Rubber Tree (Hevea brasiliensis) Bark Necrosis Syndrome II: First Comprehensive Report on Causal Stresses

Area of Science:

• Plant Pathology
• Agronomy
• Environmental Science

Background:

• Bark necrosis (BN) affects modern rubber plantations globally, with varying severity.
• Previous etiological studies have been inconclusive, but environmental factors are associated with BN.
• Epidemiological surveys indicate nonrandom disease emergence in specific plantation areas.

Purpose of the Study:

• To investigate the environmental and physiological factors contributing to the emergence of bark necrosis in rubber trees.
• To understand the relationship between soil conditions, water status, and disease development.
• To propose a comprehensive scenario for the multifactorial physiological disease.

Main Methods:

• Epidemiological surveys in African and Asian rubber plantations.
• Physical soil analyses (penetrometer) and chemical soil parameter assessments.
• Ecophysiological measurements of leaf and stem water potential using a pressure chamber.
• Comparative root counts and preliminary dye transfer studies at the rootstock/scion junction.

Main Results:

• Risky areas for BN emergence are characterized by proximity to swamps, roads, windrows, or slope breaks.
• Higher soil compaction and poorer root development were observed in BN-affected areas.
• Water stress was indicated in BN trees, suggesting nonoptimal vascular relations.
• Initial symptoms preferentially appeared near the rootstock/scion junction.

Conclusions:

• Soil compaction and reduced water availability, coupled with poor root function, are suspected primary exogenous causal stresses for BN.
• Disturbed sap flows and latex drainage may exacerbate the disease process.
• A multidisciplinary approach suggests BN is a multifactorial physiological disease potentially involving cyanide release.