There are different types of fungus that grow on plants for a variety of reasons. Gray mold on plants is caused by overhead watering, low temperatures and low humidity levels. Poor air circulation around the plant is another problem with this type of fungus, as well as low nitrogen levels in the soil. Lack of calcium in the soil is also a factor and liming the soil will help to increase those levels as well as improve the pH factor of the soil. This type of mold also causes a leaf symptom known as ghost spot, where the actual leaf is consumed by the mold and becomes transparent.
Tomato plants alone are affected by leaf mold, which occurs all over the world. If growing temperatures are below 60 degrees and humidity reaches over 85 percent without proper ventilation, this mold is likely to occur. The leaves of the tomato should be kept as dry as possible, especially before nighttime arrives. Making sure that the plants are not spaced too closely to each other helps to prevent leaf mold from growing and spreading from plant to plant. Some tomato plants are resistant but since there are 12 varieties of this fungus, it is difficult to guarantee finding a plant that is resistant to all of them.
Powdery Mildew and Late Blight
Powdery mildew grows on plants that have poor air circulation and too much nitrogen fertilizer in the soil. Late blight affects tomatoes, potatoes and eggplant. Some plants of these varieties are resistant to the blight. This fungus is easily spread when potatoes and tomatoes are planted too close together. If plants have been culled from the field for whatever reason, do not keep the cull piles too close to the still-growing plants as the fungus tends to thrive in rotting vegetation. Do not place plants too close together in the field as this encourages the spread of the fungus.