Symptoms of the disease occur only on the foliage. Symptoms initially appear as light green to yellow blotches or spots that range from 1/8 to 1/2 inch in diameter on the upper surface of the leaf. These spots eventually turn brown as the leaf tissue dies. Eventually, the entire leaf will turn brown and shrivel, but remains attached to the stem. The white, powdery growth of fungal mycelium is typically present on the lower surfaces of affected leaves.
The plant is not killed by the disease, but is weakened and productivity is greatly decreased.
Survival And Spread
In Florida, powdery mildew is caused by Oidium neolycopersici. In western regions of the U.S. and other parts of the world, powdery mildew also may be caused by the fungus Leveillula taurica.
The powdery mildew fungi are obligate parasites; they can only survive on a living host. The removal of nutrients from host cells causes the yellowing and eventual necrosis of tomato tissue.
The fungus reproduces by producing spores or conidia. These spores are transported long distances by wind, air currents, or workers. When the conidia land on tomato leaves, they germinate and enter the leaf through stomates.
The pathogen has a wide host range and survives on weeds and crops in the solanaceous family like pepper as well as other hosts or volunteer tomato plants from one season to the next.
The fungus grows at moderate to cool temperatures. Unlike other fungi, which require free water and high humidity for optimal development, epidemics have been noted during relatively dry conditions. High relative humidity favors disease development while free water from overhead irrigation may discourage disease. Mild temperatures favor infection while higher temperatures hasten the death of infected leaves.
An integrated approach should be used to control powdery mildew in the greenhouse. Maintenance of high relative humidity can help reduce disease incidence. Removal of infected foliage and severely infected plants will reduce the spread of inoculum. Greenhouses should be sanitized between crops. Registered fungicides should be applied to plants as soon as symptoms are observed.
In the field, control measures would include scouting, rouging of infected plants, use of resistant varieties, and spraying preventative chemicals such as the strobilurin fungicides. Applications of sulfur will help control new infections but sulfur should be used with caution at temperatures above 85°F.