Yellow Leaves on Tomatoes: How to Distinguish Disease from Nitrogen or Magnesium Deficiency

Yellowing leaves on tomatoes are like a fever in a human: one symptom, but a dozen possible causes. The main mistake beginners make is immediately grabbing fungicides, even though often the plant is simply "hungry." Let's figure out exactly what your tomato is trying to say.

Option 1: Nitrogen Deficiency (The plant wants to eat)

Nitrogen is responsible for green mass. If it's lacking, the bush looks tired and pale.

• What it looks like: The leaf turns yellow uniformly, starting from the tip to the base. First, the old lower leaves lighten, and then the yellowing moves upward.
• What to do: Feed the plant with diluted poultry manure, "green fertilizer" made from nettles, or a urea solution. You will see results within a few days.

Option 2: Magnesium Deficiency (Tomato "fitness")

Magnesium is involved in photosynthesis, and its deficiency is often confused with viral mosaic.

• What it looks like: Only the areas between the veins turn yellow, while the veins themselves remain bright green. The leaf starts to look like a fish skeleton or a map with roads.
• What to do: Use magnesium sulfate (Epsom salt). Foliar spraying (on the leaves) works faster than watering at the roots.

Option 3: Diseases (Blight or Alternaria)

Here you must act quickly, as the fungus can destroy the entire garden in a week.

• What it looks like: Unlike nutrient deficiencies, disease manifests as spots. They can be brown, black, often with a yellow halo. The leaves don't just turn yellow; they dry out, curl, or rot.
• What to do: Remove affected leaves (do not put them in compost!), treat the bushes with copper-based preparations or bio-fungicides.

Tip from AgroPlanner

To avoid guessing why your tomatoes are suffering, plan their location in advance in our app. Avoid planting tomatoes near potatoes (they share common diseases) and leave enough space between bushes for ventilation. Good air circulation is the best prevention for fungi!

Watch the lower leaves: if they start changing color, do not ignore this signal. Timely diagnosis is 90% of your harvest's success.