What Is It? | Facts in Depth | For the Professional Diagnostician
Early symptoms will depend on which pathogen species is present. Leveillula taurica begins on the undersides of leaves as a flat white mycelium. Bright yellow lesions form on the upper sides of leaves, and then sometimes develop into necrotic lesions with yellow halos. Oidium neolycopersici starts as white or grey splotches on the upper sides of leaves and progress to the stems. White mycelium will spread to cover the leaves and stems as the disease becomes more severe. The plant becomes very weak, which leads to rapid defoliation and fewer and smaller fruits. Fruits infected with Leveillula taurica develop irregular, bright yellow lesions until they become completely necrotic.
The pathogen is unable to overwinter in outdoor soil as mycelium on plants, but can be introduced each year through tomato transplants or contaminated soil. When a susceptible host is present, the conidiophores germinate from the mycelium to form conidia. These spores are carried by wind, water splash, workers, etc. to the tomato plants, where they land and infect the leaves. Spores land on foliage and symptoms can develop in as little as 7 days. Colonization takes place between 60°F to 80°F, and the mycelium grows out of the stomates underneath the leaves developing a white fluffy growth. The mycelium develops more conidia that spread easily to other healthy plants. This pathogen needs a living host in order to survive, so rarely kills the host and just weakens it. Photosynthesis is inhibited due to the foliar symptoms, which leaves to premature leaf drop and a reduction in fruit size and yield.
Favorable Environmental Conditions
Mild temperatures are favorable for tomato infection. Wet leaves are not necessary during this time, but aid in disease progression. During colonization, high relative humidity is conducive; however not higher than 95%. Conidia germinate from 50°F to 98°F; optimal temperature is about 70°F. Optimal relative humidity ranges from 75 – 85%. Higher temperatures hasten the death of leaves once infected.
Often Confused With
- Leaf mold – An olive-green to grey velvety mycelium develops on the undersides of tomato leaves. Other symptoms include of yellow lesions and premature leaf drop.
- Botrytis gray mold – A white to grey mycelium growths on infected leaves, stems, and fruit.
- Early blight – Necrotic lesions with yellow halos develop on older leaves first, then progresses to younger leaves. The leaves eventually die and drop from the plant.
Symptoms begin to develop on the upper and undersides of leaves. They begin as yellow lesions or white cottony mycelium. Infected leaves should be removed and destroyed to prevent any further infections.
For Leveillula taurica, action threshold information has been calculated using a disease forecasting model for field tomatoes. However, this may not be helpful for greenhouse and high tunnel growers.
- Apply Preventative Fungicides – Sulfur and copper containing fungicides can be sprayed every 7 to 14 days until the pathogen is eliminated. Oils and plant-based oils have also been effective in controlling the disease, such as Saf-T Side Spray, neem oil, or jojoba oil.
- Avoid High Humidity – Maintain proper air circulation through spacing and good pruning practices. Avoid overhead irrigation where excess leaf moisture is possible. Full-sun conditions and high temperatures will hinder the pathogen’s growth and spread to healthy plants.
- Maintaining Plant Health – Keep plants healthy by applying fertilizers regularly; however, do not raise nitrogen levels too high because this favors growth of the pathogen.