alkaline-soil chlorosis

Symptoms

Yellowing between leaf veins while veins themselves stay green (interveinal chlorosis), youngest leaves most affected, growth stunted, no pests or fungal signs.

Plants affected

• bougainvillea
• rose of Sharon
• bigleaf hydrangea

Easily confused with

• bougainvillea leaf spot

  How to tell them apart: Cercospora bougainvilleae (bougainvillea leaf spot) is a fungus: look closely for discrete, roughly round red-brown to purplish spots 2-5 mm across with a darker margin, scattered at random over the leaf blade and present on older as well as young leaves; spots may merge into larger dead-brown patches, the tissue right around each spot turns yellow, and heavily spotted leaves drop early. It worsens in warm, humid weather after summer rain and you may see fine grey-brown fungal speckling in the centre of older spots. Alkaline-soil chlorosis is an abiotic disorder with NO spots and no fungal signs at all: the leaf yellows evenly BETWEEN the veins while the vein network itself stays bright green (a fine green herringbone on a yellow leaf), it strikes the youngest leaves at the shoot tips first, often affects the whole plant or whole branches at once, and follows hard limey irrigation water or chalky soil. Decisive test: any brown-margined spots, leaf-to-leaf spread or premature drop point to Cercospora; a clean spotless leaf with green veins on a yellow ground, worst on the newest growth, is iron/manganese chlorosis from high pH.

• cotton aphid

  How to tell them apart: Alkaline-soil chlorosis is an abiotic nutrient disorder: leaves yellow BETWEEN the veins while the fine vein network itself stays distinctly green (interveinal chlorosis), worst on the youngest shoot-tip leaves, and there are no insects, no sticky residue and no curling beyond mild cupping. Aphis gossypii (cotton aphid) is a living pest: look on the undersides of young leaves and on soft shoot tips for clusters of soft, pear-shaped yellow-green to dark-green aphids 1-2 mm long, often with white cast skins, and the leaves turn a more uniform pale yellow and curl or pucker. The decisive tell is the aphid's honeydew - a sticky, shiny film on lower leaves often blackened by sooty mould and visited by ants; chlorosis leaves are dry and clean. Chlorosis spreads evenly across many plants on the same alkaline (high-lime) bed and improves with iron/manganese feeding, whereas aphid yellowing is patchy around the colonies.

• sunburn / leaf scorch

  How to tell them apart: On Hydrangea macrophylla, alkaline-soil chlorosis shows uniform pale-yellow interveinal tissue with a crisp green vein network, appearing FIRST and worst on the YOUNGEST leaves at the shoot tips, evenly over the whole canopy regardless of which way the leaf faces, with the leaf staying soft and flat. Sunburn / leaf scorch instead shows bleached whitish-to-tan papery patches only on the SUN-FACING side of EXPOSED upper and outer leaves (shaded and inner leaves are untouched), the damaged tissue feels dry, brittle and crispy and the leaf MARGINS turn brown and curl, and it can appear within hours of an intense sunny spell rather than developing slowly. Quick test: yellowing newest leaves all over = iron/manganese lock-up (chlorosis); bleached brittle patches with brown crisp edges on the sunny outer leaves only = sunburn. Both are abiotic with no spots, mould, webbing or insects.

Common questions

How do I recognise alkaline-soil chlorosis on bougainvillea?

Yellowing between leaf veins while veins themselves stay green (interveinal chlorosis), youngest leaves most affected, growth stunted, no pests or fungal signs.

Do I have to report alkaline-soil chlorosis?

No. alkaline-soil chlorosis is not a notifiable organism, so there is nothing to report to an authority. Treat it as a normal garden problem.

How is alkaline-soil chlorosis treated?

With a sequence of steps whose timing matters as much as the choice of product — and both depend on the plant, the season and your country. Dr. Phyto works them out in a free diagnosis.