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B. Destructive Activities of Saprobic Fungi 1. Destruction of Wood and Wood Products 2. Deterioration and Contamination of Foodstuff:
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It is amazing the damage that can be done by a very
tiny mushroom,
a bracket fungus, or other molds and mildews. But if adequate moisture
is present the spawn of such fungi can totally destroy wood, wood
products, fabrics, and other materials that have plant products.
Saprobic fungi also cause damage to fruits, vegetables, and other
foodstuff. They pollute the ductwork and AC systems in our homes,
resulting in allergies, or worst yet, releasing mycotoxins detrimental
to our health. Many of the fleshy mushrooms contain highly toxic
materials. 1.
Destruction of Wood and Wood Products:
Millions of dollars of damage is caused by wood rotting fungi annually, on living trees, on lumber, and on wood products. While wood decay is absolutely essential for maintaining a balance of nature; since 28 billion tons of cellulose are produced each year, the wood destroying fungi cause losses to lumber producers, to builders, and to home owners. Recently, one of the major motel chains located near Disney World experienced massive wood rot as a result of improperly built facilities that allowed the encroachment of water into the wooden subfloor (Fig. 8-1).
Fig. 8-1. Severe wood-rotting caused by water leakage.
The beams and plywood flooring were damaged to the point that they
crumbled and people were literally falling through the floor. In a previous section on decomposition of organic
material by fungi, we discussed the various kinds of rots caused by
fungi, i.e. soft rots, white rots, and brown rots.
Another kind we call dry rot is caused by fungi such as Merulius
in which when they start to actively grow, they can produce sufficient
moisture to sustain the fungus. Unfortunately, dry rot fungi can grow
slowly in completely protected lumber once it has been able to establish
growth. Even though most wood rotting is caused by Basidiomycetes in the
Aphyllophorales (the bracket fungi), a few mushroom groups (Agaricales)
and several Ascomycetes can cause wood decay. Members of the Xylariales
such as Daldinia, Hypoxylon, and Xylaria
cause slow white rots in a number of tree species. Not only does standing timber become infested, but also lumber products when stored in moist conditions may be contaminated. Wood staining is a real problem in which moist wood surfaces support growth of Penicillium, Aspergillus, Fusarium, and Rhizopus. Blue stain is common from growth of species of Ophiostoma (Ceratocystis). In the heat processing of making plywood and paneling, a pink mold Monilia can be a problem. But the most severe wood destruction in buildings is caused by species of Poria, Polyporus, Ganoderma (Fig. 8-2),
Fig. 8-2. Gandoderma lucidum, a common white rot fungus. Fomes (Heterobasidium), Stereum, Peniophora, Lenzites, Pleurotus, and Schizophyllum. Several mushrooms like Pholiota (Fig. 08-3) grow only on wood.
Fig. 8-3. Pholiota, one of the many groups of mushrooms found commonly on wood. Not only do we have to worry about building
materials that contain cellulose, but other materials such as polyvinyls
can support fungal growth whenever adequate moisture is available.
Certain kinds of electrical wiring
are susceptible to species of Cladosporium and Aureobasidium (Fig.
8-4).
Fig. 8-4. Outdoor vinyl floor covering contaminated with Aureobasidium pullulans. It is essentially impossible to control wood rotting
fungi in forests, but by cutting and destroying trees with signs of wood
decay or dieback losses may be reduced. There has been some success in
chemically treating stumps of cut trees to prevent invasion of wood
rotters that can spread to living trees through root grafts.
There has also been some success in the use of certain
antagonistic fungi such as species of Trichoderma
and Peniophora as a biocontrol
to ward off other invaders. There is a wide array of chemical preservers
used by the lumber industry. But the surest way to control wood rotting
and staining fungi in lumber is by proper drying of cut lumber and
storing it in a dry atmosphere. White
rot fungi require 25-30% moisture levels in wood before they will
infest. Keeping moisture levels below 25% is usually successful.
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2.
Deterioration and Contamination of Foodstuff:
a. Mycoflora of Processed Foods. Cereals, nuts, and spices are very susceptible to fungal
attack (Hocking, 1991. Handb. Appl. Mycol. 3:69-97). Species of Alternaria,
Fusarium, Cladosporium, Penicillium, yeasts, and smut fungi are
common on grains at the time of harvest. During storage, however,
several species of Aspergillus and Penicillium,
and certain species of Fusarium
become more prevalent. Species of these three genera are the most
damaging to stored grains. Dried fruits, jams, jellies and sugary foods are
susceptible to xerophilic fungi such as Zygosaccharomyces, Eremascus, Chrysosporum,
and Xeromyces that can grow at low moisture levels. Processed fruits usually require various types of heat treatments
in their preservation. Unfortunately, there are a number of heat
resistant yeasts and filamentous fungi that can present problems in
fruit processing. The mycelial phase of most fungi is heat sensitive and
can be eliminated, but many of these also form chlamydospores that
withstand heat treatments. Byssochalamys,
for example, has been involved in spoilage of canned lemon pie filling
and species of Penicillium in
canned blueberries (Williams et al., 1941, Food Res. 6: 69-73). Salamies, wursts, and cured meat are often spoiled by species of Penicillium, Aspergillus, and Cladosporium. Dried seafood are often contaminated by Scopulariopsis, Wallemia, Cladosporium, Mucor, and Acremonium. b. Fungal Deterioration of Fruits and Vegetables. Vegetables are perishable products that remain metabolic after harvest. Although bacteria are the main decomposers of vegetables, fungi are still important (Bulcarelli & Brackett, 1991. Handb. Appl. Mycol. 3:179-199). Several molds are associated with postharvest contamination of vegetables. These include species of Alternaria (carrots, cabbage, cauliflower, pepper, & tomatoes), Botrytis (green beans, carrots, onions, peppers), Colletotrichum (beans, onion, tomatoes), Fusarium (Fig. 8-5; Fig. 8-6) (carrots, potatoes),
Fig. 8-5. A species of Fusarium in culture.
Fig. 8-6. Long canoe-shaped conidia typify species of Fusarium. and Rhizopus (beans, carrots, & tomatoes). Citrus is especially susceptible to Penicillium (Fig. 8-7),
Fig. 8-7. Blue mold of citrus caused by a species of Penicillium. Botrytis (Fig. 8-8), and Cercospora that causes “greasy spot” (Fig. 8-9).
Fig. 8-8. The gray mold of citrus caused by a species of Botrytis.
Fig. 8-9. Greasy spot of grapefruit caused by a species of Cercospora. c. Preharvest Colonization of Grain. The most common storage fungi belong to Aspergillus, but species of Penicillium (Fig. 8-10), Chrysosporium, and Wallemia occur commonly.
Fig. 8-10. Penicillium contamination of corn Infested grain has reduced germination,
discoloration, and heating. Harvest marks a profound change in grains in
which fungi may populate. This is especially true whenever harvested
grains absorb moisture (see Bulcarelli & Brackett, ibid.).
Grains such as barley, maize
(corn), and rice have large numbers of fungi that will contaminate them before and after
harvest. |
