Alternatives to chlorothalonil for potato late blight control

Willie Kirk, and Noah Rosenzweig, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences - June 15, 2015

Chlorothalonil for potato late blight control is in limited supply for 2015. Growers should consider these fungicide modifications.

Conditions for late blight development throughout Michigan are conducive for the appearance of late blight. No late blight has been found in Michigan as of June 15, 2015, although the disease has now appeared in North Carolina and Florida on potatoes.

Chlorothalonil supply for 2015 is limited. Potato growers, therefore, have a dilemma about what to base their late blight control programs on. There are limited supplies of Bravo-based and Echo-based products available and growers should endeavor to secure supplies. In addition, chlorothalonil is available in mixed products such as Ariston (chlorothalonil equals 3.83 pounds per gallon plus cymoxanil 0.51 pounds per gallon) with an application rate of 2.0 pints per acre; Zing! (chlorothalonil equals 4.19 pounds per gallon plus zoxamide 0.71 pounds per gallon) at 24 fluid ounces per acre; and Elixir (chlorothalonil equals 0.125 pounds per pound plus mancozeb 0.625 pounds per pound) with an application rate of 2.0 pounds per acre. For more information about using Zing! for use against late blight, see “Zing! registered for use against potato late blight and early blight.”

In 2014, many growers used Elixir fungicide as the base control for potato late blight. Trials conducted at Michigan State University in 2013 indicated 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 pints per acre (see table). Note that the rate for Elixir has been increased to 2.0 pounds per acre and Michigan State University Extension does not recommend dropping to lower labeled rates of 1.2 pounds per acre under any circumstances. It is important to keep to the highest 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 MSU Clarksville Research Station
Treatment and rate per acre	Foliar potato late blight (percent)				RAUDPC^b – 37 days after inoculation
Treatment and rate per acre	Aug. 30 – 31 days after inoculation^a		Sept. 5 – 37 days after inoculation		RAUDPC^b – 37 days after inoculation
Elixir 75DF 1.5 pounds (A, B, D, F, H, J, L, N^e)	7.5	efg^f	28.8	f-j	3.9	d-i
Elixir 75DF 1.8 pounds (A, B, D, F, H, J, L, N)	3.3	fg	17.5	g-l	2.1	f-i
Bravo WS 6SC 1.5 pints (A, B, D, F, H, J, L, N)	0.5	g	0.8	l	0.2	i
Revus 249FS 5.5 fluid ounces (A, D, H, L); Bravo WS 6SC 1.5 pints (B, F, J, N)	1.0	g	1.3	l	0.4	i
Inoculated check	50.0	a	100.0	a	19.2	a

Data from 2013 Michigan Potato Research Report (page 138) and 2014 Michigan Potato Research Report (page 99).

^a Days after inoculation of Phytophthora infestans (US-22, A2 mating type, mefenoxam sensitive) on 31 Jul.

^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^oF (178 DAP).

^d Days after planting.

^e Application dates: A= 10 Jul; B= 17 Jul; C= 18 Jul; D= 24 Jul; E= 26 Jul; F= 31 Jul; G= 3 Aug; H= 7 Aug; I= 11 Aug; J= 14 Aug; K= 19 Aug; L= 21 Aug, M= 27 Aug; N= 28.

^fValues followed by the same letter are not significantly different at p = 0.05 (Fishers LSD).

Although not tested as a stand-alone product in 2014, 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 offers white mold control 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 can 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 tested over the past few years, Zampro (ametoctradin plus dimethomorph, BASF), Ranman (cyazofamid, FMC), Revus Top (mandipropamid plus difenoconazole, Syngenta), Tanos (famoxadone plus cymoxanil, DuPont), Curzate (cymoxanil, DuPont), Previcur Flex (propamocarb, Bayer), Gavel (zoxamide plus EBDC, Gowan) and Reason (fenamidone, Bayer) have provided excellent control of potato late blight, but are most effective when applied prior to the onset of late blight and should be mixed with a protectant partner (except Gavel).

Also, 2014 trials and field observations on Ridomil-based products indicated Ridomil applied protectively to crops provided excellent late blight control, as the current predominant genotype of Phytophthora infestans (US-23) is Ridomil-sensitive. Trials in 2013 indicated Ridomil-based products applied to blighted foliage did not successfully prevent further disease development. However, 2014 field observations showed when Ridomil-based products were applied to crops where about 0.01 percent of the field was affected and there were low levels of foliar blight in the affected areas (about 1-2 percent of the canopy affected), the products were effective in curtailing further development of the disease.

Full information on rates of most of these products is available on the Late Blight Risk Monitoring website.

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

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