What Is the Powdery White Mildew on My Tomato Plants?

Tomato plants exposed to mild, dry conditions are vulnerable to powdery mildew.

Powdery mildew is the common name for several types of fungus that infect the surface of plant leaves. On tomato plants in the United States, the most likely organism is Leveillula taurica or Odium lycopersicum.

Identification

The mycelium of powdery mildew are most commonly confused with pest treatments such as insecticide dust applied on the tomato plants or blown from nearby plants or fields. Powdery mildew coverage across the leaves forms small colonies or spots, and then expands. Spots on leaves yellow and then brown as powdery mildew feeds on nutrients through the stomates.

Effects

The fungus feeds very lightly on the nutrients in tomato leaves, but the colonies also block sunlight and hinder photosynthesis. Small infestations cause little concern, since the plant can produce new leaves. But when most or all of the plant is covered with powdery mildew, the plant cannot photosynthesize to produce energy, and will decline rapidly.

Control

Plant resistant cultivars such as Grace and DRW 5007. Powdery mildew favors warm days, cool nights and dry conditions. In greenhouses, keep the relative humidity high and quarantine and treat infected plants. Apply fungicides labeled specifically for powdery mildew at the first sign in the greenhouse, but only in cases of severe infestation out in the garden. Powdery mildew favors the sheltered conditions of greenhouses to outside climatic extremes.

References

• University of Connecticut Integrated Pest Management: Powdery Mildew: A New Disease of Tomato
• University of Florida IFAS Extension: Powdery Mildew on Tomato