Septoria leaf spot on tomatoes

During periods of high humidity and temperatures between 60 and 80 degrees Fahrenheit, an opportunistic fungal infection can wreak havoc on your tomato plants. Caused by the fungus Septoria lycopersici, septoria leaf spot is characterized by circular spots that range in diameter from 1/16 to 1/4 inch and appear either tan, dark brown or gray. The fungus is spread through infected plants and equipment and, if left untreated, will pass quickly and affect a whole crop, no matter what the plant’s age. Prevention, proper sanitation measures and the use of copper-based fungicides are the best ways to stop septoria leaf stop from spreading.

Septoria leaf spot, also called Septoria blight, is a very common disease of tomatoes. It is caused by a fungus (Septoria lycopersici) and can affect tomatoes and other plants in the Solanaceae family, especially potatoes and eggplant, just about anywhere in the world.

Although Septoria leaf spot is not necessarily fatal for your tomato plants, it spreads rapidly and can quickly defoliate and weaken the plants, rendering them unable to bear fruit to maturity.

Causes

Fungus spores are very good at hiding out and waiting for ideal conditions. They can travel great distances, in hopes of finding those conditions. The Septoria lycopersici fungus lives on the fallen tomato plant debris and weeds on and in the soil. It is spread to the plants by both water and wind, usually splashing up on the plants from the soil. Warm (60 - 80 F.), damp conditions are the most favorable for Septoria leaf spot to develop and that's when you should be most watchful. If caught early, it can be controlled.

Symptoms

Septoria leaf spots start off somewhat circular and first appear on the undersides of older leaves, at the bottom of the plant. They are small, 1/16 - 1/8 in. (1.6 - 3.2 mm) in diameter, with a dark brown margin and lighter gray or tan centers.

As the disease develops, the spots will get larger and may merge together. If you view them under a magnifying lens you may see the fruiting bodies of the fungus, which look like dark brown pimples, as shown in the second image. This is one of the symptoms that distinguishes Septoria leaf spot from other leaf spotting diseases.

Although the symptoms usually occur on the older, lower leaves, the disease can develop at any stage in the tomato plant's life. They may also appear on the stems, as shown in the last photo, as well as the blossoms and calyxes. They rarely affect the fruits.

If untreated, Septoria leaf spot will cause the leaves to turn yellow and eventually to dry out and fall off. This will weaken the plant, send it into decline and cause sun scalding of the unprotected, exposed tomatoes. Without leaves, the plant will not continue producing and maturing tomatoes. Septoria leaf spot spreads rapidly.

• Lower leaves show symptoms first
• Round, yellow or water-soaked spots appear on the undersides of leaves. They quickly emerge on leaf tops and turn to black or brown with tiny black dots in the center.
• Heavily infected leaves turn completely yellow, then brown, and fall.
• Spotting works its way up the plant and can infect stems.
• Septoria leaf spot does not display characteristic “target” or concentric spots seen in early blight.
• Fruit is rarely infected.

When does it attack your plants?

• It can attack at any point in the season
• It strikes frequently in early to mid-season
• It’s most common in humid weather
• It often affects tomato plants after a period of heavy rainfall

Prevention

• Rotate crops. Although it doesn’t live in the soil, Septoria can survive in garden debris, perennials, and weeds for 3 years.
• Plant tomatoes in a raised bed to improve drainage and prevent diseases from spreading.
• Choose strong hybrid varieties to increase chances of plants remaining disease-free during the season.At last check, there are no tomato varieties resistant to Septoria.
• Give tomato plants extra space (more than 24 inches) to let air to move and keep leaves dry.
• Water the soil – not the plants – to prevent splashing. Avoid overhead watering.
• Stake tomato plants for better circulation.
• Mulch with black plastic or landscape fabric to prevent fungus from spreading up onto leaves.
• Remove and destroy affected plants at the end of the season.

Treating an affected plant

1. Inspect the tomato seedling before planting. While at the home improvement store or nursery, look for the early signs of septoria leaf spot, including the tan, brown or gray rings. Only introduce plants that appear healthy and disease-free.
2. Choose a planting site featuring well-drained soil and plenty of exposure to light winds and the sun. Septoria spreads more quickly on moist leaves, so choose an area that allows the plants to remain as dry as possible.
3. Spread a thin layer of mulch around the growing tomato seedlings. The mulch acts as a protective shield that doesn’t allow septoria spores to splash onto the tomato stems or leaves during irrigation.
4. Irrigate the plants early in the day and, if possible, avoid using an overhead watering system. Watering near the base of the plant helps keep the foliage relatively free of excess moisture, which the septoria requires to thrive.
5. Monitor your growing tomato plants and pluck off any diseased leaves immediately after you notice the infection. The infection generally starts on the lower leaves and moves up. Toss or burn the infected leaves. Never mulch or compost them, as this will only spread the septoria fungus.
6. Treat the tomato plants with a copper-based fungicide at the first sign of the septoria infection, or right after the formation of fruit clusters. Apply the product to both sides of the tomato leaves and the center stock. To help control the spread of septoria to the plant’s other leaves, reapply the fungicide every seven to 14 days.

Precautions

You probably won't be able to avoid Septoria leaf spot all together. It is very wide spread and given the ideal conditions, it will try to take hold in your garden. However there are some precautions you can take to lessen its impact and prevent it from spreading further.

• Use disease free seed. There's no evidence that it is carried by seeds, but err on the safe side and don't save seed from infected plants.
• Start with a clean garden. Dispose of all affected plants. The fungus does not remain in the soil, but it can over-winter on the debris of diseased plants. So it's important to dispose of all the affected plants far away from the garden and the compost pile. Keep in mind that it may have spread to your potatoes and eggplants, too.
• Avoid overhead watering. Water aids the spread of Septoria leaf spot. Keep it off the leaves as much as possible by watering at the base of the plant only.
• Provide room for air circulation. Leave some space between your tomato plants so there is good air flow. Stake them so that they are not touching the ground and not all bunched together. Good air circulation is especially important during damp and rainy periods.
• Mulch below the plants. A layer of mulch will prevent spores on the ground from splashing up onto the lower leaves.
• Consider organic fungicide options. Fungicides containing either copper or potassium bicarbonate will help prevent the spreading of the disease. Serenade® has also shown good promise. Begin spraying as soon as the first symptoms appear and follow the label directions for continued management.
• Plant next year's tomatoes in a different section of your garden. In small gardens, it's not always practical to rotate your crops, so good clean up and sanitation become even more important.

Other options

• Remove and destroy infected leaves (be sure to wash your hands afterwards to keep fungus from spreading unnecessarily).
• Treat organically with copper spray, which you can purchase at the hardware store or home improvement center. Follow label directions. You can apply until the leaves are dripping, once a week and after each rain.
• Once blight is present and progresses, it becomes more resistant to biofungicide and fungicide. Treat it as soon as possible and on a schedule.