How to Treat Leaf Spot (Before It Spreads to Your Other Plants)

The strawberries in your garden, the succulents in your kitchen, and even the turf on your lawn—they’re all susceptible to leaf spot. But what exactly is leaf spot, what causes it, and most importantly, how can you control its spread? Read on to see what our experts advise.  

What Is Leaf Spot? 

Leaf spot is a plant disease caused by fungi or bacteria, says Matt Ramos, owner of Plant Sensation & Co. "Leaf spot is typically caused by fungi or bacteria that infects the plant through wounds, cuts, or natural openings in their leaves," Ramos says.

Leaf spots can vary in size, shape, and color, depending on the specific cause—whether a fungal or bacterial pathogen—and can even fluctuate on a single plant. Spot colors can range from yellow, red, orange, tan, green, brown, and black. Smaller spots typically indicate a newer infection, whereas larger ones have had time to mature and spread. 

Ramos says that while leaf spot is not always a death sentence for your plant, it will weaken the plant and spread to others if left untreated. In some cases, it may cause leaves to drop prematurely, causing any possible fruit to fail to mature. 

How to Identify Leaf Spot 

If multiple areas on your plants have discoloration, leaf spot might be to blame. Spots can emerge either on the upper or lower sides of leaves in the beginning stages. Some may appear almost translucent or lighter in color, but others may be darker in tone. While these places may not be perfect circles, they are often circular. If left untreated, the spots may grow enough to connect and form large patches.  

When a bacterial pathogen is the cause, the middle of the leaf spot will sometimes fall out, creating holes in the leaves. The shoots, buds, and flowers may also become infected when a bacterial pathogen is involved. 

Causes of Leaf Spot 

While almost all plants may be susceptible, there are different varieties and causes of leaf spot depending on plant type. For example, a prevalent lettuce bacterial leaf spot was first discovered in 1918 and causes small, water-soaked spots on the leaves that eventually become lesions. Meanwhile, cherry leaf spot is caused by the fungus Blumeriella jaapii and attacks not just the leaves of certain varieties of cherries but also the stems and fruit, as well.

However, there’s one common denominator when it comes to all types of leaf spot—they flourish in moist conditions. Extended periods of rain can often bring on the spread of fungal outbreaks, and high moisture levels are favorable conditions for the spread of bacterial leaf spot, as well.  

"These pathogens thrive in humidity and can spread rapidly under favorable environmental conditions," says Ramos. "Especially when the plant or leaves have wounds, cuts, and openings."

How to Fix Leaf Spot 

Early detection and action are key, so keep a close eye on your plants. Examine the underside of the leaves to identify spots early. It’s imperative to remove the infected pieces and dispose of them properly. Don’t just toss the pruned pieces nearby—seal them in a bag and remove them from your property.   

Leaf spot cannot be "cured," according to Joel Burnsed, an agricultural and natural resources agent and county extension coordinator with the University of Georgia. However, when it comes to fungal infections, "fungicides can be applied to prevent further spreading."

Ramos suggests doing additional research or reaching out to a local professional for advice before applying a treatment yourself.

How to Prevent Leaf Spot 

A key deterrent to prevent leaf spot is controlling the moisture on your plants as much as possible. Reduce overhead watering to prevent moisture from sitting on your leaves and try to water at the base only. If this isn’t possible, water your plants early in the day so they have ample time to dry out. 

Proper spacing between fully-grown plants is important to help with moisture reduction, so plant accordingly and trim when needed. Some fungi can jump and spread through surrounding weeds, so it’s imperative to keep these nuisances at bay. Also, be sure to immediately prune any infected spots and dispose of them in a sealed bag. 

"Sometimes fungicides can be applied as a preventative control to prevent leaf spots from starting," says Burnsed. While not required as often during dry periods, this can be an effective way to ward off leaf spot caused by fungal infections. 

Finally, be vigilant when inspecting plants before purchase. The last thing you want to do is add a sick plant to wreak havoc on a thriving, healthy garden.