White Root Disease in Rubber

Rigidoporus microporus: the most economically important root disease in Malaysian rubber. Recognition, expanding-focus scouting, and the cordon isolation and destroy protocol.

White root disease (Rigidoporus microporus, syn. Rigidoporus lignosus) is the number-one killer of Malaysian rubber. It spreads by root-to-root contact and by contact with infected stumps, forming expanding circular foci that can wipe out entire blocks if not stopped early. It is the top priority for the Field Walk Protocol root-collar inspection every quarter.

Cause

• Old rubber stumps left in the ground at replanting (primary reservoir)
• Adjacent forest or old rubber at block boundaries
• Root contact with an infected neighbor
• Waterlogging weakens tree defenses and accelerates infection
• Wound sites on roots from ploughing or machinery

Effect

Stage	Above-ground symptom	Below-ground symptom
Early	No visible above-ground symptom	White rhizomorph fans on root surface
Mid	Yellowing canopy on one side of the tree	Rhizomorphs spread up the root collar, root rot begins
Late	Wilting, defoliation, latex flow drop	Root system rotten, tree death within 6 to 12 months
Terminal	Tree dead, fruiting bodies (orange bracket fungi) at collar base	Complete root destruction, adjacent trees already infected

The disease spreads at 1 to 3 m per year from tree to tree. A focus that starts with one tree grows to 5 to 10 dead trees within 2 years if not stopped.

Satellite signals

Index	What to watch	Threshold
NDVI high-res	Localized declining cluster, expanding month-to-month	Direct observation
Anomaly view	Same-block year-over-year, expanding low-NDVI patch	Direct observation

Cross-check any expanding cluster against the rubber ecosystems baseline to distinguish white root from wintering.

Action

1. Confirm at the collar

   Scrape topsoil at the collar of the suspect tree. White fan-shaped rhizomorphs on the root surface confirm Rigidoporus microporus. If unsure, contact the Malaysian Rubber Board for confirmation.

2. Cordon isolation trench

   Dig an isolation trench (60 cm deep, 30 cm wide) around the focus, at least 2 m outside the last infected tree. This severs root contact and stops horizontal spread.

3. Inspect every neighbor within 5 m

   Scrape collar of every tree within 5 m in all directions from the infected focus. Root grafts spread the pathogen invisibly.

4. Fell, uproot, and burn infected trees

   Fell all confirmed and probable trees. Uproot the stumps (do not leave them in the ground; they are the primary reservoir). Burn all root material on site.

5. Apply fungicide to remaining neighbors

   Drench root collar of neighbors within 5 m with tridemorph (Calixin) or triadimefon (Bayleton) as per MRB recommendation. Repeat every 6 months for 2 years.

6. Sanitize tools

   Wash all tools with fungicide or bleach between blocks. Never move root material or stump wood off-site.

7. Follow up

   Re-inspect the treated zone every 3 months for 2 years. Expanding-focus surveillance is the only way to catch missed cases.

Recommended alert settings

Trigger	Condition	Severity
Expanding cluster (satellite)	Localized NDVI decline expanding > 1 tree per month	High
Scout confirmation	White rhizomorphs recorded on any tree	Critical
Boundary sweep	Automatic quarterly reminder for old-stump lines and forest edges	Automatic
Follow-up inspection	3-month cadence for 2 years after any confirmed case	Automatic

Related

• Red root disease
• Brown root disease
• Field Walk Protocol (root-collar inspection sequence)
• Rubber Diagnosis