Potato late blight

Late blight is a severe disease of potato, but also of tomato, which is of particularly harmful in areas with temperate climates. It can cause extremely fast epidemics,  which can entirely destroy susceptible crops within days in the absence of control measures.

Late blight is caused by a parasitic micro-organism called Phytophthora infestans. This microbe is related to algae, which explains why water (free water, as rain droplets or dew, or saturating humidity) is needed for it to infect the plants.

Symptoms

The disease causes brown, necrotic flecks on all aerial parts of infected plants :

• leaves: figures 1 - 7
• stems: figurse 8-9
• tubers:  figures 10-11.

On the lower side of these flecks, especially under humid conditions, a white to grey mould, corresponding to the mycelium and spores of the pathogen, appears (Figures 1-3). (The spores allow the reproduction, but also the dissemination of P. infestans from plant to plant, and hence cause the epidemic propagation between plots or crops).

The flecks rapidly turn necrotic (Figures 4-7). Starting from the first infected organ, they expand more or less quickly (depending on the climate and hydric status of the plant) to neighbouring organs, until the plant succumbs to complete necrosis.

On tubers, the disease manifests itself as irregular lesions, brown to violaceous on the outer side and rusty on the inside (Figures 10-11) These lesions cause tubers to rot, and make them unmarketable and unfit for consumption. They also often serve as entry points for secondary infections by other pathogens, fungal or bacterial, which can spread within and between tubers during storage.

Favorable conditions for disease onset and spread

 Primary infections can originate from several sources : volunteer plants (figures 12_14), infected compost, refuse piles (figure 15), infected seed or plantlets, which all serve as reservoirs of primary inoculum. It is critical to detect these primary infections as soon as they appear, because disease develops usually very quickly once the pathogen gets established into a plant or plot.  

The first symptoms normally appear a few days after a rainy or high humidity (dewy/foggy) period. The pathogen grows optimally around 18/20 °C ; it is able to grow as soon as temperatures exceed  8 à 10 °C, but its growth is strongly slowed down or completely stopped when temperature exceeds  25 °-28 °C.

Controlling potato late blight

• Choose the right cultivar: always prefer resistant cultivars in areas of recurrent risk ;
• Eliminate primary inoculum sources: weed out all volunteers, do not compost infected plants or fruits, manage dumps and refuse piles through plastic covers or outright destruction ;
• Carefully handle crops:
  • avoid persistant water on leaves, stems and fruits, by watering directly at the base of the plants and using mulches to avoid water projection,  ;
  • monitor the plants for the presence of symptoms, especially in risky parts of the crops ( near hedges, in wet and/or shaded parts of plots, etc…)
  • eliminate initial infections (roguing infected leaflets or secondary stems for instance) before the disease can spread.
• Spray preventative or curative products in case of high risk and/or visible outbreaks. The risk periods can be identified and forecasted through Decision Support Systems, based on local weather conditions.