Soils play an important role, participating in particular in the main cycles necessary for life on Earth, such as the water cycle and the cycles of the main nutrients (carbon, nitrogen, phosphorus, etc.). They support most agricultural, forestry and pastoral production systems and participate in climate regulation, controlling greenhouse gas emissions and carbon sequestration, but also erosion and detoxification.
Read more: Why it is so important to keep our soils healthy
However, soils can also harbor plant pathogenic microorganisms, including fungi that survive from year to year as spores or mycelium and can affect plant health for several generations of cultivation.
Remote sensing tools already allow obtaining information about the health status of plants, which can be useful for better control of these diseases. Several studies have already reported the use of remote sensing tools to assess foliar (leaf) symptoms that plant diseases can cause. In the case of aerial diseases, i.e. airborne diseases, plants are contaminated by the spread of spores by wind and rain and most often show visible foliar symptoms. But in the case of soil-borne diseases, these foliar symptoms are not always visible, at least in the early stages of the disease.
In this context, we will highlight how remote sensing techniques can offer an interesting perspective to study soil phytopathogenic microorganisms to ensure a more sustainable and ecologically correct agriculture.
Detect soil pathogens to better control them
Remote sensing is a method of collecting and analyzing data, without direct contact between the object being analyzed (plants and fields, in our case) and the instrument used (for example, satellites, cameras on board drones). Remote sensing detects the radiation emitted or reflected by the object under study; and “spectrum-imaging” is a remote sensing technology that allows you to image an object in various spectral regions ranging from 400 to 2,500 nanometers, that is, at wavelengths that may be outside the visible and that can sometimes provide more accurate information about plant health than visible wavelengths studied in isolation.
Remote sensing techniques are non-destructive: “spectral” images can capture biochemical and biophysical parameters of plants and detect changes in photosynthetic activity, pigments, water content or leaf structure caused during pathogenesis without having to destroy the plant. In fact, plants respond to infections in various ways, for example by wilting, yellowing or sometimes necrosis, and therefore in some cases it is possible to obtain a specific “spectral signature” for a given plant disease.
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For example, the presence of late blight in potato plants, caused by the soil pathogen Phytophthora infestans, can be detected using hyperspectral measurements, even before these plant disease symptoms were visible to humans. The researchers were able to distinguish between two potato pathogens that show similar symptoms. Phytophthora infestans (downy mildew, soil disease causing foliar symptoms) and alternaria solani (mildew, air disease).
Even when there are no visible symptoms
Remote sensing can be particularly interesting in the case of detection of a pathogen that does not show any visible symptoms in the plant. This is often the case for soil-borne diseases, which induce problems with physiological functioning, but not necessarily foliar disease symptoms or aerial symptoms.
Read more: New technologies to better detect plant diseases
For example, spectral imaging has been used to detect water stress induced by the pathogen Rhizoctonia solani in sugar beet fields. Water stress is caused by the abundant presence of fungal mycelium in the conducting vessels of the plant. The results demonstrated that remote sensing, combined with geographic information systems technologies, can be used effectively for the detection and mapping of stress symptoms caused by rhizoctonia crown and root rot.
A multispectral system (which acquires data at different wavelengths) has detected, for example, head burn (scab) caused by the pathogen Fusarium in winter wheat, a soil-borne pathogenic fungal microorganism and producing a highly toxic toxin more accurately than a system operating on a single wavelength. The authors showed that these techniques were able to distinguish between uninfected and infected control plots.
Thus, symptom mapping with these tools can be particularly useful to delineate sites of infection in the case of these soil diseases that are generally not very capable of propagating in soil.
Early detection of wheat Fusarium infection in seeds was also achieved using a hyper and multispectral imaging system under laboratory conditions. In this case, the remote sensing technique makes it possible to detect microorganisms that pose health risks through the production of mycotoxins, both for humans and animals. The use of these tools could facilitate the future traceability of food. It can be predicted that early detection could, for example, make it possible to harvest infected wheat grains separately from healthy wheat grains.
Detect the action of microorganisms harmful to plant pathogenic microorganisms
Spectral imaging can also offer possibilities for early detection of the action of microorganisms harmful to phytopathogenic microorganisms – these are called “bio-controls”. They could serve as an alternative to chemical control.
Recently, hyperspectral imaging has been used to detect the effect of a microscopic fungus with a biocontrol effect, Trichoderma, on phytopathogenic fungi, measuring the beneficial effects that the biocontrol provides in infected plants thanks to its “spectral signatures” (characteristic and easily identifiable). Trichoderma activity was evaluated against the soil pathogens responsible for rhizoctonia in wild arugula and sclerotinia in green and red lettuce. The authors were able to show different spectral signatures between healthy, infected and bioprotected plants.
Better understand how pathogens act on plants
In some cases, spectral imaging can even help to track the mechanisms of pathogenesis, that is, the process by which a pathogen acts on the organism and determines a disease. For example, the response of cucumber leaves to fusaric acid (a mycotoxin produced by the pathogen Fusarium) can be studied using thermal imaging.
By detecting these diseases earlier or by better understanding how they work, we can better control them by practicing crop rotations, for example, or treating them, but in a more localized and rational way.