ANTHRACNOSE: CAUSATIVE AGENT, SYMPTOMS, CONDITIONS, TREATMENTS - BIOLOGY - 2024

Anthracnose is a very common fungal disease in tropical regions that affects stems, leaves and fruits under conditions of hot and humid climate. Known as canker, canker, chasparria or burning, it is a common disease in nursery plants, agricultural crops and forest plantations.

The main symptom is recognized as a brownish spot that appears around the veins of the leaves. The lesions begin as a small circular spot, which darkens over time until it reaches total necrosis.

Symptoms of anthracnose (Gloeosporium sp.). Source: AfroBrazilian / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

The hot environmental conditions, as well as the high relative humidity with frequent rains favor their appearance. Likewise, the presence of wounds, whether caused by blows, poorly performed pruning or insect attacks, increase the possibility of infestation.

The main causative agents of this disease are certain phytopathogenic fungi belonging to the genera Collectotrichum, Coniothyrium and Gloeosporium. Its incidence reduces the useful life of plantations and limits their production, as well as can cause significant losses in productivity and quality of crops.

Causal agent

The main causative agents are ascomycete fungi belonging to the following species:

- Colletotrichum coccodes

- Colletotrichum dematium

- Coniothyrium fuckelii

- Colletotrichum gloeosporioides

- Colletotrichum orbiculare

- Glomerella cingulata

- Gloeosporium mangifera

- Destructive disc

Symptoms

Colletotrichum acutatum. Source: Robert L. Anderson, USDA Forest Service, Bugwood.org / CC BY (https://creativecommons.org/licenses/by/3.0)

Anthracnose affects the aerial parts of the plant, especially stems, branches, leaves and fruits, including the roots. The first symptoms appear as circular, sunken and brown spots, scattered on the leaf surface.

Similarly, the veins become necrotic and necrotic areas are observed along the edge of the leaflets, which is known as chasparria or burns. In case of severe infections, the plant can undergo total defoliation and progressive death.

In stems and branches, anthracnose appears as a necrotic ring that induces the death of the affected structure. This sunken, wet-looking necrosis, called canker or canker, causes wilting of the apical branches or leaf buds.

In the fruits, the first symptoms appear as small circular lesions with a gummy exudate and an oily appearance. Lesions turn brown with a yellow halo and sunken edges, eventually coalescing and spreading over the entire surface of the fruit.

The central region of the lesion, which is gray or brown in color, presents a slight subsidence, and it is common to observe the small acervuli, which contain the conidia, which are pinkish in color. As the fruit matures, the epidermis tends to soften and rot inside.

The main symptoms that a plant that is being attacked by anthracnose manifests are:

- Round dark spots on the surface and edge of the leaves.

- Rot at the level of stems and branches.

- Decay of stems, branches and leaves.

- Premature fall of flowers and fruits.

- Rounded spots and dark tones on the fruits.

Conditions for its development

The phytopathogenic fungi that cause anthracnose infest more than 65 types of hosts, including ornamental, horticultural, fruit and forestry plants. Despite being a weak pathogen, they can survive in soil or on decaying plant material for a long time.

A high relative humidity and average temperature between 20-30 ºC favors the development of the fungus on the host. Conidia and sclerotia that remain in the soil can easily spread to the host through rainfall or overhead irrigation.

Once installed on the branches, leaves or fruits, the inoculum penetrates the tissues directly or enters through physical injuries. Roots are infested when inoculation levels in the soil are very high and plants are weak due to nutritional deficiencies.

Treatment

The treatment and control of anthracnose is supported by good agronomic management. The following measures are the most recommended to prevent any type of infestation of this disease:

- Disinfection of the land before sowing: the use of ecological fungicides or the application of chemical products that prevent the development of fungi once the plantation is established is recommended.

- Efficient drainage: good soil preparation enables the soil to be conditioned so that drainage improves and the accumulation of moisture that may favor the development of the pathogen is avoided.

- Define separations between crops and plots: in areas of high temperature and frequent rainfall, it is advisable to establish planting frames. This way, contamination between infected plots or crops and adjacent areas is avoided.

- Weed control: many weeds that develop around crops are hosts for the fungi that cause anthracnose. Hence the importance of eliminating weeds to prevent any type of infestation by phytopathogenic fungi.

Anthracnose in tomato (Colletotrichum coccodes). Source: Clemson University - USDA Cooperative Extension Slide Series / CC BY (https://creativecommons.org/licenses/by/3.0)

Prevention

- Elimination of contaminated plants: once a source of anthracnose infestation has been detected within a crop, it is advisable to eliminate and burn the remains of said plant.

- Avoid wetting the foliage: under conditions of high temperature and relative humidity the application of sprinkler irrigation is not recommended. The drops of water on leaves, stems or fruits are the ideal medium for the development of fungi that cause anthracnose.

- Crop rotation: crop rotation is a very beneficial measure to prevent the spread of phytopathogenic fungi in a crop field. In addition, it allows to reinforce the presence of beneficial microorganisms in the soil, maintaining control of fungi that cause diseases.

- Disinfection of work tools: disinfection with alcohol or any other commercial disinfectant of work tools is essential to prevent the spread of the disease. Indeed, pruning, cutting or grafting tools must be disinfected every time they are used in the field.

- Eliminate the remains of previous harvests: every time a new crop is established, it is important to eliminate the residues of the previous plantation. This practice can be carried out through controlled burning, in order to eliminate possible propagules in plant waste.

- Finally, it is recommended that all the material to be propagated come from certified seeds and apply postharvest fungicidal treatments.

Anthracnose in cucurbits (Colletotrichum orbiculare). Source: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org - / CC BY (https://creativecommons.org/licenses/by/3.0)

Plants in which anthracnose is common

Avocado

The causative agent of anthracnose in avocado are the ascomycetes Colletotrichum gloeosporioides and Colletotrichum acutatum. The main symptoms are observed in branches, young shoots, buds, leaves and flowers, the incidence being greater in the fruits.

Rounded, depressed spots develop on the fruit, without defined edges and of different size, initially brown or light brown, then black. In the center of the lesions, the reddish cervules can be seen, the lesions tend to grow, join and cover the surface of the fruit. They even cause pulp rot.

Almond

The causal agents are the ascomycete fungi species Colletotrichum gloeosporioides and Colletotrichum acutatum, with a higher incidence of C. acutatum. The anthracnose of the almond tree affects leaves, buds, flowers and fruits, being frequent the presence of circular and depressed lesions in the tender fruits.

The first infestations occur during fruit set, coinciding with high temperatures and the rainy season. The affected fruits tend to mummify and remain attached to the branches, parallel to this, the branches and leaves dry out, causing the progressive death of the plant.

Citrus

Anthracnose in citrus fruits is caused by the phytopathogenic fungus Colletotrichum gloeosporioides. They generally affect ripe fruit on a wide variety of citrus fruits, from oranges and mandarins to lemons and grapefruits.

The main symptoms are manifested as depressed and dry spots with well-defined margins inside which are observed black scores or acervuli. The largest infestations occur on leaves and fruits that present physical damage caused by pruning, blows and insect bites.

Bean

The phytopathogenic fungus Colletotrichum lindemuthianum is the causal agent of anthracnose in various species of the genus Phaseolus, but mainly in Phaseolus vulgaris. The symptoms are observed in stems, leaves, pods and seeds, lesions that vary from reddish to blackish are observed on the leaves.

On the branches and stems, the lesions are oval, depressed and dark in color. The pods show lesions or rounded purple spots, even the seeds show a small lesion when they have been infested.

Anthracnose in beans (Colletotrichum lindemuthianum). Source: David B. Langston, University of Georgia, Bugwood.org / CC BY (https://creativecommons.org/licenses/by/3.0)

Mango

The causative agent of anthracnose in mango is the ascomycete Gloeosporium mangifera. The disease affects leaves, flowers and fruits, remaining latent in the tender fruits and develops when they begin to ripen.

The lesions initially appear on the skin, as the disease increases, they invade and darken the pulp, limiting its commercial quality. Usually the infestation radiates as small lesions from the peduncle towards the apex of the fruit.

Olive

The olive tree is attacked by the species Colletotrichum gloeosporioides and Colletotrichum acutatum, which cause the drying of the branches and the rottenness of the olives. The highest incidence occurs on ripe fruits, when the relative humidity is higher than 90% and there are scattered rainfall cycles.

Olive fruit affected by anthracnose. Source: RGimilio / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Cucumber

In cucurbits, such as cucumber, the main causative agent of anthracnose is the ascomycete fungus Colletotrichum orbiculare. Symptoms on cucumber leaves appear as brown spots of a more or less rounded shape and a wet appearance.

Young leaves can become deformed and present necrotic spots that cause burning of the leaflets. The petioles and stems show shallow and elongated light brown lesions. On the fruits, circular, sunken and brown spots are formed with the acervules in the central zone.

Tomato

Tomato anthracnose occurs in very ripe fruits that are in contact or close to the ground. The causative agent is the ascomycete fungus Colletotrichum coccodes. The first symptoms appear as circular and sunken lesions of 10-12 mm on the surface of the fruits.

The area around the lesion is light in color and with a granular texture. In the center of the lesion, the small acervuli that contain the conidia are observed that are released under conditions of high humidity. Usually the pulp of the fruit rots.

References

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