Why citrus leaves turn yellow: read the pattern, find the cause, then act

Three questions

Which leaves? Old leaves yellowing first often points to nitrogen or waterlogging. New leaves first, with green veins and yellow between them, suggests a micronutrient such as iron or zinc.

Where on the leaf? Uniform yellow, between the veins, or at the edges? The pattern tells an expert a lot from a photo.

Where in the orchard? All trees alike: season, water or nutrition. Trees in the low part: waterlogging. One or two trees severely yellow with twigs dying: root trouble or disease. If a gummy, sticky ooze comes out of the trunk it may be gummosis (Phytophthora), a serious disease; show it to the KVK at once.

Waterlogging

In the monsoon, water standing around the roots stops them breathing; leaves yellow and drop. Drainage first: open a channel and let the water out. Fertiliser does not fix this.

In the monsoon the satellite report gives an estimate from radar of whether water is standing; it is an estimate, not a certainty. When that message arrives, walk the low ground.

Nutrient deficiency

Nitrogen, iron, zinc and manganese deficiencies show different patterns and can be recognised from photos. With a high soil pH, nutrients present in the soil are still unavailable to the tree.

Which fertiliser, how much, how: that follows a soil test and an agronomist's advice. This guide gives no amounts.

Natural leaf fall and the bahar stress

During the bahar stress old leaves are expected to yellow and drop. A tired tree after harvest sheds some leaves too. These patterns are even across trees and tied to the season.

What the satellite shows

One satellite index (NDRE) tracks leaf chlorophyll. Where it stays low in one block of the orchard, look at that block's leaves first. The satellite does not give the cause; it tells you where to look.