Fungicides for late blight control in potatoes

Limited supplies of chlorothalonil require modifications for late blight control in potatoes with fungicides.

Posted on July 16, 2014 by Willie Kirk, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences

Due to the recall of much of the chlorothalonil supply for 2014, potato growers have a dilemma about what to base their late blight control programs on. Michigan State University Extension reports that many growers have turned to Elixir fungicide (mancozeb + chlorothalonil, 62.5 +12.5 percent, respectively from United Phosphorus) as the base control for potato late blight. Trials conducted at Michigan State University indicated that Elixir at the lower rate of 1.5 pounds per acre gave significantly poorer control of late blight than at the 1.8 pounds per acre rate, or Bravo WS 6SC applied on a seven-day interval at 1. 5 pt per acre (see table). It is important to keep to the higher labeled rate of Elixir, especially in the blight conducive conditions currently being experienced in Michigan.

Control of foliar late blight with fungicides in 2013 at the Clarksville Research Station.
Extracted data – full report on page 138 of “2013 Michigan Potato Research Report Volume 45”

^a Days after inoculation of Phytophthora infestans (US-22, A2 mating type, mefenoxam sensitive) on July 31.
^b RAUDPC, relative area under the disease progress curve calculated from day of appearance of initial symptoms to Sept. 5 (37 days).
^c Incidence of tuber late blight at harvest (150 DAP) and after storage for 28 days at 50 degrees Fahrenheit (178 DAP).
^d Days after planting.
^e Application dates: A= July 10; B= July 17; C= July 18; D= July 24; E= July 26; F= July 31; G= Aug. 3; H= Aug. 7; I= Aug. 11; J= Aug. 14; K= Aug. 19; L= Aug. 21, M= Aug. 27; N= 28.
^f Values followed by the same letter are not significantly different at p = 0.05 (Fishers LSD).

Although not tested in 2013, another product of note that could be used as a base program over the past few years includes Omega (fluazinam), which is used as the base program in Europe (Shirlan, Syngenta). Although significantly more expensive than Elixir or chlorothalonil products, Omega also offers control of white mold and can suppress early blight and gray mold. Mancozeb-based products such as Penncozeb, Manzate and Dithane may also be used as the base products and should be used in combination with translaminar or systemic fungicides. Super Tin (TPTH, UAP) should be reserved for situations where late blight is present in the field.

Of the systemic fungicides, again tested over the past few years, Zampro (ametoctradin + dimethomorph, BASF), Ranman (cyazofamid, FMC), Revus Top (mandipropamid + difenoconazole, Syngenta), Tanos (famoxadone + cymoxanil, DuPont), Curzate (cymoxanil, DuPont), Previcur Flex (propamocarb, Bayer), Gavel (zoxamide + EBDC, Gowan) and Reason (fenamidone, Bayer) have provided excellent control of late blight in potatoes, but are most effective when applied prior to the onset of late blight and should be mixed with a protectant partner (except Gavel).

The use of Ridomil-based products should be limited to situations where storage rots are a high potential risk. Trials at MSU in 2013 indicated that Ridomil applied curatively to blighted foliage did not successfully prevent further disease development. Full information on rates of most of these products is available at the Michigan Late Blight Risk Monitoring website.

Dr. Kirk’s work is funded in part by MSU’s AgBioResearch.

This article was published by Michigan State University Extension. For more information, visit http://www.cnchemicals.com/. To contact an expert in your area, visit http://www.cnchemicals.com/, or call 888-MSUE4MI (888-678-3464).