There are many reasons why your healthy tomato plant leaves may turn yellow. This plant problem can be caused by a plant fungus, nutrient deficiency, transplant shock, or improper care for your tomatoes. Learn more about what it means, how to identify the problem together with pictures.
Some causes have cures while those without might be solved by control methods. Here is why tomato leaves turn yellow.
1. Nutrient Deficiency
You may wonder why tomato plant leaves are yellowing. It happens sometimes because of poor plant nutrition. This means they do not get enough nutrients (mineral ions) they need for proper growth.
There are many different symptoms for various plant nutrient deficiencies. We have narrowed them down to yellow leaves in tomatoes.
a) Magnesium Deficiency
Magnesium plays a very crucial role in the formation of green matter. Therefore, a total lack of it causes stunted growth and poor growth. According to the South Dakota State University Extension, magnesium deficiency is the most common especially in tomatoes.
Symptoms
Older leaf blades lose chlorophyll except for the veins (see the picture below). The leaf blades appear light green or eventually turn yellow. High levels of magnesium deficiency can cause affected leaves to turn purple.
How To Manage Magnesium Deficiency
Side dress 1 tablespoon Epsom salt for each affected plant. You can use drip bottles. More importantly, avoid mixing with any other fertilizers. For example, potassium-rich fertilizers should not be applied when correcting magnesium deficiency.
Gardeners can also use Epsom salt sprays available.
b) Nitrate Deficiency
Apart from magnesium, nitrates also play an important role in plant growth. Nitrates help in the formation of green matter. The yellowing due to nitrate deficiency produces light green or pale green colored foliage.
Like magnesium deficiency, tomatoes grown in soils without nitrate are stunted. Yields are likely to drop. Fruits rarely have any symptoms but tomato fruit production is affected.
Apply manure or inorganic fertilizer with nitrate ions to solve the problem.
2. Fusarium Wilt
Fusarium wilt is one of the plant problems categorized under a group known as wilt diseases or fungal wilts. It is a soil-borne disease. Besides tomatoes, it affects many plants including potatoes, eggplant, strawberry, black raspberry among others.
What Causes Fusarium Wilt?
Fusarium wilt of tomato is caused by a plant fungus sp. Lycopersici belongs to Fusarium oxysporum species.
Symptoms of Fusarium Wilt
The first symptom of fusarium wilt is marked by tomatoes leaves starting to wilt or turn yellow. Why? The plant is poorly nourished due to blocked vessels.
The leaves may turn brown as the infection advances. Other symptoms may be analyzed after you carefully cut along the stem. Discoloration of the vessels can be seen. As the infection develops, the leaves wilt and dry up. Eventually, this disease makes the plant collapse and die.
Infection
When you plant tomatoes on an inhabited field, the fungus enters through the roots. They move up the plant water vessels destroying their cells. As a result, the water system and absorption of minerals are heavily affected because of the blockage. Infection symptoms in acidic soils are severe.
Fusarium Wilt Life Cycle
How is the fungus’ life cycle like? During its resting stage, sp. Lycopersicifungus can survive in uncultivated soil or dead plant material for years. Like many fungi, sp. lycopercisi reproduces through spores. This normally occurs during dry weather when soils are warmer with low relative humidity. The infection cycle occurs once every growing season for tomatoes.
The fungus may also be introduced into a newly prepared field through:
- infected seedlings
- contamination of planting seeds
- water and wind dispersal is rare
Is Fusarium Wilt Curable?
There are no known chemicals or registered products for fusarium wilt. This disease is currently not treatable. With that in mind, infected plants usually suffer during day time but recover at night. However, tomato plants don’t recover from fusarium wilt. They eventually die.
According to the Institute of Agriculture (University of Tennessee), the use of resistant varieties is the most effective control measure.
3. Verticillium Wilt of Tomato
Verticillium wilt is another wilt disease responsible for tomato leaves turning yellow. The fungus Verticillium dahliae causes it. Like fusarium wilt, this disease is soil-borne.
How Infection Occurs
After planting, the fungus enters the tomato plant via roots. It moves up its stem multiplying and causing serious damage. Consequently, the water conducting vessels get blocked.
Symptoms of Verticillium Wilt in Tomatoes
During the early stages of verticillium wilt invasion, older and lower tomato leaves begin turning yellow. Take a look at the pictures below. Other symptoms of the disease in plants include:
- Stunted or poor growth
- Leaves curling upwards starting from the margins
- Foliage loss
- Brown to black streaks on the water conducting vessels
- Sometimes the water vessels appear colorless
The whole diseased plant collapses with the severity of infection.
Is Verticillium Wilt Treatable?
The disease has no cure. There are no fungicides known or registered products to treat/control the fungus. However, you can opt for pest and disease control.
How to Control Verticillium Wilt
Observe and carry out these control measures to enable you do improved tomato production.
a) Select Resistant Plant Varieties
The University of California, Vegetable Research and Information Center, advises selecting seeds labeled V after the name as the most effective.
b) Use Other Control Methods
- Crop rotation – rotate tomatoes with grasses/crops less vulnerable to the Verticillium dahliae fungus
- Remove and burn all infected tomato plants
- Practice clean weeding to get rid of hiding host plants
- Subjecting topsoils to high temperature heating especially in dry areas
4. Septoria Leaf Spot of Tomato
Septoria leaf spot is a fungal plant disease. It is caused by the fungus Septoria lycopersici. The disease reproduces through spores.
How Infection Occurs
Septoria leaf spot spores are spread mainly by splashes of rainwater, insects, and equipment and through hands. Upon landing on your tomatoes, the spores will germinate. Infection begins at the point they enter via leaf stomata.
Notably, “tomatoes may often be infected with leaf spot and early blight (Altemaria solani) simultaneously,” [vegetablemdonline.ppath.cornell.edu]. However, you can each from their respective symptoms.
Symptoms
Five days or so after infection, small grey spots form on the old lower leaves. This usually marks the first symptom of infection.
These spots become larger. More leaf damage is done as the infection advances. As a result, leaf areas surrounding the enlarged spots start yellowing. Many small brown pimple-like swellings form at the center of these spots.
Later on, tomato leaves turn brown and wither. Leaves dry up and drop off eventually. You can also see these symptoms on tomato seedlings grown in greenhouses. Septoria leaf spot is one of the reasons for tomato plant leaves turning yellow with brown spots
Treating Septoria Leaf Spot in Tomatoes
You can treat this fungal disease with the use of registered control fungicides. They are sold with different names namely:
- Chlorothalonil
- Maneb
- Mancozeb
- Benomy
Managing Septoria Leaf Spot
Learning how to manage the disease is the best way to control spores from spreading. As a result, this helps stop the disease from developing and occurring in the next growing season. Control measures are mainly carried out by cultural practices. These include:
a) Grow Clean & Disease-free Tomato Seeds
Buy or obtain your sowing seeds from disease-free zones. Use clean hands as well as and cultivation tools to avoid contamination.
b) Clear Weeds and Plant Debris
Before you start a new seedbed or nursery, remove all possible host plants such as jimsonweed. Clear all tomato plant remains from the previous growing season. Collect these together and destroy them in fire or spay fungicide.
c) Rotate Tomatoes + Host Crops
If you have a potato plantation, allow a 3 – 4 year gap before you can pant tomatoes.
d) Break Fungus Cycle and Development
Consider the following:
- Proper pruning – remove lower suckers and older leaves
- Proper tomato spacing between rows and individual plants
5. Early Blight in Tomatoes
This is a plant disease caused by fungi that belong to Alternaria family. Besides tomatoes, it occurs in a host of plants such as eggplant, hairy nightshade, potatoes. It is a common tomato plant disease and can cause leaves to turn yellow. The fungi enter through wounds created by pests, injury due to pruning or poor handling.
Symptoms of Early Blight in Tomatoes
The initial symptom of early blight is the formation of irregular dark spots on leaves at the bottom. As the fungus produces more spores, the dark spots enlarge forming rings or concentric circles.
As early blight disease develops, areas around the blotches turn yellow. This occurs because the fungus kills the plant cells.
The disease is remarkable of the a “bulls-eye” pattern on infected leaves (see picture). These symptoms progress to the upper leaves.
The fruits and stems develop rot lesions with circular ring patterns. The rot normally appears sunken. Lesions can expand to cover the whole tomato fruit.
If not treated, entire tomato plant loses its foliage.
How to Manage & Control Early Blight
Disease management aims to stop the fungus from spreading throughout the various stages of tomato growing.
a) Growing Tolerant Tomato Varieties
Tomato varieties that are more tolerant to early blight include:
- Mountain Fresh Plus F1
- Juliet F1
- Cabernet F1
- Rutgers
b) Applying Preventive Fungicides
Apply preventive fungicides. Look for products labeled chlorothalonil (or mancozeb. These products protect tomato foliage against the germination of spores.
IMPORTANT: Ensure that you read the manufacturer label for information on the reapplication period and harvest interval.
b) Proper Tomato Caring
Certain routine care is crucial in dealing with the underlying problem. For instance, apply fertilizers using recommended/correct application rates. This will reduce plant stress.
c) Clearing Other Known Hosts
Another control measure is by removing alternative host plants. Clear all vegetation near your greenhouse and backyard gardens. Ensure your weed cleanup process is thorough.
d) Rotating Tomato Plants
Give your tomatoes plants a growing gap of 3 years. Avoid potatoes in your crop rotation regime.
How to Treat Early Blight
If infection symptoms are severe, apply mancozeb, chlorothalonil or copper fungicides. Always adhere to rates of application as given by the manufacturer.
6. Tomato Transplant Shock
Transplant shock is a problem every farmer and gardener has experienced. It is a temporary response to stress.
Signs of Transplant Shock
Signs are visible on plant shoot, leaves or stem (or all of them). The stems may bend over and appear weakened. The leaves wither slightly. They can start yellowing too. This yellowing is due to reduced transpiration and plant cooling.
How to Manage Transplant Shock
It may be difficult to stop the tomato plants from responding to some shock after being transplanted. However, there are things you can do to save them. This is as follows:
a) Harden Off Your Seedlings
If you have sown seeds in pots indoors, harden off by gradual exposure of seedlings. Begin by introducing them to indirect sunlight under a shade inside the first week. Bring them inside.
Move them out into the early morning sun for 1 – 2 hours per day for a few days in the second week. Then increase hours of exposure to 3 – 5 hours/day.
b) Transplant when it’s Cloudy
Transplant when it is cool, calm and cloudy. This gives your plants time to adapt and recover faster. So, rely on meteorological data and weather forecast apps.
c) Transplant Tomatoes with huge Root Balls
Use a shovel to dig out the seedling together with a huge ball of soil. Doing so helps your tomatoes to recover from transplant shock quickly.
d) Water the Transplanted Tomatoes
Good soil moisture content in the field soil minimizes adverse transplant shock. Watering may also help.
e) Control Tomato Hormones
This method involves applying a chemical substance that controls targeted hormones. For example, “abscisic acid is effective in minimizing water stress during transplanting by limiting transpiration.”[swfrec.ifas.ufl.edu]
7. Other Possible Causes
In addition to the above causes, here is what yellowing of your tomato plant leaves means.
- Underwatering
- Soil compaction
- Lack of potassium ions in the soil
- Tomato chlorosis virus
- Tomato yellow leaf curl virus
Leaves of your tomato plants turning yellow could also mean that the growing season is coming to an end.
Sources & References
- Early Blight of Tomato. https://content.ces.ncsu.edu/early-blight-of-tomato. Accessed online 31 Aug. 2021
- VERTICILLIUM WILT ON TOMATO. University of California, Vegetable Research and Information Center. https://vric.ucdavis.edu/pdf/TOMATO/tomato_verticillium-tomato.pdf. Accessed on 31 August 2021
- Septoria Leaf Spot of Tomatoes. The University of Maryland. https://extension.umd.edu/resource/septoria-leaf-spot-tomatoes. Accessed online 4th Sept. 2021
- Eliminating Transplant Shock by Hormonal Control to Improve Growth and Yield of Tomato. University of Florida, IFAS. https://swfrec.ifas.ufl.edu/docs/pdf/veg-hort/tomato-institute/presentations/ti2015/ti2015_Agehara.pdf. Accessed online 4th Sept. 2021