citrus tristeza virus

Symptoms

Vein clearing, stem pitting, dwarfing.

Easily confused with

• citrus greening (Huanglongbing, HLB)

  How to tell them apart: Candidatus Liberibacter asiaticus (Huanglongbing/citrus greening) shows a diagnostic ASYMMETRIC blotchy mottle on the leaf blade: irregular yellow and green patches that do NOT mirror each other across the midrib, often with corky, swollen, yellowed veins and a leathery upright leaf. Fruit are small, lopsided, stay green at the bottom (colour inversion), have curved aborted seeds and a salty-bitter taste. Citrus tristeza virus (CTV) instead causes vein clearing/flecking (translucent veins held to the light), seedling yellows, and most tellingly STEM PITTING - pits and grooves in the wood under the bark of the trunk and twigs (peel the bark to see them). On sour-orange rootstock CTV gives sudden quick-decline collapse with honeycombed bark at the bud union, whereas HLB causes a slow sector-by-sector dieback. Use the asymmetric mottle for HLB versus the under-bark stem pitting for CTV.

• citrus root and foot rot

  How to tell them apart: Citrus tristeza virus (CTV) decline shows up in the canopy first, with no rot at the trunk base: leaves turn pale and yellow with vein-clearing or vein-yellowing, the foliage cups and thins, and on susceptible sour-orange rootstocks the whole tree wilts and collapses suddenly (quick decline). Peel back bark just above the bud union on such trees and you see honeycomb-like pitting and a brown line of dead tissue at the graft, while the trunk base stays firm and dry. On grapefruit and some sweet-orange wood you also find stem pitting: long narrow grooves under the bark on twigs and branches. There is no gum, no soil-line lesion. Phytophthora citrophthora attacks from the soil and trunk base: look for dark, water-soaked bark cankers at the crown/foot that ooze amber-brown gum (gummosis), with the bark drying, cracking and sloughing off to expose dead brown wood right at or just below soil level. Scrape the feeder roots and they are blackened and rotten with a sour smell, and after rain you may see brown rot lesions on low-hanging fruit. The yellowing canopy and dieback here are a downstream consequence of the rotted roots and girdled trunk, so the tell-tale damage is low (collar, roots, soil line), not in the bud-union wood. No honeycomb bark pitting.