WOOD ROT IN SRI LANKAN TEA (CAMELLIA SINENSIS): THE CAUSE, NATURE OF TISSUE DAMAGE AND YIELD LOSS
A BALASURIYA1 and NKB ADIKARAM2
1PI. Path. Div., Tea Res. Inst. of SL, Talawakelle, Sri Lanka; 2Bot. Dept., Univ. Peradenlya, Sri Lanka
Wood rot in tea has been reported from several countries including India, Kenya, Malawi, Zimbabwe and Sri Lanka  and implicated to Hypoxylon serpens, H. investiens, H. vestitum, H. asarcodes, H. nummularium, H. truncatum, etc., the most common being H. serpens. In Sri Lanka, the first record of Hypoxylon in tea was that of Petch [1, 2], in 1906. He observed H. vestitum, on a tea branch in a Nuwara Eliya district estate.
Presently this wood rot is found concentrated in several tea estates in the same district and as sporadic cases in a very few estates outside. Significantly, all these estates lie at or above 1500 m. The characteristic symptoms normally appear when the bushes are about 15 to 20 years old. Suddenly, the foliage of a branch will show wilting followed by scorching. Such branches snap-off easily near the base. Sometimes intact lower branches with green foliage in these bushes could snap-off similarly, especially when workers walk through during field operations. The bases of such branches have characteristic black encrustations (stroma). The causal organism is now identified as Nemania diffusa Syn. H. vestitum) [Ju Y-M, personal communication]. The pathogen appears to be disseminated mainly through pruning knives and the infections progress from prunmg cuts during wet weather.
Majority of fields affected consist of vegetatively propagated (VP) clones. A very few old fields raised from seedlings too carry heavy infections. Among the available clones, only a few are susceptible to the disease. Others, growing under identical conditions do not show any symptoms while the susceptible clones show 100% infection. Some attributes of the resistant and susceptible stem wood were compared. These included, density of wood, mineral composition, pH, biochemical extractives and wood anatomy. Of these, only the wood anatomy showed any significant difference, Susceptible clones contained about double the number of fibre cells than resistant clones. Microscopical studies have revealed typical 'soft rot' nature of the affected wood.
Under natural field conditions, infections at 60-90% and over 90%, could bring about 27 and 36% yield reductions respectively, showing that the disease can cause significant damage to the local tea industry.