Septoria leaf spot is a common foliage disease known for its impact on tomato plants and other members of the nightshade family. The first visible signs are small, circular lesions, typically starting on the lower leaves, which develop gray or tan centers surrounded by a dark brown border. As the disease progresses, these spots may contain tiny, dark specks, and the surrounding leaf tissue turns yellow, leading to premature defoliation. This affliction is caused by a fungal pathogen.
The Fungal Culprit
The specific fungus responsible for Septoria leaf spot in tomatoes is Septoria lycopersici, a microscopic organism that feeds on living plant tissue. While this species primarily targets tomato, other Septoria species cause similar leaf spot diseases in different host plants, such as celery and parsley. The most distinguishing feature of S. lycopersici is the formation of specialized, flask-shaped structures called pycnidia within the infected leaf spots. These pycnidia function as the fungus’s fruiting bodies and are visible as the small, black specks in the center of the lesions. Inside these structures, the fungus produces asexual spores, known as conidia, which are released to initiate new infections.
Pathogen Survival and Primary Sources
The pathogen survives between growing seasons by relying on primary sources of infectious material, or inoculum. The most significant survival mechanism for S. lycopersici is overwintering within infected plant debris, such as fallen tomato leaves and stems left in the garden. The fungus can remain viable in this dead material for up to three years, providing a ready source of spores when conditions become favorable. The fungus does not persist in the soil itself but rather on the debris lying on or buried shallowly beneath the surface. Solanaceous weed hosts, such as horsenettle or nightshade, are another important source that can harbor the pathogen throughout the year. Furthermore, spores can be carried on contaminated equipment like stakes and cages, or on infected seeds, which can introduce the pathogen to new planting areas.
Environmental Triggers for Infection
Successful infection by the Septoria fungus is dependent on specific environmental conditions. The spores require a period of prolonged leaf wetness to germinate and penetrate the host tissue. This wet period must be extensive, often requiring approximately 48 hours or more of continuous 100% relative humidity, or cumulative periods of high moisture, for successful infection. The disease is also favored by warm, but not excessively hot, temperatures. The optimal range for spore germination and disease development is between 68 and 77 degrees Fahrenheit (20–25 degrees Celsius). When these conditions of moderate warmth and high moisture coincide, the fungal spores rapidly germinate and penetrate the leaf surface, often through natural openings like stomata, leading to visible lesions within five to ten days.
Methods of Transmission
Once the fungal spores are produced within the pycnidia, they must be moved from the primary source or an established lesion to healthy plant foliage to cause secondary infections. Water is the main vehicle for this movement, with rain splash being the most common and effective method of transmission. Raindrops or overhead irrigation physically dislodge the sticky spores from the pycnidia and splash them onto nearby leaves, stems, and other plants. The pathogen can also be spread mechanically through human activity. Gardeners working among wet plants can transfer spores on their hands, clothing, or contaminated tools, such as pruning shears or stakes. Wind can carry water droplets containing the spores over short distances, and certain insects have also been shown to aid in the movement of the fungal inoculum.