Bamboo is a major raw material in the pulp and paper industry in Asia. Large quantities of bamboo culms are used also in traditional industries. Usually, extraction of bamboo culms from the forest is often restricted to 6-8 months in a year and hence, culms for 6-9 months' pulp production have to be maintained by pulp mills. Bamboo culms for use in traditional industries, as well as for structural purposes, also have to be stored for long periods of time. Possible loss of bamboo clumps owing to gregarious flowering, often occurring extensively, also necessitates the storage of bulk quantities of culms as a precaution against shortage.
Bamboo, although one of the strongest structural material available, often succumbs to fungal decay and biodeterioration during storage. The natural durability of bamboos is low, and varies from 1 to 36 months, depending on the species and climatic conditions (Purushotham et al. 1954; Chandra and Guha 1979a,b, 1981; Tewari and Singh 1979; Guha et al. 1980; liese 1980; Kumar et al. 1994). In tropical humid areas, enormous quantities of bamboo culms stored in forest depots, mill yards, etc. decay and deteriorate. The severity of decay and biodeterioration depends on the duration of storage, bamboo species, and environmental and storage conditions. During storage for up to 12 months, about 20-25% damage of culms has been reported in India (Varma and Bahadur 198O). Most bamboos used for structural purposes in rural and tribal housing deteriorate within a couple of years, and the demand for frequent replacements puts a heavy pressure on the resource.
Decay and biodeterioration of culms are caused mainly by fungi, and these include: soft rot, white rot and brown rot. Bacteria also deteriorate culms under storage, with one or more of these organisms attacking the culms in succession. Colonization by the microorganisms and the severity of attack depend on the moisture content and nutrient status (starch content) in the culm, ambient temperature, humidity,etc. Bamboo consists of 50-70% hemicellulose, 30% pentosans and 20-25% lignin (Tamolang et al. 1980; Chen et al. 1985). Ninety percent of the hemicellulose is xylan, with a structure intermediate between hardwood and softwood xylans. Bamboo is known to be rich in silica (0.5-4%), but the entire silica is located in the epidermal layers, with hardly any silica in the rest of the cell wall. Although bamboo also has minor amounts of resins, waxes and tannins, none of these have enough toxicity to impart natural durability; furthermore, the large amount of starch makes bamboo highly susceptible to attack by staining and decay fungi ( Harmada 1962; Mathew and Nair 1990; Gnanaharan et al. 1993). The sclerenchyma fibres which are responsible for the strength of bamboos are attacked by fungi and its strength is reduced considerably.
Fungal staining in stored bamboo culms develops as dark colouration. The affected culm becomes coloured in shades of brown to black. Although staining is usually superficial and may be easily brushed or scraped off, stains penetrating deep into the culm also occur. Staining is caused by fungi, which belong to Ascomycetes or Fungi Imperfecti.Fungi causing decay of bamboo culms are grouped as white rot, brown rot and soft rot. White rot and soft rot cause more serious damage than brown rot. In white rot, both lignin and cellulose are attacked. White rot of culms is characterized by a bleached appearance caused by the utilization or modification of chromogenic material in the culm. In brown rot, cellulose and its associated pentosans are attacked, while lignin remains unaffected. The lignin-rich residue of the decay imparts a brown colour to the decayed culms. In soft rot, cellulose is removed like in brown rot, but the mechanism of action on the cell wall is different. The fungi causing soft rot, like those causing staining, belong to Ascomycetes and Fungi Imperfecti, although the species involved and their action mechanism are different. Bacterial degradation of culms also occurs, but it is a slow process unlike that caused by fungi. Many bacteria may attack cellulose, while some may attack the lignified cell wall.
A large number of fungi causing decay and deterioration of bamboo culms in storage have been reported from different countries in Asia (Giatgong 1980; Wang 1985; Sutathip 1988; Arunee 1989; Shojiro et al. 1989; Mohanan and Liese 1990). In Japan, the decay fungi recorded on Phyllostachys edulis, P. heterocycla var. pubescens, P. lithophila, P. nigra, Dendrocalamus latifolius and Chimonobambusa quadrangularis include: Irpex lacteus, I. consors Berk., Tyromyces palustris, Pycnoporus coccineus ( = Trametes sanguinea), Poria vaporaria, Schizophyllum commune Fr., Polyporus versicolor (= Coriolus versicolor), etc. Of these, Tromyces palustris, the brown rot fungus, and Irpex lacteus and Polyporus versicolor, the white rot fungi, are the important ones.
In India, the decay fungi recorded on stored bamboos are Polyporus zonalis Berk., P. tenuiculus (Beauv) Fr.,P. grammocephalus Berk., Polystictus steinheilianus Berk. et Lev.,Poria diversispora Berk. et Br,P. rhizomorpha Bagchee, Pleurotussp., Lenzites elegans (Fr.) Pat.,Fomes durus (Jung.) Cunn.,F. hypoplastus Berk.,Schizophyllum commune, Stilbum erythrocephalum Ditm .,S. lateritium Berk., Tetraploa aristata Berk., Thelephora palmata(scop.) Fr., Trametes persoonii Fr.,Tremella fuciformis Berk.,Gloeophyllum striatum (Fr.) Murr.,Cyathus limbatus Hall.,Sphaerostilbe bambusae Pat.,Sporidesmium nilgirense Subram.,S. leptospermum, Cribaria intricata Schard., Flammula dilepsis Berk.et Br., Apiospora montagnei Sacc.,Lacellina graminicola (Berk et Br.) Petch, Phellinus gilvus (Schw.) Pat., etc. (Mathur 1936; Patel et al. 1949, 1951; Banerjee and Ghosh 1942; Subramaniam 1956; Bakshi et al. 1963; Kar and Maity 1971; Harsh N.S.K. 1995 personal communication ;Mohanan 1994, unpublished observation) (Figs. 132-134).
White rot fungi recorded on different species of bamboos in Thailand include: Daldinia concentrica (Bolt. ex Fr.) Ces et de Not., Trametes lactinea Berk.,Lentinus sp., Pycnoporus sanguineus L. ex Ft., Gloeophyllum sepiarium (Fr.) Karst., G. subferruginosum (Berk.) Bond. et singer, Trametes cervino-gilvus Aosh. and Haploporus ljubarskyi (Pil.) Bond.et singer, Brown rot fungi reported on different species of bamboo from Thailand are: Fomitopsis Pinicola (Sw. ex Fr) Karst,Favolus sp. and Schizophyllum commune Fr. (Sutathip 1988; Arunee 1988). staining and decay fungi recorded on stored culms of Bambusa blumeana, B. vulgaris var. striata, Schizostachyum lumampao and Bambusa spp. in the Philippines include: Penicillium spp., AspergillusspP., Trichoderma spp., Schizophyllum commune, Poria sp., Polyporus sp., Lenzites striata, etc. (Marcelina 1995, personal communication).
Among the soft rot fungi, Xylaria spp. and Chaetomium globosum Kuntze et Schm. are the most important. C. giobosum has bcen observed as associated with soft rot of culms of Phyllostachys reticulata and Phyllostachys spp. in Japan (shojiro et al. 1989). Split bamboo is more rapidly destroyed by the fungi than round bamboos (Liese 1959; Gnanaharan et al. 1993).
Decay and deterioration has been considered a serious problem in bamboo culms stored for making pulp. Decay fungi seriously affect the pulp yield (up to 25% loss over one year storage) and pulp strength is reduced by 15-40% (Guha and Chandra 1979; Bakshi et al. 1960). White rot-affected bamboo culms can be used for pulping, however, the additional bleaching chemicals required cause low pulp yield and lower physical strength. In the case of brown-rot culms, the pulp yield becomes low and the Permanganate number is high so that bamboo culms affected by brown rot are undesirable for pulping. Advanced brown rot results in 25% loss in yield and produces unbleachable pulp.
Fungal staining of the culm does not appreciably affect the yield; however, strength properties of pulp may be slightly lower than that of non-stained culms (Bakshi et al. 196O). Fungal staining also affects the brightness of the pulp, and an increase in bleach consumption(to the extent of 10%) to bring the stained pulp to acceptable brightness has been reported (Bakshi 1976).Hence, it may be said that, in general, fungal attacks increase pulping costs (Singh 1977).
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Fig. 132 : Decaying culms of T. oliveri; note the fructifications of different decay fung
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