Anthracnose - Fungal disease of various genera and specific species of host plant: Isolation in culture; Molecular diagnosis; Species identification (sequencing).
Anthracnose (canker or chancre) is a symptom of plant disease in hot and humid areas, caused by some species of fungi of various genera, of the division Ascomycota, family Glomerellaceae, genus Colletotrichum (syn. Gloeosporium) or species Coniothyrium fuckelii The causative fungi are host specific, that is, each species of fungus produces anthracnose in one plant, but not in another.
The affected plants show the following signs: round and dark spots on the leaves; rot; decay of leaves and stems; premature fall of fruits and flowers; round and dark spots on the fruits. Leaf lesions are usually associated with venation or margins.
It is rare that anthracnose can cause serious damage in most plant species, but it is a serious economic damage due to the aesthetic damage it causes in fruits, which are no longer suitable for sale due to the signs they exhibit. Therefore, and the fact that the pest especially affects farms, it is a much more serious problem for farmers than for gardening enthusiasts or small garden owners.
Although the signs of the affected plants may be variable, they all have a common element of these infections corresponding to the appearance of fruiting structures (where propagation conidia appear), in the form of a "cup" formed by fungal mycelium, called acervuli. The acervuli are formed in the damaged tissue and can be observed microscopically. Conidia are colorless, unicelular and ovoid shape.
Anthracnose fungi spend the winter in leaf remains on the ground and/or in dead areas of the tree´s bark, called chancres. At the beginning of the spring, the conidia of the fungus are generated and dispersed by rain splashes, sprinkler irrigation, etc. These conidia infect leaf buds, or in some cases, young leaves. The infection process is favored by relatively cold temperatures and prolonged periods of leaf moisture. After infection, the causal fungus colonizes the leaf tissue and begins to produce new fruiting and conidia structures capable of reinfecting the leaf tissue in expansion. The development of the disease can continue during spring until early summer if a favorable climate persists. These diseases tend to be less problematic during the hot and dry summer weather.
These fungi can be symbionts of plants, without causing damage, or be phytopathogenic, injuring the leaves, stems or fruits to obtain nutrients from the host tissues.
The conidia are dispersed by rain or sprinkler irrigation, adhering to the aerial parts of the plant to infect them. Heavy rain can spread the spores up to 4.5 meters from the host plant. Conidia germinates in the new host and forms a short germinal tube that will develop an apresorium (organ to attach). As the germ tube grows, it connects the conidia and the apresorium, causing an indentation in the plant cell wall that will allow an infectious hypha to penetrate through the cell wall, grow and become a vesicle of infection.
The first stage after infection is known as the biotrophic phase, and consists of a broad primary hypha, which develops from the vesicle of the infection. Primary hypha occasionally penetrate through additional cell walls through the use of mechanical force, but generally will not grow very far from the vesicle of infection. It always remains along a wall, so that half of the circumference of the hypha is in contact with the cell wall at all times. The primary hypha do not penetrate the plasma membranes of the host cell, but grow between it and the cell wall, not killing any cells. In these early stages of infection, proteins are released by the vesicle of the infection that suppresses the host´s defense responses and allows the growth and development of the fungus without hindrance. During the biotrophic phase, the pathogen obtains nutrients by transferring hexoses and aminoacids from the living host cell to the fungus.
Approximately 48 to 72 hours after inoculation, depending on the environment, the biotrophic phase ends and the necrotrophic phase begins, marked by the development of many thin hyphae, known as secondary or necrotrophic hyphae, which develop from the primary hypha and the infection hypha. Unlike much larger primary hypha, these secondary hyphae develop freely through the host in all directions, penetrating cell walls and membranes equally. To spread and branching more easily throughout the host tissue, hyphae release enzymes that break down host-specific proteins. Some of these enzymes include chitinases, proteases, pectin and pectate lyase. An enzyme, endopolygalacturonase, is a highly specialized enzyme that degrades the plant cell wall and is critical for mycelial growth. Endopolygalacturonase is normally produced by fruiting plants and induces fruit ripening by degrading the polygalacturonan present in the cell walls. In addition, since the mycelium must spread rapidly during the necrotrophic stage to supply nutrients to the fungus, it will thrive more easily in younger plants, which have softer tissues than their older counterparts. While there is very little discoloration of the host cells before the development of secondary hyphae, the discoloration quickly becomes evident approximately 100 hours after infection. Black spots begin to develop on the surface of the plant and grow radially outward, leading to the manifestation of anthracnose disease. These are the first visual symptoms of an infected plant, and they usually occur along leaf veins on the underside of the leaf. As the lesions grow, conidia begin to develop.
Tests performed in IVAMI:
- Culture of simples from lesions of leaves, stems or fruits to isolate the fungus.
- Molecular identification of isolated fungi in culture (sequencing).
- Direct molecular diagnosis (PCR) in plant sample for some fungus (Colletotrichum gloeosporioides and Colletotrichum acutatum).
Sample recommended:
- Leaves, stems or fruits with lesions suspected of anthracnose.
Storage and sending of the sample:
- Refrigerated (preferred) for less than 2 days.
- Frozen: more than 2 days.
Schedule for results:
- Culture of lesions of leaves, stems or fruits for fungus isolation: 7 to 14 working days.
- Molecular identification of isolated fungi grown in cultures (sequencing): 4 to 5 working days.
- Direct molecular diagnosis (PCR) in plant samples for some species (Colletotrichum gloeosporioides and Colletotrichum acutatum): 24 to 48 hours on weekdays.
Cost of the test:
- Culture of lesions of leaves, stems or fruits for fungus isolation: Consult to ivami@ivami.com.
- Molecular identification of genera and species of isolated fungi (sequencing): Consult to ivami@ivami.com.
- Direct molecular diagnosis (PCR) in plant samples for some species (Colletotrichum gloeosporioides and Colletotrichum acutatum) (each): Consult to ivami@ivami.com.