Argentina (Mendoza): Research on management strategies against late blight of potato

Late blight of potato, caused by Phytophthora infestans, remains a challenging disease for  crops worldwide, even in irrigated areas like western Argentina. Its epidemic behavior, strongly influenced by environmental conditions, affects management strategies and guides decisions to maintain crop health beyond pre-established schedules. It is a polycyclic disease, meaning it can recur multiple times within a single  growing season .

Typical symptoms begin with yellowing (chlorosis) of the leaves, usually from the edges toward the center. The spot progresses inward, leaving brown areas surrounded by a yellow halo. In severe outbreaks, plants may appear "burned," resulting in the common name "blotch."

Field studies conducted during the 2025-2026 campaign in the Uco Valley (Mendoza) provide valuable information on how the disease progresses under local conditions, when the environment becomes favorable, and what strategies can slow its advance.

The environment plays a crucial role; the development of late blight depends not only on the pathogen but also on the presence of susceptible crops and predisposing environmental conditions. Temperatures between 10 and 20 °C and high relative humidity (above 90%) for several consecutive hours accelerate epidemics.

During the 2025/26 season, temperature and relative humidity were continuously monitored to identify periods conducive to Phytophthora infestans. Data were measured at the canopy level to minimize bias, in a main production area (La Carrera, Tupungato).

Many hours were observed with temperatures between 10 and 20 °C and relative humidity above 90%, especially during February and March, mainly concentrated in nighttime and early morning hours when the foliage remains wet.

These prolonged conditions increase the risks, even without heavy rainfall. Furrow irrigation and high humidity events create microclimates conducive to infection. The accumulation of conductive hours is key to the spread of blight. Not every day is critical, but the combination of relative humidity and optimal temperatures increases the risk of disease. As the season progresses (February-March), the risk of blight increases, supporting the recommendation for early planting to avoid predisposing conditions.

The incidence, or number of affected plants, was not found to be significantly different among the treatments tested, including the control without fungicides. However, when analyzing the cumulative severity over time and the rate of disease progression, clear differences emerged between the strategies employed (with/without fungicides).

Phytosanitary strategies using fungicides significantly reduced overall disease severity and, especially, the rate of disease progression, with a nearly 30% decrease in epidemic speed compared to the control. This is key for polycyclic diseases like late blight, as reducing the rate of progression helps control the epidemic by lowering inoculum production and secondary cycles, allowing for more flexible application intervals and reducing pressure towards the end of the cycle.

From a production standpoint, no statistically significant differences were detected between treatments. The incidence in tubers was practically nil due to the good overall management of the plot and the harvesting conditions.

This text underlines a familiar but often overlooked notion: health strategies tend to have more influence on system stability and risk management than on pure performance in terms of weight, where their influence is more subtle.

Recommendations

To manage late blight, it’s essential to consider the environment in addition to the crop itself. Monitoring relative humidity and temperature is crucial for predicting critical periods, even before symptoms become apparent. If possible, using leaf moisture sensors and conducting frequent crop monitoring are also beneficial.

Furthermore, it is important to devise tactics that slow the rate of disease progression. Strategies that slow disease progression are especially valuable in prolonged cycles or during campaigns that favor the disease.

Another recommendation is to adapt decisions according to the context. In situations with a high accumulation of predisposing factors, choosing tools with proven effectiveness against severity can be decisive (from resistant cultivars to chemical fungicides).

Approach

Plant protection requires a dynamic approach and constant interaction between the environment, the pathogen, and  crop management . Incorporating health monitoring, collecting environmental data, and analyzing epidemic progress allows for a shift from reactive management to a more strategic and efficient approach.

For more information, you can contact the Phytopathology Laboratory of the EEA La Consulta INTA.

*By Pablo Caligiore Gei and Federico Fuligna, researchers at EEA La Consulta - INTA Mendoza