Symptoms
Dark, sharply defined, sunken spots appear on the leaves, stems and fruit, with collapsed, rotting patches on the fruit. In wet weather, pinkish, slimy masses of spores can be seen in the centre of the spots, often arranged in concentric zones. The spots enlarge and merge, the leaves die, and the fruit of beans, peppers, squash or strawberries rots; the disease intensifies in warm and humid summers. The most distinctive feature is precisely this salmon-pink, slimy coating of conidia filling the moist spots - these are the fungus's spore-producing clusters (acervuli), not a dry, powdery mould. On bean pods and fruit the spots are round, deeply sunken and sharply edged with a dark brown, sometimes reddish border; on leaf veins and stalks elongated, brownish-black, sunken streaks appear. On strawberry the disease attacks the fruit (hard, brown, sunken spots) as well as the crown and the stalks, leading to the dieback of whole plants. On cucurbits the young spots may be watery and translucent, later turning brown and cracking. Infected fruit is not suitable for storage, because the disease continues to develop after harvest.
Treatment
Removing and destroying infected parts and plant residues and, in addition, preventive fungicide sprays during periods that favour infection. Working among the crop when the plants are wet should be avoided, as it promotes the spread of spores. Fungicides act only protectively - they safeguard healthy tissue but will not cure spots that have already formed - so they must be applied before infection occurs and repeated every 7-14 days in warm, humid weather and after rain that washes off the layer of contact product. At the first symptoms, infected leaves, shoots and fruit should be cut out and removed from the crop to limit the source of spores. In commercial crops, products with different modes of action are used in alternation (rotation of FRAC groups), because Colletotrichum readily develops resistance, especially to strobilurins. In the home garden, crop hygiene and an airy site remain the basis, while chemical protection is of supplementary importance.
Prevention
Healthy, certified seed and planting material, crop rotation, airy spacing, watering at the base without wetting the leaves, removing crop residues and growing less-susceptible cultivars. Seed material is of key importance here - especially in beans the fungus is carried with the seed, so seeds should not be collected from infected plants, nor should uncertified material be used. Crop rotation with a 2-3 year break in growing susceptible species on the same site allows the residues carrying spores to decompose. Mulching the soil (particularly under strawberries) reduces the splashing of spores from the ground onto the fruit during rain, while switching from overhead watering to watering at the base shortens the leaf wetness needed for infection. Harvesting in dry weather and deep ploughing-in or removal of crop residues after the season further reduce disease pressure in the following year.
How to tell it apart from look-alikes
| Feature | Anthracnose | Alternaria blight |
|---|---|---|
| Sporulation within the spot | Pinkish, slimy, moist masses of spores (acervuli) in the centre of the spot | A dark, dry, velvety-powdery coating of black spores |
| Nature of the spot | Distinctly sunken, sharply defined, often with concentric zones of acervuli | A brown-black spot with distinct concentric rings (target spot), usually shallower |
| Symptom on fruit | A sunken, rotting patch with a pink, slimy coating of spores | A dark, dry, sunken spot with a black powdery coating, often near the stalk |
| Pathogen | Colletotrichum spp. | Alternaria spp. |
| Favourable conditions | Warmth and high humidity, rain washing away the conidial slime | Warmth, alternating moisture and dew; more often on weakened plants and older leaves |
Control — organic and chemical methods
Organic and biological methods
| Method | How it works | When to use |
|---|---|---|
| Crop hygiene and removing the source of infection | Cutting out and destroying infected leaves, shoots and fruit and clearing crop residues reduces the pool of overwintering and secondary spores. | Ongoing during the season and after harvest |
| Healthy seed material and crop rotation | Certified, pathogen-free seed (in beans the fungus is carried with the seed) and a 2-3 year break in growing susceptible species break the disease cycle. | Before establishing the crop |
| Cultural practices limiting leaf wetness and splash | Airy spacing, watering at the base instead of overhead, mulching and working with dry plants shorten leaf wetness and hinder the spread of the slimy spores. | Throughout the season |
| Growing less-susceptible cultivars | Choosing cultivars with increased tolerance reduces infection, but this is only a supporting method - it does not replace crop hygiene or protection in years that favour the disease. | — |
Chemical methods
| Method | How it works | When to use |
|---|---|---|
| Contact fungicides (e.g. copper-based) | They form a protective layer on the plant surface; they act only preventively, require reapplication after rain and must be applied before infection. | Preventively in warm, humid weather |
| Systemic fungicides (triazoles, strobilurins/QoI) | They penetrate the tissue and give deeper, more lasting protection; rotation of FRAC groups is necessary, as the pathogen quickly develops resistance, especially to strobilurins. | Preventively, in alternation with contact products |
We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.
Sources
- Compendium of horticultural plant diseases
