Powdery mildew (Oidium neolycopersici, Leveillula taurica) can attack tomatoes at all stages of development, even small seedlings in the cotyledon stage, but it is most commonly seen in older plants. Unlike most tomato diseases, which are aggravated by the presence of abundant moisture, powdery mildew can also develop during dry periods. The spores of this fungus can be easily lifted by the wind and carried to neighboring plants. High humidity does exacerbate the onset and worsening of symptoms.

Conidia spores (asexual reproductive spore of fungi) germinate most intensively at a temperature of 71°F – 82°F (20°C – 28°C) and relative air humidity of 75-100%. When temperatures remain at 68°F – 82°F (22°C – 28°C) and air humidity is moderate, conidial stalks with spores capable of causing a new infection are formed just 4-5 days after infection.
At the initial onset of the disease, small, round, white spots appear on the upper side of the leaves. These spots grow by merging with each other and gradually cover a significant part or even the entire surface of the leaf blades with a white, powdery coating. Over time, the powdery coating is also visible on the stems, petioles, and calyx sepals. Sometimes it covers the underside of the leaves.

The mycelium of O. neolycopersici only grows on the surface of plant tissues and never grows inside the leaf. Yellow spots can sometimes appear on infected leaves. The oldest leaves on the plant are the first to be infected, but the disease develops quite quickly and begins to affect the higher parts of the plant. Affected leaves become chlorotic (insufficient chlorophyll) and eventually necrotic (tissue degeneration).
To biologically protect tomatoes against powdery mildew, use a preparation containing lecithin. The most effective chemical controls are sulfur, triflumizole, dinocap, chlorothalonil, daconil, and imazalil. For chemical powdery mildew controls to be effective, spraying should be started as soon as possible after noticing single white spots on the leaves.