Septoria Leaf Spot Guide: How to Identify and Treat

When most people hear the term “fungi,” they envision mushrooms quietly growing in the forest, while consuming dead wood and other organic debris. But some fungi are actually pathogens, who not only feed on living organisms but cause serious health problems too. 

Today, we’re going to examine one particularly interesting and noteworthy fungus, which goes by the common name of Septoria leaf spot. We’ll discuss the details of the fungi, explain some of the symptoms it causes, and outline the basic treatment strategy needed to help the trees it infects. 

Septoria: The Basics

Septoria leaf spot is a catchall term applied to diseases caused by fungi in the genus Septoria. Thirty-odd species have been formally identified, and each tends to infect a relatively well-defined group of plants or trees. 

For example, Septoria dianthi primarily infects carnations, Septoria lactucae is chiefly a pathogen of lettuces, and Septoria pistachiae usually infects pistachios. But at least one species – Septoria musiva – attacks poplar trees of the genus Populus. 

The lifecycle and method by which the fungus spreads varies slightly from one species to the next. However, most are spread when the wind blows spores from an infected plant or tree to one that is not yet infected. Grafting may also allow the fungus to spread, and it is possible – though not yet confirmed – that seeds can become contaminated with the fungus too. 

The leaves of a plant or tree are typically the first location that becomes infected, with those closest to the ground usually being the first target. Over time, the infection often spreads upward, eventually involving most or all of the affected organism. 

Like many other fungi, Septoria species tend to thrive best in warm, relatively wet conditions. These fungi usually overwinter in the ground, but they can also remain on tools. This helps illustrate the importance of cleaning all shears, saws, and other gardening or tree-care implements regularly. 

Spotting Leaf Spot: Symptoms of the Septoria Disease

The primary symptom of Septoria leaf spot is – wait for it – spots on the leaves. However, the symptoms do vary slightly from one species to the next. 

Nevertheless, the spots generally look pretty similar in most cases. They’ll usually feature a dark brown, circular outline, with a khaki to grey-colored center. You’ll also see small, dark-colored fruiting bodies in the spots. Most Septoria spots are fairly small, with the largest ones measuring about ¼ inch in diameter. 

Many Septoria species eventually cause defoliation, but this doesn’t necessarily affect yield. As an example, consider Septoria glycines, which infects soybean plants. This fungus causes almost complete defoliation, but it rarely affects the number of seed pods produced in a given season. 

However, on the other hand, some Septoria species do cause damage to the fruit of a plant or tree and reduce total yield, similar to what Apple Scab can do to apple trees (although a different fungus). Septoria citri, for example, infects citrus trees and causes the fruit to develop numerous dark-colored spots. 

If left untreated, Septoria fungi can also cause more significant structural damage. Infected trees, for example, will often develop cankers after losing their leaves. This can not only expose the tree to secondary pathogens (such as bacteria and more dangerous fungi), but it can significantly weaken the trunk, causing total collapse. 

How to Get Rid of Septoria Leaf Spot

The precise method by which you’ll need to treat Septoria leaf spot will vary slightly depending on the tree or plant species you’re growing. But in general terms, the treatments all follow the same basic strategy. 

Step 1: Manual Removal

You’ll want to start by removing any infected leaves. With luck, you’ll have noticed the infected leaves quickly, before the fungus can spread and infect a significant portion of the foliage. You’ll have to use care if leaves above existing fruit are infected, as the removal of these leaves may expose the fruit to more sunlight than it can handle. 

If you are battling Septoria on tall trees, you’ll likely need professional help, as it isn’t safe for laypersons to remove large branches or work above the ground. This generally isn’t a problem with relatively short trees (such as citrus trees), but it can be a problem with poplars and other tall species.  

Be sure that you dispose of the leaves properly, so they can’t spread the fungal spores throughout the environment. Burning them in a safe manner is likely the best choice, but you can bury them deeply if you prefer. Just be sure not to introduce them to your compost pile. 

Step 2: Pruning & Maintenance

After removing the infected leaves, you’ll want to ensure that the plant or tree in question is receiving plenty of air flow. This will help support the tree’s health and vigor, while simultaneously making life hard on the fungus. It’s also wise to mulch around the base of the plants and trees to prevent water from splashing spores off the soil and onto your plants. 

Additional management strategies that are generally helpful include things like using soaker hoses rather than overhead irrigation strategies. It’s also wise to eliminate weeds promptly, as many can carry Septoria spores which will then infect your plants. 

Step 3: Fungicides for Leaf Spot

If nothing else works, you can try using fungicides to help reduce loses and better support your plants and trees. Fungicides vary in terms of efficacy when trying to treat these leaf spot diseases, so you’ll need to select one specifically labelled for the crops you’re growing. It is also important to note the directions regarding fruit or vegetable harvesting, as some may render your crops inedible. 

Nevertheless, copper-based fungicides, such as Bordeaux mixes, are often effective. However, Bordeaux mixes can be harmful to fish, livestock and earthworms, so discretion is obviously important. Always apply these types of fungicides as directed by the product label, but most are designed to be applied to the plant or tree’s leaves at intervals of about two weeks. 

Chlorothalonil is another popular fungicide used to treat Septoria leaf spot. It is applied in the same basic manner as copper-based fungicides, and it too must be re-applied regularly. It is also harmful to fish and wildlife, and it can cause skin or eye irritation, so care is required during the application process.

Actinovate is another fungicide that is often effective against Septoria leaf spot. And unlike some other fungicides, which rely on toxic chemicals, Actinovate relies on beneficial bacteria. This means it not only works systemically, but also that it is approved for organic use. Actinovate is also applied to the soil, rather than the plant’s above-ground structures, which many growers find more convenient and appealing. 

It is important to note that fungicides are not effective for treating leaves or fruit that is already infected. Think of them as preventative treatments, rather than anything that will cure your plants and trees. 

Finally, you’ll always want to rotate crop species after dealing with Septoria leaf spot. If, for example, you end up battling the pathogen on your tomato plants, you’ll want to avoid planting tomatoes in the same area for several years. 

Preventing Leaf Spot

Like most other plant and tree diseases, the best way to defeat the pathogens is by preventing them from becoming established in the first place. Aside from implementing good growing and plant-management practices, you can also try to do so through the use of cultivars and varieties that are resistant to Septoria leaf spot. 

Not all Septoria targets are available in resistant forms, but there are several available. For example, native eastern cottonwood trees are naturally resistant to the fungus, while hybrid poplars are not. So, if you are considering planting poplars on your property, you may want to consider using the native variety rather than man-made hybrids. 

In other cases, completely resistant varieties do not exist, but partially resistant forms are available. Tomatoes provide a good example of this, as the Iron Lady tomatoes are partially resistant and often better prepared to fight off the fungus on their own. 

Like many other pathogens, Septoria leaf spot is a serious disease that demands swift and decisive action to eliminate. You’ll obviously need to tailor your treatment strategy to the plant or tree species suffering from the infection, and even then, you may not be successful. This illustrates the importance of implementing good gardening and tree-care practices (such as cleaning your tools after each use and regularly inspecting your plants) to help – if you’ll forgive the expression – nip the infection in the bud.