Septoria Leaf Spot: What It Is And How To Fix It



You’ve noticed a disturbing trend. The lower leaves on your tomato plants have developed spotting. Some are turning yellow and are falling off. And it looks like it’s spreading. If this sounds familiar, you may have just discovered septoria leaf spot.

But while this is likely the most common tomato disease known to home gardeners, it’s fixable. It’s also not limited to tomatoes — it may pop up in many places you wouldn’t expect!

Let’s talk about what septoria fungi are, what they cause damage to, and how to treat it. You don’t have to watch your plants drop leaves until there’s none left… you can fix this!

Products To Treat This Leaf Spot:

What Is Septoria?

Septoria leaf spot is a common problem in home gardens. Source: Scot Nelson

Spots on tomato leaves can be a sign of septoria. Septoria leaf spot on tomatoes is caused by a fungus, 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 can cause damage to many different plant types. Among those are other popular garden staples like potatoes and eggplants. Any Solanaceae family plant may be affected by this particular fungal species.

The Septoria genus of fungi is quite large, with over a thousand fungal species. And 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, as an example. Septoria glycines impacts soybean crops. Septoria cucurbitacearum affects cucurbits. The list goes on and on!

No matter which species of septoria is discovered, it’s absolutely 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 itself and lying in wait. The fungus can also overwinter on nearby weeds.

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

The center of the lesions on the leaves contain the pycnidia, the fruiting bodies of the fungus. The pycnidia are where spores form. As the damage to lower leaves becomes severe and they fall, these fruiting bodies may work their way into the soil. This means that spores can later be re-spread by water, wind, or insect transmission, or even on human hands or tools.

The fungi does best during periods of warm and wet or humid weather. It 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.

Symptoms On Tomatoes, Potatoes, & Eggplants

Early signs of stippling on leaves
The stippling or dotting on these leaves may be an early sign of septoria infection. Source: verymom

Initially a stippling effect across tomato plant leaves may be observed. 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. However, it’s a good sign that you have the beginning of a problem.

As it matures, water-soaked lesions on the leaves appear. Over time, they grow in size, and can grow up to 1/4″ across. 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 will multiply and form spores which look like tiny black dots. These spores can be spread to other leaves by insects, wind, water, or human touch. Leaves will yellow as the lesions grow larger. Eventually the leaves will drop, recolonizing the soil with more spores.

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

Symptoms On Other Plants

Septoria spp. on blackberry leaf
Septoria species fungal infection of a blackberry leaf. Source: ssimp92

On soybeans, the stippling may be brown or reddish in color. Spotting can range from pinpoint in size to almost 1/4″ 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 shows as 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 which may be at risk from septoria species include mints, strawberries, cucurbits, sweet potatoes, citrus, pecans, cane berries like raspberry or blackberry, lettuce, hazelnuts, wheat, corn, hops, and peas. Flowers which may be damaged include azaleas, bellflowers, carnations, sunflowers, hydrangeas, and rhododendrons.

Damage from all forms of the fungi are visible on both sides of the leaf. Spores will be darker in color and will 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

Septoria on chrysanthemum leaf
Chrysanthemum leaves are susceptible to a form of septoria too. Source: Scot Nelson

As with so many other diseases, there’s 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.

Let’s go over both of these aspects in detail. Wiping out this fungal irritant is possible, and if handled correctly, you may not see it again!

Septoria Leaf Spot Treatment

Before you can treat plants with leaf spot, it’s important to be sure what type of leaf spot you’re dealing with. Another leaf spot disease, alternaria, is also common in home gardens. You can generally assume that if the spotting begins close to the ground, it’s likely to be septoria.

Assuming that it’s septoria, begin by removing damaged leaves from the plant. Be sure to wash your hands or sterilize any tools you were using to remove the leaves afterward. This makes sure you aren’t spreading any fungal spores. Be sure to also pick up any fallen leaves beneath the plant. Don’t compost the infected plant material – destroy it instead.

For all treatment methods, please 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 which works against septoria leaf spot is copper fungicide. There’s 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 has effect on a wide variety of other plant diseases. It’s a great addition to any gardener’s arsenal!

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 also have shown some effectiveness. With both of these, be very sure to keep an eye on the label, as it may be significantly longer before edible harvesting can be resumed.

Prevention

Septoria on maize
Maize leaves develop a related disease called Septoria leaf blotch. Source: CIMMYT

There’s many prevention techniques that work against this and other fungal diseases. Let’s explore your options!

First, 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 tools you use to remove debris if you suspect fungal disease may be present.

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 around your plants. This has a dual effect. It prevents soil splashing up onto the leaves, which can spread spores to your plant. It also reduces the likelihood of weed development. A 3″ to 4″ layer of mulch will also aid in water retention in the soil, which is an added bonus!

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 3 years. Crop rotation can reduce the likelihood of reinfection year after year.

Ensure there’s proper airflow. Plants which are tightly packed together are more at risk from 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 as well.

Keep pests at bay. Remember, 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 which is resistant to septoria will reduce its occurrence.

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.

Frequently Asked Questions

Q: Can you eat tomatoes from plants with Septoria leaf spot?

A: Septoria generally only affects the leaves. The fruit is usually intact. Depending on your treatment method, you may have a delay between spraying and harvest. This enables the spray used to break down safely before you consume the fruit.

If you find spotting on the fruit, it’s likely not septoria-related. The fungi don’t colonize fruit.

Q: Can leaf spot be cured?

A: Most fungal leaf spots can be treated. Good management can reduce the likelihood of fungal spread. Treatments prevent spores from infecting new leaves, but will not cure existing damage. Some bacterial leaf spots are not treatable at all. Thankfully, if damaged material is removed and treatments applied, both septoria and alternaria leaf spots can be managed or eliminated!


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. With good stewardship of your garden and occasional treatments, your plants will thrive. Just keep a watchful eye for the symptoms and be ready to act when necessary!


The Green Thumbs Behind This Article:


Lorin Nielsen
Lifetime Gardener



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