Anthracnose is a general term for a group of fungal diseases characterized by the appearance of dark, sunken lesions or spots, blights, and cankers.
It is a common disease that affects many species and causes major economic losses worldwide. The disease affects economically important plants such as cereals, fruits, various vegetables, ornamental plants, and medicinal plants.
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Causal Agent of Anthracnose
More than one fungal genera are able to cause the disease. The most important genera include- Colletotrichum sp., Gloeosporium sp., and Discula destructiva.
Colletotrichum sp is the asexual stage, is most abundant in tropical and subtropical regions, and causes major crop losses. This pathogen belongs to the order Melanconiales under the class Deuteromycetes. The sexual stage of this fungus is Glomerella sp. which plays a minor role in the disease cycle.
Gloeosporium sp. infects woody plants and causes symptoms similar to those of Colletotrichum sp. Discula destructiva. Infect dogwood plant.
Each pathogen has a different range of hosts and generally thrives in hot and humid environmental conditions.
Symptoms of Anthracnose
The disease can affect various plant parts such as leaves, stems, flowers, and fruits and shows symptoms such as spots, lesions, cankers, etc.
Spots– It is the most common symptom and usually dark brown or black in color with a sunken appearance. They are usually small in the beginning, which later increases and merges to cover the whole infected area. The shape may be circular or irregular and in some cases, they are surrounded by yellowish halo. The spots usually cause wilting and withering of infected plant parts. In severe conditions, all the leaves or sections of the leaves may die, a phenomenon known as blighting.
Lesions– Anthracnose causes large sunken lesions on fruits, which may enlarge, leading to fruit rot. This lesion usually affects the tip portion, which gradually increases and spreads inwards. It may sometimes lead to fruit deformation.
Cankers- Anthracnose also leads to cankers and stems on twigs cutting off nutrient flow and causing dieback.
Defoliation- In ornamental plants, anthracnose leads to defoliation during the growing season, which leads to weakness and increases susceptibility to other pests and pathogens.
Disease Cycle of Anthracnose
Overwintering phase
During winter, the fungal mycelium remains dormant in the debris such as fallen leaves, twigs, and infected parts of the living plants. With the onset of the spring season, the mycelium starts growing and produces asexual spores called conidia. These spores are dispersed by wind, rain, etc. Conidia act as inoculum for infection.
Infection
The infection is initiated by attaching conidia to the host surface. Once the pathogen is able to attach, it germinates to form a specialized hyphal structure called appressoria, which penetrates the host epidermis.
During infection, melanization and cutinization take place, giving the infected areas a characteristic brown-black appearance.
The fungal hyphae spread around the host tissue, forming fruiting bodies called acervuli. This structure produces sticky conidia, is pinkish in appearance, and causes further infection.
Secondary spread
The secondary spread takes place during the wet season. Secondary conidia are produced from the infected areas that infect the same host or spread to another healthy plant for infection.
The spread of disease varies within the hosts. In some species, appressoria and secondary conidia are formed within the host cells, which further spread inside the host. This is known as the biotrophic disease cycle. In some species, the pathogen shows subcuticular intramural necrotrophy by developing wide hyphae in the anticlinal and periclinal walls of the epidermis. However, in some other species, the infection is hemibiotrophic, in which the pathogen directly infects the vesicles.
As the favorable season ends, the fungus prepares for the overwintering phase remaining dormant in fallen debris and infected plant parts.
Factors affecting the disease cycle
Anthracnose outbreaks generally take place during warm and humid weather. Prolonged periods of wetness are important for spore germination. The dispersal of spores on wet surfaces makes it easier to stick to the host plant parts. Weakened or stressed plants are more susceptible to infection. The optimum temperature required for anthracnose is 25°-30°C. Cool temperatures can slow the rate of fungal growth. Frequent rainfall helps in the dispersal of spores. Anthracnose germinates easily on wet plants, more than 12 hours of wetness favors pathogen growth. The shaded area also favors the fungal growth.
Preventive Measures of Anthracnose
Cultural practices– Rotating susceptible hosts with non-susceptible crops is important to break the disease cycle and helps reduce the viability of spores in the soil.
Removing and destroying infected plant parts will effectively reduce the inoculum present in the area.
Dense foliage, infected leaves, and branches should be pruned regularly to limit the disease outbreak. Crops should be planted so that there must be sufficient space between them. The infected materials should be disposed of properly to prevent spores from spreading.
Irrigation should be done using the dripping method, which helps keep the foliage dry and reduces the chances of infection. Watering should be done early, which allows the plant to dry during the daytime, thus reducing the moisture on the plant’s surface.
Mulch acts as a barrier between the fungus and the plant. Mulching is usually done with the help of straws, wood chips, or compost, which suppress weeds, retain soil moisture, and reduce the risk of spore dispersal.
Resistant varieties–
Different resistant varieties of crops, vegetables, fruits, and flowers, such as Alphonso mango, GBUEL06 capsicum, Drake Chinese elm, etc., are more tolerant to the disease. Planting such resistant varieties is one of the effective ways to manage the disease.
Fungicides-
Fungicides are necessary to control anthracnose. Copper-based fungicides such as copper diammonium diacetate, Mancozeb, Benomyl, etc., are commonly used to prevent anthracnose. Fungicides should be applied regularly based on plant growth stages and weather conditions. Fungicides should be rotated to prevent the fungus from developing resistant varieties.
Fertilization– Soil tests should be done to determine nutrient levels, and fertilizers should be applied as needed. Nitrogen-containing fertilizers may promote the growth of pathogens, so they should be avoided.
Sunlight exposure– Planting sites should get abundant sunlight to reduce humidity. Pruning is to be done in densely planted areas.
References
- The Editors of Encyclopaedia Britannica. (1998, July 20). Anthracnose | Description, Symptoms, & Control. Encyclopedia Britannica. https://www.britannica.com/science/anthracnose
- Stone, J., & Putnam, M. (2004). PATHOLOGY | Leaf and needle diseases. In Elsevier eBooks (pp. 777–785). https://doi.org/10.1016/b0-12-145160-7/00066-1
- Anthracnose | Disease Treatment | Bayer Crop Science. (n.d.). https://www.crop.bayer.com.au/pests/diseases/anthracnose
- Anthracnose diseases of trees. (n.d.). CT.gov – Connecticut’s Official State Website. https://portal.ct.gov/caes/fact-sheets/plant-pathology/anthracnose-diseases-of-trees
- The Morton Arboretum. (2024, June 22). Anthracnose | Symptoms, management Disease cycle | The Morton Arboretum. https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/anthracnose-of-shade-trees/#management
- Anthracnose diseases of trees | NC State Extension Publications. (n.d.). https://content.ces.ncsu.edu/anthracnose
- Blaedow, R., Heath, B., Langston, W., Lawing, C., Moan, J., Trickel, R., & Oten, K. (2013). Forest Health Handbook (3rd ed.). North Carolina Forest Service.
- Stone, J., & Putnam, M. (2004). PATHOLOGY | Leaf and needle diseases. In Elsevier eBooks (pp. 777–785). https://doi.org/10.1016/b0-12-145160-7/00066-1
- Saxena, A., Raghuwanshi, R., Gupta, V. K., & Singh, H. B. (2016). Chilli Anthracnose: the Epidemiology and Management. Frontiers in Microbiology, 7. https://doi.org/10.3389/fmicb.2016.01527
- Anthracnose of Pepper | NC State Extension Publications. (n.d.). https://content.ces.ncsu.edu/anthracnose-of-pepper
- Anthracnose. (2009, March 6). [Slide show]. SlideShare. https://www.slideshare.net/slideshow/Anthracnose/1112256
