Septoria, commonly known as septoria leaf spot, is a crop disease caused by the fungus Septoria lycopersici. This fungus tends to begin its attack on the lower, older leaves and stems where the fruit set. Fungus typically develops on the leaves of the plant, but can also occur on the calyx, stem, and petioles. Initially, the leaf spots appear in large quantities as small brown spots on the underside of the leaf. As the spots develop, their centres slowly turn grey to tan and have a dark brown border. As the circular spots age, they often enlarge and form into one mass. When spots become large in quantity, affected leaves turn yellow, begin to shrivel, brown, and eventually drop off. Starting with the oldest leaves, defoliation rapidly spreads up the plant to new growth. Substantial yield losses can occur from early leaf-drop, leading to sunscalding of fruit when plants are defoliated in their early stages. This fungus is typically spread to leaves by splashing rain, windblown water, and overhead irrigation. Pickers can also spread this fungus through contact with clothes, hands, and cultivation equipment. Certain types of insects facilitate the spread of this fungus such as tomato worms and Colorado potato beetles. Spores can also overwinter in diseased plant debris and leaf fragments in the soil.
Septoria Identification and Habitat
Upon infection, all aerial parts of the plant are susceptible to infection, aside from the fruit. Foliage tends to be the easiest and most common part of the plant to identify a septoria leaf spot infection. When identifying the disease, look for the distinct small, circular spots covering the tissue of the leaves. These dark brown spots with greyish centers will appear in large numbers on the surface of the leaf and will spread quickly throughout the plant. Leaves will turn yellow and begin to shrivel soon after infection. Eventually leaves will turn brown and fall. Early detection of leaf spots is necessary in avoiding complete defoliation of the plant, which can cause unwanted damage to crops.
Septoria leaf spot develops best in warm, humid, and wet conditions. Typically, this disease first appears in early to mid-August when foliage has become dense enough to restrict air circulation in the plant’s canopy. This high density foliage causes any water on leaf surfaces to dry slower and it causes higher humidity. After 48 hours in conditions with a relative humidity of 100%, infections are likely to begin development. Septoria develops best in temperatures ranging from 68 to 77 F. Low leaves are more susceptible to early infection due to high humidity levels and attraction of fungal spores.
Septoria Management and Control Methods
To avoid disease, always try to use disease-free seed when planting. Spacing plants properly is a great way to improve air circulation to help plants dry faster and maintain proper humidity levels. Watering plants early in the day is another way to promote faster drying. Avoid overhead watering when possible to reduce moisture on the plant leaves. If a plant is infected, hand-picking the infected leaves is important to lower the amount of spores spreading further infection. However, avoid working with the plants while they are still wet as this can aid the spread of disease. Be sure to dispose of picked leaves effectively as spores can still spread after leaves have been removed from the plant. Practicing crop rotation is a good way to avoid any further spread of diseases within crops. At the end of the season, try to remove excess crop debris from the soil as this fungus can overwinter and infect future crops. If removing all debris is not possible, plowing over debris into the soil can help break down microorganisms.
The use of fungicides on this disease can help to avoid defoliation and damage to the plant’s vigor. Some effective fungicides include: Quadris, Daconil, and Ziram. Be sure to read the label carefully before use to ensure proper application and safety procedures. Fungicides should be applied immediately after detection of Septoria leaf spot symptoms due to the rapid spread of this disease. In combination with the cultural practices listed, fungicides can prove to be an effective solution to stop the spread of this disease.
Alternative Septoria Names