Potato late blight update and recommendations for mid-late 2014 season

Michigan potato growers should continue scouting for late blight on a weekly basis, and treat fields aggressively in areas where late blight has been reported.

Many more fields in the Michigan counties of Montcalm, Isabella, Gratiot, Kent and Allegan have been reported with late blight over the past week. On Aug. 11, 2014, there were further reports of late blight on potatoes and tomatoes in Ingham County and on potatoes in St. Joseph County. Growers should be aggressively treating all fields throughout Michigan on a minimum five-day schedule. Applications should include a translaminar fungicide in combination with a protectant (chlorothalonil or mancozeb) and Super Tin.

Elixir, a combination of chlorothalonil plus mancozeb, should not be applied at reduced rates. Although a few fungicides have yet to be added, the rates of fungicides can be found at the Michigan Late Blight Risk Monitoring website. The amount of disease is generally light in some fields, although others are quite badly affected and those areas should be desiccated immediately with Reglone plus Super Tin. Areas in fields that are particularly vulnerable are field margins, especially those close to tree lines, raised cable lines and where water can accumulate such as around pivot tracks and tractor wheel lines.

The genotype of the Phytophthora infestans isolate responsible for all late blight confirmations in potatoes throughout Michigan has been the US-23 genotype, and we are continuing to receive and genotype samples at the Kirk lab. This was confirmed by GPI allozyme analysis. This genotype is Ridomil-sensitive; however, the recommendations for treatment include treating with one of the translaminar fungicides listed on the Michigan Late Blight Risk Monitoring website. Ridomil Gold Bravo has proven to be quite effective in fields this year.

Conditions remain conducive for late blight in irrigated potato crops given weather conditions experienced over the last few days. Forecasts and disease severity value (DSV) accumulations can be checked daily at the Michigan Late Blight Risk Monitoring website. For the next five days, for example, most parts of Michigan have a high risk forecast.

Michigan State University Extension recommends growers continue to scout on a weekly basis where late blight has not been reported and treat fields aggressively in areas where late blight has been reported (see table for recommendations). Growers should sample symptomatic plants and send them for diagnosis and continued application of residual protectant fungicides. Recommendations for appropriate late blight control where late blight has been confirmed or is suspected should include approaches suggested in the table below and include desiccation of infected areas.

Suggestions for appropriate fungicides for late blight control including semi-systemic fungicides under different late blight conditions in susceptible potato varieties

Disease category

Late maturing, especially storage varieties

No senescence - early senescence

Mid - late senescence

A) None

Curzate or Tanos or Zampro or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil

Quadris or Headline or Gem + EBDC or Gavel or Omega

Champ or Kocide can be added to enhance activity

Curzate or Tanos or Forum or Previcur Flex or Revus Top or Ranman + EBDC or chlorothalonil +TPTH or Chlorothalonil  fb EBDC+TPTH or Chlorothalonil + TPTH five-day

fb Chlorothalonil various + ZN) or Gavel or Omega

Champ or Kocide can be added to enhance activity

B) Few random lesions even distribution  throughout field (0-1% foliar infection)

Curzate or Tanos or Zampro or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH or Chlorothalonil fb EBDC+TPTH or  Chlorothalonil + TPTH five-day

fb Chlorothalonil (various + ZN) or Gavel

Curzate or Tanos or Zampro or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil +TPTH or Chlorothalonil  fb EBDC+TPTH or Chlorothalonil + TPTH five-day

fb Chlorothalonil various + ZN or Gavel

C) One or more (up to five) loci spreading from the edge of the field or from several centers within the field (1% overall field infection, but locally heavily infected plants 5-10%)

Curzate or Tanos or Zampro or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH

Kill infected area with Reglone****

fb EBDC+TPTH or Chlorothalonil + TPTH every five days until vines dead

Curzate or Tanos or Zampro or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH

Kill infected area with Reglone

fb EBDC+TPTH or Chlorothalonil + TPTH every five days until vines dead

D) Partial crop infection, large areas infected with up to 20% loss of  GLA evenly distributed throughout the field or large areas of the field

Curzate or Tanos or Zampro or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH

Chlorothalonil (various + ZN) + TPTH

Kill infected area with Reglone

fb EBDC+TPTH or  Chlorothalonil + TPTH every five days until vines dead

Curzate or Tanos or Zampro or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH

Chlorothalonil (various + ZN) +TPTH

Kill infected area with Reglone

fb EBDC+TPTH or  Chlorothalonil + TPTH every five days until vines dead

E) 20-100% crop infection with large   loss of GLA***

Kill infected area with Reglone

fb EBDC+TPTH or Chlorothalonil + TPTH every five days until vines dead

Kill infected area with Reglone

fb EBDC+TPTH or Chlorothalonil + TPTH every five days until vines dead

fb=followed by
GLA=Green Leaf Area
* TPTH has seven day post-harvest interval (max 11.25 ounces per acre per season).
** Chlorothalonil has seven day post-harvest interval.
*** Protectant applications of an EBDC or chlorothalonil-based fungicide should be maintained on a five-day schedule until the vines are completely dead.
**** Infected areas should be treated last and a fungicide should be applied during the exit from the field.

The indication of high risk was communicated early in the season to the industry. This included information from the University that winter temperatures were likely to have resulted in a high likelihood that volunteer tubers would have survived the winter, as well as feedback from scouts that survival of volunteer potatoes was profuse. This intense, early scouting resulted in late blight being reported early and the flurry of early protection which, in my opinion, resulted in many acres of potatoes being spared from late blight in 2014.

In 2014, the early action of the Michigan Potato Industry Commission (MPIC) in conjunction with MSU to contact growers alerted growers to the possibility of late blight risk in 2014, which would have had a mitigating effect on the progress of the disease. One further fact for 2014: in many areas of Michigan, the accumulated late blight DSV for 2014 are among the highest we have recorded in the past 10 years.

For more information on fungicide recommendations, see the recent article circulated by MPIC to Michigan potato growers, “Potato late blight update and recommendations,” or the MSU Extension article, “Late blight confirmed in potatoes in northern Indiana July 6, 2014.”

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

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