What Is It? | Facts in Depth | For the Professional Diagnostician
Passalora fulva mostly causes foliar symptoms, although it can occur on fruit. The fungus first appears as an olive-green to gray velvety mycelia on the undersides of older leaves. Within the mycelia are many conidia (spores). As the disease progresses, pale green lesions coalesce into larger yellowish-brown lesions with poorly defined margins. The leaves then become very chlorotic and curl and begin to prematurely drop. Defoliation continues to younger leaves until the entire plant has died.
Foliage infections are most common, but occasionally the disease spreads to the stems, petioles, blossoms, and both immature and mature fruit. On the blossoms or fruit, symptoms may appear as bronze lesions. As the disease progresses on the fruit, a leathery rot forms with irregular margins on the stem-end.
Passalora fulva overwinters as conidia or as sclerotia on plant debris or in seeds or soil. These sclerotia germinate to form conidia under favorable conditions. The conidia serve as the source of primary inoculum on tomato plants and are spread by water and by infected tools and workers. The conidia germinate in water droplets on the foliage under high humidity and warm temperatures. The pathogen enters through leaf stomata and spreads to the underside of leaves as a thick olive-green mycelium, where more conidia are produced. Symptoms can appear 10 days after infection occurs with spore formation occurring a few days later. There are many races of the pathogen because Passalora fulva mutates very rapidly. The pathogen can survive at least one year in the absence of a susceptible host on plant debris or in seeds and soil.
Favorable Environmental Conditions
This pathogen needs excess water on foliage or high relative humidity (>85%) and grows best at temperatures between 40 – 94°F. The conidia spores germinate ideally at 75 – 78°F, where spore production is most abundant at relative humidity 80 – 90%.
Often Confused With
- Botrytis gray mold: Dark grey mycelia form with visible conidia on foliage and other plant parts that resemble the mycelia of Passalora fulva.
- Late blight: Large yellowish-brown lesions with irregular margins for on the foliage of tomato plants. Coppery-colored lesions on fruit are similar to fruit symptoms caused by P. fulva.
- Powdery mildew: Yellow, irregular lesions form on the upper surfaces of leaves; whitish- grey mycelium develops on the both sides of the leaves. These leaves eventually drop prematurely; fruit rarely develop symptoms.
Look for moldy growth appearing on the undersides of older leaves. If the disease has progressed there will be an overall decline in plant health, starting with chlorosis and defoliation. If plants have early symptoms of leaf mold, prune out and destroy symptomatic leaves to prevent further infection, if practical.
Currently, there is no threshold information available for leaf mold.
- Reduce Relative Humidity – High relative humidity and free moisture on leaves are the major driving forces for leaf mold disease progression. Limiting overhead irrigation and handling of plants during periods of high humidity will help in managing leaf mold. Increasing air circulation by increasing spacing and pruning should aid in keeping humidity lower than 85%. It may be necessary to heat the greenhouse or high tunnel during cool nights to keep the temperature higher inside than outside to reduce humidity and prevent water condensation inside the structure.
- Use Sound Cultural Practices – Sanitation of tools and workers’ hands and clothing will prevent the pathogen from spreading in high tunnels. Symptomatic leaves and plants should be removed and destroyed.
- Use Resistant Varieties – There are several tomato cultivars available that are partially or highly resistant to leaf mold. However, the pathogen is variable and the use of resistant should not be relied upon as the sole means of managing this disease.
- Start with Clean Seed – Tomato seeds can be sanitized before planting; simple treatment with dilute bleach will kill bacterial and fungal pathogens on seeds. Refer to the “Hot Water and Chlorine Treatment of Vegetable Seeds to Eradicate Bacterial Plant Pathogens” Fact Sheet.
- Use Foliar Fungicides – There are a number of fungicides that when used in combination with other disease management tools aid in controlling the spread of leaf mold on tomatoes. Note that many fungicides approved for use in open field are not permitted in greenhouses, high tunnels or other “protected culture” systems.