Alternaria in agricultural crops: symptoms, prevention, and complete treatment

Introduction to alternariosis in crops

Alternaria is one of the most significant and destructive fungal diseases that can affect a wide variety of agricultural crops. It is caused by different species of the genus Alternaria, among which stand alternaria alternata y Alternaria SolaniIts presence represents a significant challenge for farmers and producers, as it can cause significant losses in crop yield, quality, and commercial value when environmental conditions are favorable for its development.

Alternariosis can attack from the beginning of the crop cycle until the storage of the crops., so it is essential to thoroughly understand its symptoms, mechanisms of spread, the factors that facilitate its spread, as well as effective prevention and treatment strategies.

What is alternariosis and how does the Alternaria fungus act?

Alternariosis is a disease caused by phytopathogenic fungi belonging to the genus Alternaria. These species develop a purely asexual phase and survive mainly as conidia (spores) and mycelium, which can overwinter for several years in the soil or on plant debris. In this way, Alternaria can remain dormant for long periods and cause primary infections when favorable conditions exist.

The appearance of alternariosis is especially frequent in humid environments and moderate to warm temperatures, typically in the range of 20 to 30°C. The fungus grows on dead organic matter, crop debris, and fallen leaves, from where it releases spores that can be transported long distances by wind, water (rain and irrigation), or by insects and birds.

One of the main dangers of Alternaria It is its high capacity to rapidly colonize plant tissues, expand to different organs and spread to other neighboring crops if appropriate management measures are not taken.

Risk factors and conditions that promote alternariosis

• Humid and warm environmental conditionsAlternaria thrives when humidity is high and temperatures remain temperate or warm.
• Dense crops and poor ventilation: Lack of air circulation and excessive plant density cause the leaves to remain wet and increase the risk of the fungus appearing.
• Wounds on plantsAny damage to tissues caused by pests, hail, or cuts facilitates the entry of spores and infection.
• Plant debris and infected soil: The permanence of residues from previous harvests is a common source of inoculum.
• Neglect of hygiene and agricultural work: Failure to remove waste, failure to detect early symptoms, or use of contaminated plant material increases the risk of the disease developing and spreading.

Crops most affected by alternariosis

Alternariosis affects a wide range of economically important crops.Its manifestation and severity depend on the species, variety, and environmental conditions, although some crops are more severely affected by the disease:

• TomatoAlternaria, also known as early blight, is one of the most common tomato diseases. It can attack leaves, stems, and fruit.
• Potato: Early blight of potato caused by A.solani can drastically reduce yield, affecting both the leaves and occasionally the tuber.
• Broccoli and other cruciferous vegetables: It manifests as small spots that enlarge on leaves and stems, compromising the quality and quantity of production.
• Mandarin, citrus and fruit trees: Produces black spots on fruits, premature fall and commercial depreciation.
• Apple: Causes dark lesions on fruits and affects quality.
• Khaki: It is one of the most sensitive fruits, showing the disease even in post-harvest with severe damage.
• It is also reported in wheat, carrots, papaya, asparagus, and stone and pome fruits.

The following table summarizes some common crops and symptoms:

Symptoms and characteristic signs of alternariosis

Early recognition of symptoms is essential for making a correct diagnosis and implementing control strategies quickly. Signs vary by crop, but there are common patterns in most affected plants.

• Necrotic spots on leavesCircular, brown or black lesions, often surrounded by a chlorotic halo. On tomatoes and potatoes, the spots may present concentric rings with a bull's-eye appearance.
• Premature fall of leaves and fruits: Defoliation reduces the photosynthetic capacity and overall yield of the plant.
• Fruit spots and rot: The lesions can join and cover a significant part of the surface, causing the skin to rot while the interior remains healthy, as occurs in persimmon, pomegranate, or citrus fruits.
• Stem necrosis and structural weakening: Especially in severe attacks or in young plants.
• Tissue collapse and deathIn advanced stages, large areas may become necrotic and dry.
• Damage in storage or transport: Some infections may not manifest until after harvest, causing post-harvest losses.

In crops such as pomegranates, two types of symptoms can be distinguished: external spots and the so-called "heart rot," which affects the interior of the fruit and is detected only after harvesting.

Diagnosis and differentiation: how to identify alternariosis

Visual diagnosis is crucial, but it must be differentiated from other fungal diseases. Among the most characteristic signs of alternariosis are:

• Stains with concentric rings (in tomato and potato) dark brown or black.
• Presence of a yellow or chlorotic halo around the spots.
• Vascular necrosis on old leaves or close to the ground.
• Lesions that progressively join together, forming large necrotic areas.
• In persimmon and pomegranate, internal rot recognizable only after opening the fruit.

It is advisable to periodically inspect crops, especially during wet periods, to detect any abnormal spots or lesions.

Prevention and cultural management to minimize alternariosis

Cultural practices are the first barrier against the emergence and spread of alternariosis.Key recommendations include:

• Crop rotation: Alternating species from different botanical families reduces the accumulation of inoculum in the soil.
• Removal and burning of infected plant debris: Essential to prevent the survival and dissemination of spores between campaigns.
• Use of healthy and certified plant material: Reduces the risk of introducing the disease into new areas.
• Maintaining adequate planting density: Facilitates ventilation and drying of foliage after rain or watering.
• Avoid sprinkler irrigation, preferring drip irrigation to avoid wetting the leaves.
• Check and discard affected plants, fruits or branches during pruning or harvesting..
• Balanced fertilization to keep plants vigorous and less susceptible to fungal attack.

In addition, it is recommended to inspect the crop during the flowering and fruit development phases, as these are critical periods for infection.

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Physical control and integrated measures against alternariosis

• Ventilation and aeration suitable, especially in greenhouses.
• Removing fallen foliage and post-harvest residues to reduce sources of inoculum.
• Use of physical barriers in high-value crops to prevent the access of vectors that can carry spores.
• Disinfection of tools pruning and handling.

Integrated management that combines cultural and physical practices keeps inoculum pressure low and reduces the need for chemical treatments..

Chemical and biological treatments: when and how to apply them

The use of fungicides may be necessary in situations of high disease pressure, but it is always part of a responsible strategy.

Conventional chemical fungicides

• Contact fungicides: They create a barrier on the surface of the plant and prevent spore germination (e.g. copper oxychloride).
• Systemic fungicides: They are absorbed by the plant and protect it internally (e.g. propiconazole, thiophanate methyl, azoxystrobin, pyrimethanil).

It is essential to alternate active substances and respect the doses and time of application. To prevent the development of resistance, preventative applications are usually recommended before flowering and at the onset of the first symptoms.

Biological control and sustainable solutions

• Commercial biological solutionsThere are innovative products, such as those based on beneficial microorganisms, that compete with Alternaria on the leaf surface, strengthen the plant's immune system, and create a less favorable environment for the development of the fungus.
• Vegetable extracts: Certain formulations based on horsetail extracts (Equisetum arvense), with phenolic compounds and saponins, have shown preventive and curative effects without leaving chemical residues.

An example is the application of biological agents via foliar application at regular intervals, especially during critical stages, always under conditions of low solar radiation to maximize their effectiveness.

Additional preventive measures to avoid alternariosis

• Keep the soil well-drained to avoid water stress and the development of cracks in lesions and fruits.
• Have early warning and detection systems: Surveillance and monitoring allow for rapid action when the first symptoms appear.
• Carefully sort and select the fruits after harvesting to prevent infected specimens from infecting the rest during storage.
• Harvest at the right time to minimize injury and pathogen entry.

Specific cases: Alternariosis in persimmon, pomegranate and post-harvest

Persimmon and pomegranate have particularities in the manifestation of alternariosisIn persimmons, the disease can remain superficial and asymptomatic until postharvest. Once stored, the fruit develops superficial black spots, cell collapse, and rot, severely affecting its commercial value.

The main difficulty lies in the fact that it is possible to harvest apparently healthy fruit that already contains the developing fungus. Therefore, postharvest management must include rigorous selection, removal of damaged fruit, and, if necessary, low-toxicity fungicide treatments such as the use of approved fumigant fungicides in storage chambers.

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Alternaria is a persistent threat to a variety of economically important crops. Its versatile nature, ability to persist in the environment, and potential to cause damage at various stages of production require comprehensive and proactive management based on prevention, constant monitoring, and the responsible use of chemical and biological tools. Adopting adapted agronomic practices, investing in soil health, and maintaining agricultural biodiversity are essential steps to minimize the impact of this disease and ensure the long-term sustainability of agricultural production.