Fungi are the cause of many diseases in plant pathology, which are often called awkwardly "cryptogamic diseases". This generic term evokes hidden reproduction and relates mainly to the order of fungi, but also to that of algae and that of protists (single-celled organisms). It would probably be more accurate to speak of fungal diseases or plant yeasts when talking about fungi.
Notions of mycology
The vegetative apparatus of fungi, called thallus, consists of mycelium, that is to say of generally branched filaments, compartmentalized or not, on which the sexed or asexual organs are differentiated. It is the observation under the microscope of the reproductive organs, which makes it possible to characterize and classify fungi in different families. You should know that mushrooms are unable to use carbon dioxide to synthesize the organic substances necessary for their growth. This is why they live, either as saprophytes, that is to say they take the organic matter they need from plant or animal debris, or as parasites on living organisms (plants or animals ). Finally, some species live in symbiosis, associated with algae (lichens) or higher plants (mycorrhizae). During winter, fungi keep very well in plant debris or in the soil, sometimes in the form of specialized organs, which makes their elimination very difficult.
Soil fungi, plant parasites
Many species of plant parasitic fungi live in the soil, each with varying degrees of nuisance. Here are some common examples. Collar disease and damping off The soil fungi attacking the collar of plants are very polyphagous (harmful action on a wide variety of plants). They develop in the soil, at shallow depth, and contaminate neighboring plants, causing destruction by outbreaks. At the seedling stage, the action of these fungi causes a considerable loss, it is the melting of seedlings (ex: botrytis). Fungal pathology at the level of underground organs There is a large category of parasitic fungi of the roots, bulbs and tubers, responsible for necrosis or rots. The scenario is always the same, growth slowed down or stopped, with the consequence of wilting, until the death of the plant (ex: rotten). Vascular disease or tracheomycosis Other fungi, more specific, penetrate the tissues of underground organs and seal the conductive vessels of the raw sap, with a rapid death of the affected subjects (eg fusarium wilt).
Parasitic fungi in the aerial organs of plants
This category represents the action of a large number of fungi, in the form of spores or conidia (asexual spores), transported by wind, rain or by means of various movements. Some find on the surface the substrate necessary for their development (ex: sooty mold on aphid honeydew). Others have a different mode of action and penetrate through stomata or lenticels (natural orifices for gas exchange), or else benefit from insect bites and various injuries. The observable damage from this parasitic intrusion is extremely diverse. Mention may be made of spots on the aerial organs, necrosis and premature fall of leaves or flowers, fruit rot, canker, drying of twigs, leading more or less to dieback of the plant. A good number of these pathogenic agents are at the origin of diseases whose cause is easily identifiable (ex: powdery mildew, anthracnose, blister of the peach tree…). There are less specific symptoms that can be confusing with bacterial or viral conditions that require laboratory control.
The main principles of fungal disease control
To avoid the development of plant mycoses, it is necessary to be attentive to the good cultural techniques, and to stick to common sense practices which it is useful to remember. If possible, use new equipment and tools, or at least clean and disinfected. The seeds, plants, bulbs or tubers must be healthy, or even treated preventively if necessary. Choose the varieties most resistant to disease. To promote good aeration of the plot, it will be good to work the soil well, to ensure adequate space between plants, to practice mulching ... The crop rotation must be long enough. Avoid sprinkler spraying which can contribute to the spread of fungal diseases. Drainage must be considered without reserve, each time it is necessary to reduce excess water and thus preserve the root system ... Everyone knows that plant fungi are almost always associated with excess moisture. Of course, crop waste should be removed by burning, especially if it has been contaminated. In intensive crops (nurseries, greenhouses), it is sometimes necessary to disinfect the soil by heat or by a chemical. Finally, before or during the course of vegetation, the use of a natural fungicide by spraying, such as nettle liquid or so versatile Bordeaux mixture, can be of great help. This should make it possible to avoid the last resort to the synthetic product, necessary however under precise conditions, and after consulting the phytosanitary index (ACTA). By Claire Schutz Croué