Deficiency or disease? The vein test

A leaf turns yellow. The reflex is often to spray. In one case out of three there is nothing to spray: the plant is hungry, not infected.

The first question to ask

Look at the veins. On their own they answer half the cases.

Veins still green, blade yellow between them. That is interveinal chlorosis, the signature of an iron, magnesium or manganese deficiency. No fungus respects a pattern that geometric.

Veins yellowed along with the rest, fading uniform. That is a nitrogen deficiency, or a root problem — asphyxia, waterlogging, nematodes.

Veins green but the blade irregularly spotted. Now you are in pathological territory.

The second question: which leaves?

Elements that move within the plant — nitrogen, magnesium, potassium — are pulled out of old leaves to feed the young ones. A nitrogen deficiency therefore starts at the bottom.

Elements with poor mobility — iron, calcium, boron, sulphur — stay where they are. Their deficiency shows up at the top, on young leaves that never got their share.

A disease does not follow that logic: it starts wherever the inoculum landed, often low and on the outside, but without the even gradient of a deficiency.

The three most common profiles

Iron. Young leaves, clear yellow between veins that stay firmly green, in a very sharp network. Almost always an availability problem in calcareous soil, not an absence of iron in the soil.

Magnesium. Older leaves, interveinal yellowing starting from the margins, sometimes tinged red on tomato and vine.

Potassium. Older leaves, the margin of the blade browning and drying into a clean border while the centre stays green.

Legitimate doubt

Two situations call for a second opinion: a long-standing deficiency eventually necrotises and then looks like a disease; and a plant weakened by a deficiency really is more susceptible to pathogens — the two can coexist.

In those cases, a photo of the whole leaf with the stem, taken in morning light, lets an agronomist decide. The top-to-bottom gradient, invisible in a close-up, is often the deciding detail.

What the mistake costs

Spraying a deficiency with fungicide costs the product, the application time, a pre-harvest interval imposed for nothing — and above all two lost weeks during which the real cause keeps eating into the yield.