the pathogen over long distances.
Using a protectant fungicide on a regular schedule with the application method best suited to your needs can be done economically, and it can provide good protection against late blight. It is important to choose the right fungicide, apply it properly, and maintain a good program while vines are actively growing. If late blight is confirmed in or near your fields, using additional products with extra activity against late blight would be recommended.