Late Blight in potatoes

What is Potato Late Blight?

Phytophthora infestans, more commonly known as Potato Late Blight (PLB) is one of the oldest recorded plant diseases in human history. It is  generally spread by briding between seasons on tubers as volunteers, unharvested crop, cull piles or home saved/commercial seed. It may also be soilborne in sexually derived resting bodies called oospores, though in practice the pathogen population faced at the outset of one season is comprised of the strains which ended the previous one. 

During early infection, PLB is made visible by brown, moist lesions on the green leaf tissue, often outlined with the typical white mildew layer on the underside of the leaf, typical of Oomycetous diseases. 

This white layer is predominantly made up of sporangia; asexual spores that are fundamental to the primary stage of infection and indirectly to secondary infections. 

At this point, PLB can spread systemically within the plant and via the production of prodigious numbers of spores. These are carried by air currents or splashed onto neighbouring plants, germinating to infect tissue directly or via the release of mobile zoospores that spread over the leaf surface, depending on temperature. Indeed it is the production of motile zoospores during the lower temperature period in the latter part season which may be washed into the soil and can enter tubers via lenticels or weak spots such as wounds. 

Here, PLB sporangia can overwinter and survive until the next season, or cause significant problems in storage.

Both Zorvec™ active and Cymoxanil (Option) have curative activity, providing strong antisporulant activity as a result. Additionally, by targeting the pathogen early in the epidemic it will revent the build up of spore numbers and more effectively prevent PLB sporangiaphores being produced at the leaf surface.

The conditions favouring P.infestans are those used in the Decision Support System (DSS) behind the ‘Hutton Criteria’ is met. A Hutton Criteria occurs when two consecutive days have both a minimum temperature of 10°C and at least six hours of relative humidity (RH) at or above 90%. This is calculated by looking at the relative humidity at or greater than 90% and the minimum temperature, on each of two days.

• If RH > 90% occurs for 5 hours on either day, or the minimum temperature is 9°C on either day then a near miss is illustrated on the map, as the full criteria isn't met for Hutton.

Source: https://blightspy.huttonltd.com/#/

It is worth noting that the weather data is collected from standard meteorological sites across the potato growing regions of the UK and while an invaluable guide, will not reflect the exact conditions which arise within the canopy of a specific area of a field crop.

Why is Late Blight a problem?

• The pathogen can cycle very quickly once infection sets in and the symptoms can appear in as few as 4-5 days under the right conditions. With PLB sporangia being capable of overwintering, if infected tubers are stored, the disease can further develop there, leading to store breakdown and huge losses. 
• The main method of fungal dispersal is through air-borne sporangia from already infected crops, volunteer potatoes and cull piles.
• In recent years, PLB has become more agressive and harder to control through the evolution of strains which produce ever greater spore numbers and cycle faster, as well as evolving mutations which confer resistance to some fungicide modes of action. It is for this reason that advice to alternate modes of action and maintain spray intervals as well as mixing active ingredients at each application are now strongly advised. See Corteva's Best Practice Advice for more information.