Symptoms
Numerous small, round spots appear on the leaves with a lighter, greyish centre and a darker, brown margin. Within the spots tiny black dots are visible – pycnidia, the fruiting bodies of the fungus, which distinguishes this disease from other leaf spots. The infection progresses from the lower, oldest leaves upwards; with heavy infection the spots merge, the leaves yellow, dry out and drop prematurely, weakening the plant. Affected plants include tomato, currant, phlox and many perennials. The spots are usually 2–5 mm across; the decisive identifying sign is the dark pycnidia at their centre – best seen through a hand lens, and in high humidity exuding whitish 'tendrils' of spores. Leaves densely peppered with spots look as if dusted. The disease is favoured by warm (about 20–25°C), humid weather and water lingering on the leaves for a long time (rain, dew, overhead watering). On tomato the infection is confined almost exclusively to the leaves and leaf stalks – the fruit usually stays healthy – but the loss of the photosynthetic apparatus leads to smaller fruit and exposes it to sunscald.
Treatment
Removing and destroying infected leaves and plant debris reduces the source of spores; they should not be composted. In heavily infected crops, sprays with contact and systemic action are used, started after the first spots appear and repeated according to recommendations. Watering at the root, without wetting the leaves, reduces the spread of the fungus. It is worth repeating the sprays every 7–14 days during humid weather and alternating products with different modes of action, so as not to lead to resistance in the fungus. It helps to remove the lowest, most heavily infected leaves at the first symptoms, and to disinfect tools and stake the plants for better airflow. Because the fungus overwinters on debris and on related weeds (on tomato – on plants of the nightshade family), removing them limits reinfection.
Prevention
Spacious planting and an airy site promote quick drying of the leaves, which hinders infection. It is worth mulching the soil to limit the splashing of spores from the ground onto the lower leaves, and removing infected debris in autumn. Crop rotation and choosing less susceptible varieties (e.g. of tomato) further reduce the risk. It is worth starting from healthy material – certified seed and symptom-free transplants – and avoiding overhead watering, especially in the evening, when the leaves stay wet for a long time. Staking, tying and formative pruning improve air circulation, while disinfecting stakes, supports and tools between seasons removes overwintering spores. Systematic weeding (especially of nightshades when growing tomato) deprives the pathogen of a reservoir between successive crops.
How to tell it apart from look-alikes
| Feature | Septoria leaf spot | Early blight (Alternaria leaf spot / Alternaria blight) |
|---|---|---|
| Appearance of the spot | Small (2–5 mm), round, uniform: greyish centre and dark-brown margin | Larger, with concentric rings (a 'target board' effect), often with a yellow halo |
| Sign within the spot | Tiny black pycnidia (dots) at the centre of the spot – visible through a hand lens | No pycnidia; in humidity a dark, velvety coating of spores on the surface |
| Pathogen | Septoria spp. | Alternaria spp. |
| Organs affected | Almost exclusively the leaves (rarely the leaf stalks); fruit usually spared | Also attacks stems and fruit – dark, sunken, concentric spots at the fruit base |
Control — organic and chemical methods
Organic and biological methods
| Method | How it works | When to use |
|---|---|---|
| Sanitation (removing infected leaves and debris) | Removes the source of spores – pycnidia overwinter on debris; do not compost. Works as a support measure, on its own it will not control a heavy infection. | From the first spots and in autumn after harvest |
| Watering at the root and an airy site | Irrigating to the soil without wetting the leaves, together with spacious planting, shortens the leaf-wetness period that the fungus needs to germinate its spores. | Throughout the season |
| Mulching the soil | A layer of mulch limits the splashing of spores from the soil by raindrops onto the lower leaves, where the first infections begin. | In spring, before the disease develops |
| Crop rotation and less susceptible varieties | A 2–3-year break from growing susceptible species in the same place, together with choosing less susceptible varieties (e.g. of tomato), lowers the pathogen pressure. | — |
Chemical methods
| Method | How it works | When to use |
|---|---|---|
| Contact fungicides (copper, multi-site) | Protective products forming a barrier on the leaf surface; they act preventively, require thorough coverage and repeat sprays after rain. Copper is also permitted in organic growing. | Preventively and from the first spots |
| Systemic fungicides (triazoles/DMI) | They penetrate the tissue and act systemically/curatively; the mode-of-action groups should be rotated so as not to select for resistant strains of the fungus. | Under heavy pressure, alternating 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
- Phytopathology. Diseases of horticultural plants
- Institute of Horticulture – PIB
