Root and Stem Rots caused by Basidiomycetes

Pages 593-614

Basidiomycetes cause a wide range of root rot, stem rot, wood rot and decay diseases. These diseases are of particular importance to forestry and timber concerns but there are two very serious basidiomycete pathogens of agricultural crops, that are effectively sterile.

Rhizoctonia and Sclerotium!

These are wide -spread soil inhabitants that can grow saprophytically and do not usually produce any spores. They have been made to fruit in the lab under certain conditions and then they are shown to be Basidiomycetes in the genus Thanatephorus (Rhizoctonia) and Athelia (Sclerotium).

Both cause damping off and root and stem rot of many plants including both monocotyledonous and dicotyledonous. Both overwinter as sclerotia free or in plant material and also as mycelium.

Including black scurf of potato and many different kinds of diseases.

Disease cycle Figure 11-154

Both genera follow this life cycle. Both form sclerotia

They are necrotrophs and exceedingly common on all plants. Rhizoctonia is an aggressive fungus/saprophyte that can grow through the soil to a living host or fresh organic matter. It can also spread by root to root contact and to a lesser extent by tillage

Rhizoctonia mycelia is generally brown and has diagnostic right angle branching and a septa after the branch. Rhizoctonia has been separated on the basis of pathogenicity on specific hosts and Anastomosis Groups (AG). Fusion of two mycelial isolates indicates an anstomosis group. There are AG 1-9, and tester strains to help type the strains.

Different AG's cause different types of diseases, often geographically separated but this is still under investigation.

The fungus is cosmopolitan and temperature optima follow the local conditions usually.

Control as always sanitation. Favored by moist conditions, disease free seed, rotation to non-susceptible crops (cereals), standard avoidance for damping off (fast seedling emergence etc.), fungicides.

Fungicides work for specific diseases e.g. damping off in nursery situations etc.

Biocontrol is also being studied.


Sclerotium rolfsii

Behaves similar to Rhizoctonia, but generally produces black mycelia and produces rhizomorphs which are cord-like strands composed of many mycelia. Theses are often the colonizing entity. It grows on the surface of the ground under leaves or other litter, and will grow from plant to plant. It is slightly less aggressive than Rhizoctonia and doesn't produce damping off, but does cause stem rots. Is also a soil inhabitant, but less successful there than Rhizoctonia. It produces a simple disease and life cycle - mycelia-sclerotia-mycelia. Control is same as for Rhizoctonia.


Root Rot of Trees: Armillaria

The mushroom tree killer, very common in forest soils and forms honey colored gilled mushrooms around the base of affected trees. Causes root rot of forest and orchard trees. It is a common disease with a broad host range- any forest tree. Takes out one tree at a time. It grows up into the base of the tree under the bark, then grows in to and kills cambium and sapwood, then produces the mushroom around the tree. It overseasons as mycelia or rhizomorphs on roots in pathogenic phase. However the mushrooms still flourish after the tree dies. Rarely the basidiospores from the mushroom can infect, however most disease is spread through rhizomorphs and root to root.

The disease cycle is simple and the fungus survives as rhizomorphs (have structure black outside with white inside) and in dead wood.

Disease cycle Figure 11-158

Define rhizomorphs:

Control not warranted in forests. In orchards, removal of affected trees and fumigation of soil where the tree was. Also remove dead stumps. Fungicides don't work.

Trees rely on fungi to live and grow. Ectomycorrhize are found on forest trees. These include many types of mushrooms and puffballs whose spores are wind disseminated. The fungi grow all over the feeder roots of the trees producing a mantle (blanket) that helps with nutrient absorption.


Wood Rots

Basidiomycetes also cause wood rots. These cause huge losses to forests and to timber nurseries.

Heartwood rots - central wood of living trees. Can also infect the outer wood, sapwood of cut trees.

Brown rots - the fungi utilize the cell wall polysaccharides leaving the lignin and so brown and often cubical pattern and crumbly, infect softwoods (conifers)

White rots - decompose all components leaving spongy or stringy white material, infect hardwoods (oaks, maples etc.)

Wood staining fungi - blue stain fungi, and soft rot fungi are Ascomycetes and Deuteromycetes.

Heterobasidion (Fomes) - causes root and butt rot of living trees, has perennial basidiocarp (conk)

Polyporus - heart rot of living trees, also dead trees

Phellinus (Poria) - root rots on conifers and some hardwoods, brown cubical rot on lumber, annual basidiocarp

Peniophora - causes decay on conifer logs

Ganoderma - root and basal rot in conifers and hardwoods

Life cycle - All enter trees as mycelium or basidiospores, usually through wounds. The fungi spread up and down more than radially. Since they rot the center of the trunk, the tree may fall over with wind. These diseases are slow, and variable may take up to 50 years to kill a tree or may cause major damage in 5 years. The spores are carried by wind or rain.

Disease Cycle Figure 11-159

Control - Prevent introduction of pathogen into healthy stands of trees. Minimize wounding and prune with a flush cut. Treat wood in contact with soil with creosote or copper naphthanate. Can use biocontrol for Heterobasidion annosum - antagonistic bacteria applied to stumps or chain saw oil.

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