Fungal decay

Most wood species are subject to some form of biological attack, a hazard not encountered by most other constructional materials. Fungal decay and attack by termites and other insect borers and marine borers are the main problems. The heartwood of many timber species and the sapwood of most species can be impregnated with wood preservatives to prevent these problems. 

For the remedial treatment of fungal decay and insect attack on timber in buildings, clear solvent-type preservatives applied by spraying all available surfaces are generally used, supplemented with wood preservative pastes to obtain deep penetration. Water-based emulsions are also used for woodworm control. There has been a general move towards the use of those... 

Bacterial attack is an early stage in the degradation of wood exposed in wet or moist conditions. Bacteria can be the dominant form of attack when fungal decay is suppressed by a wood-preserving treatment. Bacteria can attack the cell wall of wood by tunnelling, cavitation or erosion mechanisms (Eaton and Hale, 1993). 

Chen, G.C. (1992c). Fungal decay resistance and dimensional stability of loblolly pine reacted with 1,6-diisocyanatohexane. Wood and Fiber Science, 24(3), 307-314. 

Chen, G.C. (1994). Fungal decay resistance of wood reacted with chlorosulfonyl isocyanate or epichlorohydrin. Hol orschung, 48(3), 181-185. 

Boron rods provide an alternative wood preservative, which is considered safe for people and the environment. They are made of boron compounds that have been subjected to high temperatures to form water-soluble, glassllke rods. These rods are Inserted Into drilled holes In the wood, strategically positioned where decay is most likely the holes are then plugged to keep the rods In place. When the wood becomes wet, boric acid is released, which prevents fungal decay. Boron pastes have a similar effect. Manufacturers of boron rods are seeking to make these rods available for home use. 

Initial CA treatments with N2 gas have been shown to improve cranberry storage life by reducing fungal decay (12). 

Both ripening and reducing fungal decay in many species of harvested fruits and vegetables have been studied in relation to food irradiation alone or in combination with other treatments [39]. 

From intact vegetable crops, apart from tuber, bulb, and root crops, tomatoes and asparagus show potential for radiation treatment. The development of fungal decay in tomatoes caused by Alternaria, Botrytis, or Rhizopus spp. can be controlled using doses of approx. 3 kGy. However, softening and the loss of characteristic flavor may occur. Doses of 0.1 kGy and above result in a delay of ripening of tomatoes [43]. 

Figure 1. Free radical content of fungally decayed sweet gum...

Fungal Decay of Woods. Blocks of sweet gum and southern pine sapwood were inoculated with test fungi by the standard soil-block method (7). The test fungi were the brown rots Poria monticolla and Lentinus lepidius and the white rot Polyporus versicolor. As a control, one block of pine and one block of gum were left in the sterilized soil-block chambers in which the fungus had been started on feeder blocks and then sterilized. 

Newell, S. Y., Arsuffi, T. L. Palm, L. A. (1996a). Misting and nitrogen fertilization of shoots of a saltmarsh grass effects upon fungal decay of leaf blades. Oecologia, 108, 495-502. 

The treated specimens without the ageing procedure did not suffer from fungal decay. After leaching, the samples lost approx. 5% of their initial weight. However, after evaporative ageing of the samples, toe mass loss was below 1%. 

In terms of the performance of coatings, the developed system proved to be comparable to phenol-formaldehyde. The board appeared to be less susceptible to fungal decay in preliminary studies, as compared to phenol-formaldehyde board, but had high mold susceptibility. According to ASTM 631 72 corrosion test, the board was initially highly corrosive to metals but after 16 d the corrosiveness dropped and was only slightly higher than that of untreated wood flakes (102). 

A few brown-rot fungi, notably Serpula incrassata (Berk, and Curt.) Dank, and Merulius lacrymans (Wulf.) Fr. have the unusual ability to conduct liquid water from moist soil or other sources of moisture into dry wood (36). Similarly, subterranean termites can attack very dry wood and, if they have access to water, can transport it through the tubes they construct between moist soil and wood. But chemical soil treatments around wood buildings can prevent attack by even these organisms. Construction practices that thwart both fungal decay and insect attack have been described (36, 38). 

T Nilsson. Defining fungal decay types A proposal for discussion. Intern Res. Group on Wood Preserve Document IRGAVP/1264, 1985. 

Doster, M.A. and Michailidies, T.J. 1999. Relationship between shell discoloration of pistachio nuts and incidence of fungal decay and insect infestation. Plant Dis. 83, 259-264. 

It seems that water absorption serves as one of the key parameters in microbial growth in WPC materials. When a whole cross section of a composite deck board is tested by immersion in water, water absorption after 24 hr is typically between 1 and 3% by weight, after 7 days between 3 and 10%, after 20 days about 8 to 15% (see Chapter 12). These values depend on temperature, and the lower the the temperature, the lower the water absorption [1]. However, water absorption by the top layer of a composite board (1 mm in depth, 50 50 mix of woodiplastic) was in excess of 15% after 24 hr [2]. On other data, water absorption by the 5-mm top layer of Trex deck board in the temperature range from 5 to 25°C was 45 and 60%, respectively. This level of moisture content is well in excess of that necessary to support fungal decay. In fact, authors [1] noticed that when the 25°C trial was run for 30 days using Trex samples, a thick microbial film was developed on the surface of the material. This is a rather common observation in the course of long-term water absorption studies. 

ASTM D 7031-04 says (5.21.1) Resistance to fungal decay shall be determined in accordance with accepted methods. Test Method D 2017 (or its alternate Test Method D 1413) is commonly used for this purpose. It also says (Note 11) This is an accelerated laboratory decay test. Results are subjective and comparisons between tests and materials should be used with caution. However, mean specimen weight losses greater than 5% or significantly greater than controls should be cause for concern. ... 

Resistance to fungal decay shall be determined in accordance with Test Method D 2017 or D 1413. Note 4 in ASTM D 7032 repeats the Note 11 in ASTM D 7031. 

Borates (such as boric acid, borax, disodium octaborate tetrahydrate) have been used as broad-spectrum wood preservatives for over 50 years. Zinc borate has a shorter history as wood preservative, but it is well accepted in that capacity. There is a belief that, generally, zinc borate is more effective against fungal decay compared to mold inhibition, though systematic studies, using different composite materials, are apparently lacking. 

Fungal Decay of Woods. Blocks of NaOH-treated and untreated sugar maple were inoculated with three test fungi—a brown, a white, and a soft rot—by the standard soil-block method (8). At various intervals, the test blocks were removed, the fungal growth washed off, the block sterilized, and ovendried to obtain weight losses. 

Tributyltin compounds applied to timber by dipping, spraying or vacuum impregnation methods have been well established as wood fungicides, both as protective and remedial treatments against fungal decay and insect attack. 

Soft rot is a type of decay caused by ascomycetes and Fungi Imperfecti. The term soft rot was proposed by Savory (54) for a form of fungal decay where characteristic chains of cavities with conical ends were produced within wood cell walls. He recommended that the term soft rot should be used for decay caused by cellulose-degrading microfungi to distinguish it from the brown and white rots caused by the wood-destroying basidiomycetes. ... 

Wood degradation by bacteria is clearly of great importance in archaeology. Except for dry wood, most ancient wood that has survived until the present will have been exposed to the threats of microbial decomposition. Only wood that has been exposed to conditions restricting fungal decay, conditions that often favor bacterial attack, will survive for any length of time in a recognizable condition. 

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