“In recent years we have witnessed an evolution in the diseases that we are regularly confronted with,” says Thierry Langin, research director at CNRS and head of the cereal disease team at UMR GDEC in Clermont-Ferrand.
On the one hand, new strains of known diseases are emerging that are better adapted to changing climatic conditions and, on the other hand, there is a high risk of emerging or re-emerging diseases for which control strategies are limited or non-existent.
The bad news is that these unusual diseases appear to be more devastating than the ones we are familiar with. In any case, they have a bigger potential economic impact for farmers.
This first observation, quite alarmist, is not the result of climate change alone. While it obviously plays an important role in the evolution of diseases, strains and their respective virulence, we must not forget the impact that changes in farming practices have had on the evolution of pest pressure. "The ban on active substances and the agroecological transition are also contributing to the evolution of pathogens raging in fields across France. “
Water stress leading to more fragile physiological conditions
“The more stressed and weakened a plant is, the greater the likelihood that it will be impacted by pests/diseases, affirms Gwénola Riquet, head of the disease unit at Terres Inovia. Climate change, which has brought drier and hotter Springs as well as late frosts, has contributed to crop stress during part of their cycle.”
The impact of water stress on crops is also very well documented by Alain Kleiber, plant nutrition expert at the Auréa laboratory. At the Agroforum organized by the Agora cooperative at the beginning of February, he said :
"We have been evaluating the evolution of the mineral content of field crop leaves since 1985 through analyses conducted between March and May, i.e., almost exclusively on small grain cereals. The nitrogen and iron content in plants has been gradually decreasing over the past 35 years. However, this decline is not significant. On the other hand, the decline in potassium, manganese and boron is significant.For potassium, for example, the decrease has averaged 3 to 4 % per year over the last 35 years. However, these last three elements, plus nitrogen, are taken up by the plant passively, i.e., through the water absorbed by the plant. This mechanism accounts for 65 to 85 % of these minerals which makes them very good indicators of water flow in the plant.”
Their downward trend over the past 35 years is largely due to climate change, especially water stress during the sampling period. A lower concentration of certain minerals can lead to natural or induced deficiencies and imbalances between different elements which can explain why a plant is less resistant to pests/diseases.
According to Gwénola Riquet, "it is therefore essential to first give the plant the best conditions to offset the impact that climate change has on exposing crops to pests. This involves careful planning (sowing dates, structural status of soil, etc.)”.
This work is all the more essential as the transitions between weather episodes are often more pronounced than before. Intense heat and dry conditions often appear very quickly. Rain can also come in the form of torrential downpours.
Milder Winters favor inoculum
Winter weather is also undergoing major changes. In recent years, Autumns and Winters have been mostly mild with significantly fewer frost days. Moreover, Serge Zaka, an agroclimatologist at ITK, says that "frost days are becoming increasingly rare… between 1959 and today, the number of frost days has declined by 50% and it appears that this trend will continue through the year 2100. Some parts of France may have already experienced their last frost”.
This means that Winters will be increasingly mild. This is good news for household energy consumption, but much less so for vernalization, which is essential for many crops, and the evolution of pest/disease pressure on them.
“With the milder Winters that we have been experiencing and the ones to come, we will have to pay particular attention to viral diseases, says the CNRS research director. The insect vectors of these viruses prosper in mild Winters. If their cycle is not interrupted in Winter, insect populations at the end of the cold season will be much more abundant leading to more virus inoculum and therefore a higher risk of an epidemic associated with an earlier impact on crops. If a seedling is affected early, the damage may be significant. A physiologically weakened plant is less able to protect itself and complete its cycle. Also, the early onset of a disease gives it more time to multiply over several generations. The more the pathogen is given the opportunity to multiply, the greater the risk of more aggressive strains appearing by means of genetic mutation.” Mild winters are therefore formidable allies of crop disease.
Late frosts also do damage
Mild winters have another potentially adverse effect on crops: they accelerate the development cycle generating water and heat stress in early Spring and Summer.
This acceleration of the development cycle means that a late frost can happen when plants are more fragile. While a late frost can harm crops, it can also promote the development of disease.
Gwénola Riquet, from Terres Inovia, takes the example of pea blight on winter peas: "It has long been known that the disease is always more prevalent on cold-sensitive varieties after winter frosts. Today it has proved to be a problem during Spring frosts. While plants are turgid and therefore more sensitive to frost, freezing temperatures weaken it making it more susceptible to bacterial infection by blight. And as this is a repeating phenomenon, the inoculum increases and so does the disease.”
While one could expect that more frequent droughts and higher temperatures would lower the impact of crop disease, climate change is introducing new types of pressure never before experienced.
