Black spots spread across the leaves, the foliage blackens, and within days an entire crop can be lost: that is tomato late blight (Phytophthora infestans), a fungal disease driven by heat and moisture. Space your plants so air moves freely and keep the leaves dry, and you give the harvest its best chance.
As soon as black spots appear on leaves, you should prepare for this worst of tomato blight, the late tomato blight
Here is how to avoid this blight and treat this disease with organic techniques.
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Tomato blight is a fungus with latin name Phytophthora infestans that results in a devastating fungal disease related to downy mildew.
As for all mushrooms, two factors meet to provide the most favorable conditions to its growth: heat and moisture.
Once contaminated, tomato plant leaves start to blacken. If not treated, the disease can spread to an entire crop in just days and destroy the harvest.
Since there is no curative treatment, the easiest way to stay ahead of late blight is to grow varieties bred to resist it. All of the following are widely sold in garden centres and seed catalogues.
| Variety | Fruit type | Blight resistance | Where it grows best |
|---|---|---|---|
| ‘Crimson Crush’ F1 | Large round slicing fruit, 200 g and up | Very high late blight resistance | Outdoor beds and open ground |
| ‘Mountain Magic’ F1 | Cocktail-size, crack-resistant | Resists both late and early blight | Outdoors, cordon-trained |
| ‘Lizzano’ F1 | Small cherry, heavy trusses | Late blight tolerant | Hanging baskets and containers |
| ‘Losetto’ F1 | Cherry, trailing bush habit | Strong blight resistance | Baskets, pots, no staking |
| ‘Defiant PhR’ F1 | Medium slicing fruit | High late blight resistance | Open ground, humid summers |
| ‘Legend’ | Large red slicer | Good late blight tolerance | Cool, damp outdoor sites |
| ‘Fantasio’ F1 | Round to slightly flattened, 150 g | Good blight tolerance | Outdoors, well-spaced rows |
Most important to avoid tomato blight is to space tomato plants well apart. If air circulates well, you will guarantee quicker drying of leaves.
Tightly packed tomato plants lead to a closed, stuffy environment that is exactly what downy mildew needs to grow. Downy mildew spreads along wet leaves, so the longer they stay wet, the higher the risk of becoming contaminated by the tomato blight.
When watering, never wet the leaves: only bring water to the foot of the plant.
Lastly, do all you can to avoid having leaves or fruits touching the soil. Soil propagates diseases and fungus such as downy mildew very well. Plants touching the ground will potentially be infected with mildew.
As of today, there is no curative treatment against downy mildew. Observing your plants very regularly is the best manner to keep downy mildew from spreading.
As soon as black or brown spots appear on the leaves, remove and destroy them to eliminate any trace of the fungus.
Use clean pruning shears sterilized with alcohol after each cut to avoid contaminating health plants.
After that, treat with Bordeaux mixture and repeat this surveillance on a regular basis until about 2 weeks before harvesting the tomatoes.
Another solution is to treat with baking soda soap spray: 1 table spoon baking soda and 1 table spoon liquid beldi soap for 1 quart (1 liter) water.
