Yellow Spots on Tomato Plant Leaves – What’s Causing Them?

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Tomato plants are one of the most common fruits in the world and are classed as a delicacy in many different cultures. 

This plant’s popularity comes from its ease of care and its ability to thrive with little maintenance needed. 

However, one of the downsides to tomato plants is that they’re disease susceptibility. 

The tomato plant’s leaves are usually the first to show signs of infection, with any color changes usually meaning that your plant is suffering.

If yellow spots have appeared on your tomato plant’s leaves then this is definitely a cause for concern, so read ahead to find out all about it!

 

Yellow spots on tomato plant leaves

If you notice yellow spots on your tomato plant’s leaves, this could either be a sign that you need to make adjustments to the care you are giving your plant, or that it has been affected by a disease. This can often be resolved by using fungicides, fertilizer, or repotting your plant. 

 

Causes of yellow spots on tomato plant leaves

 

Tobacco Mosaic Virus

Tobacco Mosaic Virus (TMV) is a plant virus that causes light patches of yellow and brown to appear on the leaves of your tomato plant.

In some cases, the yellow patches of discoloration will appear to create a slight mosaic pattern on the leaves.

Unlike most other tomato diseases, TMV impacts both the plant and its fruit. If affected, the tomato fruit will grow with light brown patches on the skin. 

The virus is often mistaken for other diseases, due to having similar symptoms to that of nutrient deficiency and overwatering. 

It can be very hard to diagnose plants affected until symptoms appear on the fruits. 

This can particularly be an issue because of how fast the virus can spread to other plants. The virus is extremely tolerant to all kinds of weather and can last from between months to years on plant seeds as debris. 

As a result of this, a plant suffering from TMV should be pulled from the ground immediately, and it is suggested that the infected plant is then disposed of or burned. 

Although there, unfortunately, isn’t a cure for this virus, staying on top of weeding your garden is one of the main ways to prevent it, as this will reduce the risk of an infected weed spreading the disease to your plant. 

You should also use sterile gardening tools and ensure that you wash your hands when handling tomato plants. 

This is extremely prevalent if you are a smoker, as the virus can be spread by bacteria left from cigarettes.

 

Overwatering (root rot)

Yellow spots on your tomato plants’ leaves alongside soil that is wet or soggy to touch are symptoms of overwatering.

When the bottom leaves of the tomato plant start turning yellow, it’s an early symptom of overwatering. As time progresses, other leaves will also begin to grow yellow spots, and the stem will become mushy and too weak to hold your plant up. 

If your tomato plant is consistently overwatered, it will lead to root rot, which is a fungal disease that attacks the roots. 

You can diagnose root rot through a touch test, by using a finger to gently push the soil away from the stem of your tomato plant so that you can examine the roots.

If the roots are soft and discolored or you can notice a foul odor coming from the soil then it is likely that your plant is in the throws of root rot. 

It is particularly hard to save a tomato plant from this disease if it is not caught in the early stages. Tomato plants are very delicate and will no longer be able to bear fruit if the roots have been severely damaged. 

However, if you have been fortunate to catch root rot early, there are a few things you can do to reverse the root rot damage. 

Start by very carefully removing your plant from the soil so that you can examine how far the damage has gone. 

Then, wash off any excess soil from the roots. Don’t panic if you start to notice that parts of the roots are falling off, these are the parts that have decayed and are no longer providing nutrients to your tomato plant. 

Use a sterile pruning tool to remove any black or brown parts of the roots that have become soft or mushy, as this will prevent the disease from spreading to the rest of the root ball.

You should now use a gentle fungicide on the roots, which can be purchased in most plant nurseries. This will help kill any remaining bacteria left by the fungus. 

Finally, you can repot your tomato plant into a new one containing a fresh batch of soil. Be sure to dispose of the infected soil and avoid putting it onto a compost heap. 

 

Septoria Leaf Spot

Septoria leaf spot is a very common disease that all gardeners should be aware of, as it is highly likely that they will come across it at some point when growing tomatoes. 

It is caused by a fungus living in the soil which attacks your plant from the roots upwards. 

This results in circular-shaped spots forming on the undersides of the leaves, particularly in the more mature and older leaves. 

As the leaf spot disease progresses, these circular-shaped spots become a dark brown color with a faint yellow ring around them. These spots will grow over time, and eventually merging and taking over the entire leaf. 

As this is quite a common problem, it is rather easily treated. A simple organic or chemical fungicide can be purchased online or in a plant nursery. 

However, the downside to this disease is that is it relatively fast spreading. Due to it mainly affecting the undersides of the leaves first, it is often the case that Septoria isn’t caught until it has progressed quite far. 

In cases when the disease has become extreme, it might be best to dispose of your tomato plant to avoid the infection spreading to the rest of your crops.

 

Nutrient deficiency

Nutrient deficiency can be a cause of yellow spots on tomato plant leaves. Tomato plants need a good balance of nutrients, especially the NPK nutrients that are included in most-store bought tomato fertilizers. 

The most common nutrient deficiencies that cause the yellowing of the leaves are nitrogen and potassium.

A tell-tale sign that your plant is experiencing nitrogen deficiency is the older and more mature leaves gradually turning a yellow color. 

In extreme cases, the leaves may even turn a bright white yellow. Growth on your tomato plant will then start to slow, and some leaves may not grow at all, staying spindly and small. 

If your tomato plant has been affected by potassium deficiency, the leaves will have spots of yellow and brown on the edges which will become dry and crispy to touch. 

Eventually, the edges will start to roll inwards, and it will almost look like the tips of the plant have been burnt.

Potassium’s main function is to aid in producing blossoms, and without a source of potassium, your tomato plant will not thrive or bear fruit. 

Nutrient deficiency is usually caused by your plant not being fertilized regularly, or not having enough nutrients in the soil. 

Tomato plants should be grown in a compost mix because it is full of organic matter that naturally contains everything your plant needs.

Use a general tomato food fertilizer to introduce nutrients into the soil again. From there, recovery should be almost immediate.

Be sure to follow the guidelines on how much fertilizer to add. Adding too much could result in giving your plant a fertilizer overdose, which causes the leaves to revive themselves but no blossoms to grow. 

 

Preventing yellow spots on tomato plant leaves

The most important way to prevent yellow spots is to practice a good level of care for your tomato plant. 

As previously mentioned, this means creating a watering schedule that is right for it, and ensuring that you fertilize your plant when needed. 

Your tomato plant also needs to be getting at least 5 hours of sunlight a day. The happier and healthier your plant is, the lesser the chances that it’ll be affected by the disease.

Leaves that have become too moist will also be susceptible to diseases, watering from the base of your plant in oppose to from the leaves downwards will promote healthy leaves. 

 

Frequently Asked Questions about Yellow Spots on Tomato Plant Leaves

 

Why do to yellow spots on my tomato plant have brown rings around them?

The appearance of rings around the yellow spots usually means that your plant is being infected by some sort of disease or virus.

 

Should I remove the yellow leaves from my tomato plant?

Though yellow leaves can sometimes mean that your tomato plant’s experiencing an ailment, do not remove any yellow leaves that are still alive. You should only remove leaves that start to feel crispy to the touch, and those that are dead.