Tomato Leaf Mold (Cladosporium fulvum)
Leaf mold on tomato foliage is usually not a problem in field crops, Cladosporium Fulvum, on the other hand, does pose a threat to crops in enclosed environments. In protected culture, there is higher air humidity, which favors the growth of many different types of fungi. Very often, the leaf blades are the only infected part of the plant. Shoots, flowers, and fruit are not normally affected by this disease, but in severe cases, it is possible, which directly reduces the yield.
The first symptom of leaf mold is small, yellowish spots, 5-10 mm in diameter, on the upper side of the leaves. These spots gradually turn brown and grow larger. There is no defined margin between diseased and healthy tissue.
On the underside of the leaves, in the middle of yellow spots, a profuse, olive-green to brown, velvety mold forms, consisting of conidiospores (spores that are formed externally). As the disease progresses, the affected leaf blades on the lower branches begin to die first.
Leaves sometimes fall off the plant, but more often remain attached. When the disease progresses and no countermeasures are taken, flowers and fruits can also become infected. The flowers turn black and fall off. Fungal infection on fruit begins at the stem and increases as the disease progresses. The tissue collapses, then becomes dry and leathery. Ripe spores in a drop of water and above 85% relative air humidity germinate at about 41°F to about 86°F (5 to about 30°C). Spore germination takes 3-4 hours. At 89° F (32°C) and strong sunlight, the fungus stops growing.
The most important ways to stave off infection are the combination of appropriate levels in air humidity and temperature. If the temperature is 59°F (15°C), contamination is only possible when the air humidity exceeds 90%. On the other hand, when the temperature is 68°F (20°C), the relative humidity at which contamination can occur is about 75%. If the air humidity drops to 60-65%, the fungus does not develop and infection does not occur. The greatest risk of developing C. fulvum occurs when the air humidity is above 85% and the leaves are moistened for an extended period of time. Development of the disease occurs when there are no protective measures taken to limit increased air humidity in greenhouses during a significant drop in temperature, which usually occurs overnight. When the fungus infects plant tissue, humidity levels no longer affect further development.
When conditions are optimal for the development of the fungus, the first symptoms appear about 10 days after infection. In a dry environment, the spores remain viable for 9-10 months, so any fallen leaves or plant debris left over from an infected crop should be thoroughly removed and burned. The fungus can survive in the form of conidial spores or fragments of mycelium. It can also survive on structural elements of a greenhouse or above the soil line.
In order to minimize the risk of disease after a previous infection, select cultivars resistant to leaf mold. Be aware that there is a possibility of breaking immunity, it’s easy for C. fulvum to create new physiological races. This is why it is so important to ventilate tunnels and greenhouses intensively, keep relative air humidity below 75%, and maintain appropriate spacing between plantings so as to ensure proper air circulation. Remember to remove the lower leaves from the plants early on for better circulation. These measures will significantly reduce the risk of leaf mold on your tomato plants.
If you decide to chemically protect crops, spray the plants as soon as you notice symptoms of the disease. Spray with fungicides containing chlorothalonil or thiophanate-methyl every 7-10 days. The last spraying should be made about three weeks before harvest.