How to Identify and Treat Septoria Leaf Spot

You’ve noticed a disturbing trend. The lower leaves on your tomato plants have developed spotting. Some are turning yellow and falling off. And it looks like it’s spreading.

If this sounds familiar, you may have just discovered septoria leaf spot.

While this is likely the most common tomato disease known to home gardeners, it’s fixable. It’s also not limited to tomatoes, possibly popping up in a few places you wouldn’t expect.

You don’t have to watch your plants drop leaves until there’s none left. Follow this guide to identify, treat, and prevent septoria leaf spot on tomatoes, or whatever plant is afflicted in your garden.

What Is Septoria?

Spots on tomato leaves can be a sign of septoria, a fungus scientifically known as Septoria lycopersici. Attacking at any stage of development, this fungi is one of the most damaging tomato diseases, although not one of the deadliest to plants.

It doesn’t just affect tomatoes, either. Septoria lycopersici damages many different plant types. Among those are other popular garden staples like potatoes and eggplants. Any Solanaceae family plant may be affected by this fungal species.

The Septoria genus of fungi is quite large, with over a thousand fungal species. As would be expected from such a large genus, other forms of this fungal disease exist too.

Septoria pistaciarum causes leaf spotting in pistachio trees, while Septoria glycines impacts soybean crops. Septoria cucurbitacearum affects cucurbits. The list goes on.

No matter which species of septoria is discovered, it’s essential to treat it. Without treatment, it can rapidly spread.

Life Cycle Of Septoria

Fungal spores can linger and survive in infected plant debris on the soil’s surface. The fruiting bodies (or pycnidia) can also be buried in the soil, lying in wait. The fungus overwinters on nearby weeds, too.

Wind, splashing water, or insects can carry these spores onto the lower leaves of your plants. They attach to leaves, gradually causing spots to appear. These spots will be distinctive, with a greyish center and brownish edges.

The center of the lesions contains the pycnidia, the fruiting bodies of the fungus. As the damage to lower leaves becomes severe and they fall, these fruiting bodies work their way into the soil. From here, they spread again by water, wind, insect transmission, or even on human hands or tools.

The fungi appreciate moderate and wet or humid weather. Septoria often appears during the spring and can reoccur throughout the summer. It becomes less common in the fall and winter as the weather is less conducive to sporulation.

Signs of Septoria Leaf Spot

To effectively treat septoria leaf spot, you need to ensure it is the problem you’re dealing with in the first place. Plants in the Solanaceae family are the most common victims, but there are many other plants susceptible to the disease.

Look out for these signs to identify a septoria problem and tackle it immediately.

Tomatoes, Potatoes, and Eggplants

Initially, you may notice a stippling effect across tomato plant leaves. This stippling resembles damage done by other insects or diseases and looks like tiny yellow or brown spots. It may not immediately be identified as septoria-related, but does signal the start of a problem.

As it matures, water-soaked lesions on the leaves appear. Over time, they grow up to a quarter inch in size. The center will be a greyish-white color, with a darker margin. Often there will be a patch of brown surrounding the lesion.

Over time, the lesions multiply and form spores that look like tiny black dots. These spores spread to other leaves by insects, wind, water, or human touch. Leaves yellow as the lesions grow larger. Eventually, the leaves will drop, recolonizing the soil with more spores.

This process usually begins close to the ground with the lowest leaves of a plant. If not treated, the fungi can spread higher through the plant. Too many leaves impacted reduce the plant’s ability to photosynthesize light. This can become deadly to the plant.

Other Plants

On soybeans, the stippling may be brown or reddish in color. Spotting can range from pinpoint in size to almost a quarter inch across. With these spots, as with tomato or other solanaceous plants, leaves will begin to yellow and drop. The only significant difference is the initial color of the spotting.

In pistachio trees, septoria produces distinctly brown spotting. There may be hundreds of spots per leaf. Over time, leaves will brown and drop. If the fungal infection is severe, the entire canopy of a tree can become damaged.

A form of septoria also infects cannabis plants. Spots begin as yellowish or brownish, gradually forming a darker greyish center as they enlarge. These too will rapidly spread upward on the plant.

Other plants that may be at risk include:

• Mint
• Strawberry
• Sweet potato
• Citrus
• Pecan
• Corn
• Azalea
• Sunflower
• Hydrangea

Damage from all forms of the fungi is visible on both sides of the leaf. Spores will be darker and form at the center of the leaf spots. In most cases, the center of the spot will be grey or greyish-white. In all cases, the leaves will yellow and drop over time.

Controlling Septoria Leaf Spot

As with so many other diseases, there are two lines of defense. Once it’s appeared, treatment is necessary to prevent further spread. But prevention is an important secondary measure, as it stops it from happening at all.

Septoria Leaf Spot Treatment

Before you treat plants with leaf spot, ensure you know what type of leaf spot you’re dealing with. Another leaf spot disease, Alternaria, is also common in home gardens. If the spotting begins close to the ground, it’s likely septoria.

Begin by removing damaged leaves from the plant. Wash your hands or sterilize any tools used to remove the leaves afterward. This ensures you aren’t spreading any fungal spores. Also pick up any fallen leaves beneath the plant. Don’t compost the infected plant material – destroy it instead.

For all treatment methods, pay close attention to the labeling on the container, especially if treating edible plants. Different formulations of fungicide may require a short delay before it’s safe to harvest your produce. This gives the fungicide time to break down.

An organic fungicide that works against septoria leaf spot is a copper fungicide. There are many copper-based fungicidal sprays on the market. Ideally, a copper diammonia diacetate complex is best for treatment. Copper octanoate/copper soap may also work, but is a weaker treatment method.

Some biological fungicides are also effective. Fungicides using Bacillus subtilis have been shown to eliminate many forms of fungi. This beneficial bacteria also affects a wide variety of other plant diseases.

If organic methods like copper fungicide and Bacillus subtilis aren’t working, don’t panic. There are chemical controls as well. Chlorothalonil-based fungicides are effective against septoria leaf spot. Mancozeb fungicides have also shown some effectiveness.

With both of these, keep an eye on the label, as it may be significantly longer before edible harvesting can resume.

How To Prevent Septoria Leaf Spot

Many prevention techniques work against this and other fungal diseases. Let’s explore your options.

Remove Debris

Remove all fallen plant debris from your garden beds. Fallen leaves can harbor spores and allow them to overwinter. Be sure to clean your hands and sterilize the tools you use to remove debris if you suspect fungal disease may be present.

Weeding

Keep weeds in check. This is especially true with Septoria lycopersici. Many solanaceous weeds can harbor fungal spores. Diseased weeds will spread the disease to nearby plants.

Mulch

Mulch around your plants. This has a dual effect. It prevents soil from splashing up onto the leaves, which can spread spores to your plant. It also reduces the likelihood of weed development. A three to four-inch layer of straw mulch will also aid in water retention in the soil.

Water Carefully

Water the soil, not the plants. Drip or soaker hose irrigation will reduce the likelihood of spore spread.

Rotate Crops

Septoria fungi can survive in diseased plant debris, weeds, or perennials for up to three years. Crop rotation can reduce the likelihood of reinfection year after year.

Improve Airflow

Ensure there’s proper airflow. Plants that are tightly packed together are at higher risk of contracting diseases.

Trimming excess foliage will provide better airflow. It also reduces the ease of spore spread, as leaves are spaced further apart. Staking plants or securing them in sturdy tomato cages can help with airflow too.

Check for Pests

Keep pests at bay. Pests may accidentally carry spores from plant to plant. Reducing your pest population will also reduce spore transmission.

Select Disease-Free Varieties

While tomatoes are all susceptible to septoria, other plants have resistant hybrids. Picking a variety resistant to septoria will reduce its occurrence.

Trim

Remove and destroy damaged material. If you see signs of reappearing fungal damage, trim it off before it can spread. If you remove it before it forms spores, it won’t have time to spread further.

Be sure to sterilize any pruning tools you use to remove damaged leaves and wash your hands before touching the healthy parts of the plant.

Final Thoughts

Septoria can cause lots of damage to your tomatoes, and maybe some other plants. But it doesn’t have to spell the end of a successful garden. Keep a watchful eye out for the symptoms and be ready to act when necessary.

Frequently Asked Questions