Powdery mildews, also known as oidium or white mildews of plants, are a group of phytopathogenic fungi belonging to the Erysiphaceae family. They are characterized by producing a whitish powder, which characterizes them, on the surfaces of infected plants. These fungi affect a wide variety of plants, including agricultural and ornamental crops, and are very easy to develop resistance to phytosanitary products, which makes them a real concern.
# # # Symptoms of infection # # #
The symptoms of a powdery mildew infection are quite characteristic and may include:
White spots or powder: The most obvious symptom is the appearance of white powder or spots on the surface of leaves, stems and sometimes on fruits. This powder is actually the visible part of the fungus, the spores or conidia. When we see it, the fungus already has the advantage and is established in the plant.
Stunted growth: Infected plants may show slower growth than normal as the fungus takes nutrients from the plant to feed on as well as hindering photosynthesis.
Leaf deformation: It is rare, but sometimes the leaves may become yellowed, curled or deformed.
Defoliation: In severe cases, infected leaves may drop prematurely.
Affected Fruits: In fruit plants, the fruits may show spots and develop irregularly.
Secondary infections: infections caused by powdery mildew favor the appearance of opportunistic pathogens since the plants are weakened and the defenses are inhibited by the fungus, which opens the door to other pathogens.
# # # Development of infection # # #
Powdery mildew infection develops in conditions of high humidity (60 - 90%) and moderate temperatures (10 - 30ºC). The life cycle of the fungus includes the following stages:
Spore Deposition: Infection begins when a powdery mildew spore is carried by the wind and deposits on the surface of the host plant. Once deposited on the leaf surface, the spore must recognize a suitable environment to germinate. This recognition is based on chemical and physical signals from the environment.
Spore Germination: When conditions are favorable (high humidity and moderate temperatures), the spore germinates and emits a germ tube. This tube is an elongated structure that grows on the surface of the leaf in search of a suitable place for penetration.
Development of the Appressorium: The germ tube forms a specialized structure called the appressorium. The appressorium adheres firmly to the leaf surface using an adhesion matrix, which ensures a stable connection with the plant epidermal cell. This structure is crucial for effective penetration of the fungus.
Cell Wall Breakdown: The appressorium generates mechanical pressure and secretes lytic enzymes that degrade the plant cell wall at the point of contact. These enzymes, such as cellulases and pectinases, break down the structural components of the cell wall, facilitating the penetration of the fungus.
Formation of Haustorium: After breaking the cell wall, the fungus penetrates the plant cell and forms a structure called haustorium. The haustorium is a specialized bulb-shaped structure.
This structure allows the fungus to extract nutrients from the host efficiently without destroying the cell completely, creating a sink effect, drawing nutrients from the leaf to the place where the fungus is growing. In addition, it secretes proteins called effectors that inhibit the plant's defense mechanisms, hide the pathogen and take control of the plant cell.
If you want to know more about the haustorium, effectors and ways of evading pathogens, we leave you this article
Propagation and Reproduction: Once the haustorium is established and begins to extract nutrients, the fungus can grow and expand. In this phase, it produces more spores on the surface of the leaf, forming the characteristic white powder that we see with the naked eye. These new spores are released and dispersed by the wind, starting the infection cycle in other parts of the plant or in nearby plants.
Resistance and survival: During cold months or adverse conditions, powdery mildew can form resistant structures called cleistothecia (sexual spores) that can survive until conditions are again favorable for germination and reinfection.
# # # Most important species of powdery mildew and plants it infects # # #
There are a multitude of different species of powdery mildew that affect different plants. Some species are host-specific while other species infect a wide variety of plants. Some of the most common and relevant that we can find are:
Podosphaera xanthii and Erysiphe cichoracearum: They mainly affect plants in the Cucurbitaceae family, such as pumpkins, cucumbers, zucchini and melons.
Sphaerotheca pannosa: It causes powdery mildew in roses, one of the most common problems in these ornamental shrubs.
Uncinula necator: It is the species responsible for vine powdery mildew, seriously affecting vineyards and grape production.
Blumeria graminis: It attacks cereals such as wheat, barley and oats, being a significant threat to these crops.
Podosphaera leucotricha: It mainly infects apple and pear trees.
Podosphaera tridactyla: This species attacks stone fruit trees, especially peach, almond, plum, cherry, apricot and nectarine.
Laveillula taurica: Unlike many other powdery mildews, it can infect a wide variety of plants. Vegetables such as tomatoes, peppers and onions, cereals and legumes such as chickpeas, sunflowers and beans, tobacco and cotton plants and ornamental plants such as gerberas, chrysanthemums, petunias and dahlias.
# # # Control and treatment of Powdery Mildew # # #
There are several approaches to managing this disease, including chemical treatments, biological control, and integrated control strategies.
CHEMICAL TREATMENTS
Chemical treatments are a common and effective way to control powdery mildew. However, they should be used with caution to avoid the development of resistance.
Systemic Fungicides:
-Triazoles (e.g., penconazole, tebuconazole, difenoconazole): They act by inhibiting the synthesis of ergosterol, a vital component of the fungal cell membrane.
-Strobilurins (e.g., azoxystrobin, trifloxystrobin, pyraclostrobin): They inhibit the mitochondrial respiration of the fungus, preventing energy production.
-Pyrimidyl ethyl benzamides (e.g., fluopyram): Inhibits mitochondrial respiration of the fungus by blocking electron transport in the respiration chain at the level of succinate dehydrogenase (Complex II - SDH Inhibitor)
-Pyrazoles (e.g., fluxapyroxad): Inhibits succinate dehydrogenase in complex II of the mitochondrial respiratory chain, inhibiting spore germination, growth of germ tubes and mycelia of the target fungal species.
-Pyridine-carboxamide (e.g. boscalide): Inhibits the enzyme succinate ubiquinone reductase (SDHI) in complex II of the electron transport chain in the mitochondrial membrane. It prevents the development of the fungus by depriving the cells of their energy source.
Authorized treatments against powdery mildew:
-AZOXYSTROBIN 20% + DIPHENOCONAZOLE 12.5%
-AZOXYSTROBIN 25%
-DIFENOCONAZOLE 25%
-FLUOPYRAM 40%
-FLUXAPYROXAD 7.5% + DIFENOCONAZOLE 5%
-PENCONAZOLE 20%
-PIRACLOSTROBIN 6.7% + BOSCALIDE 26.7%
-TEBUCONAZOLE 25%
-TETRACONAZOLE 12.5%
-TRIFLOXISTROBIN 50%
Contact Fungicides:
-Sulfur: Effective against powdery mildew and relatively inexpensive. It acts as a contact fungicide, drying out the fungal cells.
-Neem Oil and Horticultural Oils: These products interfere with the respiration and metabolism of the fungus.
-Formulated with EUGENOL, GERANIOL and THYMOL: These compounds have insecticidal, insect repellent, fungicide and bactericidal properties.
BIOLOGIC CONTROL
Biological control uses living organisms to reduce the population of powdery mildew pathogens.
Microbial Antagonists:
-Trichoderma spp.: Beneficial fungi that parasitize powdery mildews and compete for nutrients and space.
-Bacillus subtilis and Bacillus amyloliquefacines: These bacteria are the biocontrol agents par excellence. These bacteria act against powdery mildews by inhibiting their growth through the production of natural antibiotics and lipopeptides, competing for space and nutrients and stimulating the plants' immune system.
-Ampelomyces quisqualis: is a fungus that has the ability to colonize the mycelium of powdery mildews, competing with it for nutrients and space. In addition, it produces enzymes and metabolites that inhibit the growth of the pathogen.
Beneficial Insects:
-Predatory mites (e.g., Amblyseius swirskii): They feed on the spores and mycelium of powdery mildew.
Resistance Inductors:
-Potassium Phosphite: Promotes the plant's natural defenses, making it less susceptible to infections.
-Laminarin: Stimulates the natural defense mechanisms of plants.
# # # The Problem of Resistance Development in Powdery Mildew # # #
The development of resistance in powdery mildews is very common and has become a major challenge for agriculture and horticulture. The ability of these fungi to quickly adapt to fungicides has led to the appearance of resistant strains and the cancellation of phytosanitary compounds due to the loss of effectiveness, which makes control difficult. Resistance to phytosanitary products can develop through different mechanisms, including genetic changes that affect the sensitivity of the fungus to fungicides.
Genetic Mutations: Changes in the genes that encode the target proteins of the fungicide can reduce the affinity of the fungicide for its target, decreasing its effectiveness.
Gene Overexpression: Increasing the production of enzymes that degrade or expel the fungicide out of the cell can neutralize its effect.
Metabolic Alterations: Changes in the metabolic pathways of the fungus can reduce the accumulation of the fungicide in the fungal cells.
Alternative metabolic routes: some fungi have alternative routes for the synthesis of a certain protein so that the inhibition of one of the routes does not affect the development of the fungus. Some of these routes remain unknown.
# # # What is Cross Resistance? # # #
Cross-resistance occurs when a fungus that has developed resistance to one fungicide also shows resistance to other fungicides, often due to similar mechanisms of action. There are two main types of cross resistance:
-Positive Cross Resistance: Here, resistance to a fungicide confers resistance to other fungicides that have a similar mechanism of action. For example, if a fungus develops resistance to a fungicide that inhibits ergosterol synthesis, it may also become resistant to other fungicides that affect the same biosynthetic pathway.
-Negative Cross Resistance: In this case, resistance to one fungicide increases sensitivity to another fungicide with a different mechanism of action. This phenomenon is less common but can be exploited in resistance management strategies.
# # # Strategies to Avoid the Emergence of Resistance # # #
To prevent the development of resistance in powdery mildews, it is crucial to implement integrated management strategies that include a variety of approaches:
Fungicide Rotation: Using fungicides with different modes of action in rotation can reduce selective pressure on fungi and minimize the development of cross-resistance. It is important not to use products from the same chemical group consecutively.
Mixture of Fungicides: Applying mixtures of fungicides with different mechanisms of action can be more effective than using a single product. This makes it difficult for fungi to develop simultaneous resistance to both fungicides.
Use of Biological Fungicides: Incorporating biological fungicides and biological control methods, such as the use of microbial antagonists (beneficial bacteria and fungi) that inhibit the growth of powdery mildew, help reduce the continued use and dependence on chemical fungicides.
Correct use of fungicides: The application of the amount and doses indicated according to the crop and the species of powdery mildew is vital for the fungicides to act correctly and avoid the appearance of resistance.
# # # How are Phytosanitary Resistances analyzed? # # #
The identification and analysis of resistance to phytosanitary products in powdery mildews and other pathogens is essential to develop effective management strategies and reduce the use of chemical fungicides. This process can involve both traditional phenotypic assays and advanced molecular techniques.
In Vitro Tests: These consist of plate growth tests or spore germination tests in culture media containing different concentrations of fungicides. This allows the effectiveness of the fungicide to be determined.
In Vivo Tests: These tests are done on infected plants. It is the type of test that is used on powdery mildews since they do not grow in artificial culture media. Different concentrations of the fungicide are applied to infected plants and the evolution of the infection is observed.
Molecular Techniques: Molecular techniques allow precise identification of the genetic mechanisms responsible for resistance and can be faster and more specific than phenotypic assays. Some of the most used techniques are PCR, qRT-PCR (real-time PCR) and sequencing.
In summary, powdery mildews, also known as powdery mildews or powdery mildews, represent a significant threat to a wide range of agricultural and ornamental crops. Its characteristic symptoms, which include white powdery formation on leaves and delayed plant growth, are early warning signs of infection. Furthermore, the development of resistance to chemical fungicides presents an additional challenge in the control of this disease, which requires integrated management that includes biological control strategies, rotation and mixing of fungicides, and correct use of chemical treatments. Knowing this disease, the symptoms, the resistance mechanisms and the implementation of surveillance controls are essential to prevent the appearance and spread of powdery mildew.
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