Potato late blight control recommendations for southern Idaho in 2020

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The potential for another outbreak of late blight in 2020 remains high. This is primarily due to the presence of the disease in Southcentral Idaho in 2019, followed by the mild winter conditions across the whole Snake River Valley which enabled the survival of volunteer potatoes over winter.  Infected volunteer potatoes, cull piles and seed tubers along with any cool wet weather conditions in May and June could all potentially contribute to the development of a late blight epidemic again this year.  As such, it would be prudent to plan ahead for the management of any potential late blight outbreak before or shortly after the emergence of the potato crop in many areas of southern Idaho. 

            Effective management of late blight requires the implementation of an integrated disease management approach, including strict sanitation practices (e.g. management of cull piles), good irrigation management, cultural practices and the proper timing and application of effective fungicides.  All these practices together can reduce the chances of a late blight outbreak.  Cultural practices are the first line of defense against late blight.  Before planting growers should take several measures to control late blight.  Firstly, it is important to eliminate potential sources of inoculum.  These are likely to be infected potatoes in cull piles, infected volunteer potato plants that survived the winter and infected seed tubers.  For information on volunteer and cull potato management see the Potato Progress XVI (4) article (http://bit.ly/1XSWHsS).  Field scouting should begin after emergence when the first plants are 4 to 6 inches tall.  Field scouting should be a vital component of a grower’s IPM program.  Scouting allows growers to make informed disease and pest management decisions and provides early detection of other problems that may be present in the field, such as nutrient deficiency or herbicide injury.  By using information collected by scouts, growers can time fungicide applications for optimal effectiveness.  This is especially important for the control of late blight as fungicides are most effective when applied to foliage before infection occurs or when the disease is in its very earliest stages of development and no symptoms are visible.  In the irrigated fields of southern Idaho, late blight can be very difficult to manage once infections become established as the humid microclimate within the canopy favors further disease development after irrigation.

There are a wide range of fungicides labeled for use against potato late blight (Table 1).  Each fungicide is different and will have specific conditions for use listed on the label with additional details regarding application rates, re-entry intervals and total product amounts that can be applied in a season.  Some may even include information on how to minimize the risk of fungicide resistance developing.  Fungicides that are effective for the control of late blight tend to have one of three modes of action: germination inhibition (they prevent germination of spores and thus plant infection), inhibiting mycelial growth (they block pathogen colonization of the plant cells), anti-sporulation activity (they prevent the pathogen from sporulating and thus limit spread of the disease).

Recommended programs for late blight control are not straightforward.  The product(s) of choice will depend on whether the crop is at low risk, high risk or already has some disease and from where the disease has developed.  The appropriate placement of translaminar and other systemic products in a fungicide spray program should be determined by the mode of action of the product in relation to the host and disease development.  However, all products are best used within a preventative protectant program.  For example, Previcur Flex, Forum, Revus, Curzate, Reason, and Gem (all plus protectant fungicide [EBDC or chlorothalonil]), or Gavel or Tanos may be applied while the canopy is expanding but before senescence.  Forum is most effective during canopy expansion and as a post-senescence product and can be applied up to late crop senescence.  Results from fungicide resistance screening of P. infestans isolates (US23) collected in the 2019 growing season indicated that the US23 strain in Idaho had developed resistance to mefenoxam.  As such, Ridomil Gold SL and other products containing mefenoxam only, may not be effective in the control late blight in Idaho in the 2020 growing season.  Their use is not recommended for late blight control until we have collected and tested enough P. infestans isolates to determine whether isolates involved in any 2020 outbreak are resistant or sensitive to mefenoxam.  If they are sensitive, then mefenoxam-based products will be an effective tool.  Ridomil has been shown to have curative properties and may halt a late blight epidemic if it is applied early enough in an outbreak (less than 1% field infection). If you are using mefenoxam based products in a program for control of pink rot and pythium leak you should continue to do that as there are no know issues with resistance in these pathogens in Idaho.  Table 2, shows suggestions for late blight control using protectant, systemic and semi-systemic fungicides under different late blight risk conditions prior to any reported late blight outbreak. For more information on a mid – late season spray program to manage late blight click here (http://bit.ly/1KWFt6a).

Table 1. Common potato fungicides registered for control of late blight in Idaho.  This list does not include seed or in-furrow treatments and is not comprehensive. For a more complete list click here (http://bit.ly/1TXPSDr).

Table 2.  Suggestions for late blight control using protectant, systemic and semi-systemic fungicides under different late blight risk conditions in susceptible potato varieties, assuming that late blight has not been found in the state.^‡