The underground part of the plant, the root system, may suffer from diseases which generally remain unnoticed until manifested by symptoms on the above-ground parts. Diseases may be due to fungi, bacteria, nematodes etc. Fungi parasitic on plant roots and root infecting fungi constitute an important group of organisms causing root diseases of forest trees.
Root diseases are of two types:
I. Vascular wilt.
II. Root rot.
I. Vascular Wilts:
Wilt is a systemic disease where the entire individual or its part exhibits wilting of the foliage in acropetal succession up the shoot. The leaves become yellow, lose turgidity and fall off. The affected plant or the branch soon dies. A growing imbalance of water economy in affected plants is common to all wilt diseases.
A characteristic symptom of several wilts is the vascular discoloration in the outer layers of the sapwood, green streaks in verlicillium wilt in many hardwoods and pink stain in fusaral wilt. The vascular browning is ascribed as due to enzymatic action of the wilt fungus. Such discolouration may not be, however, diagnostic and cultural studies are necessary to diagnose the cause of wilt.
Control:
Fungicides like mancozeb, carbendazim and Thiram are good seed dressing chemicals. Bioagents like Trichoderma and Psedomonaus fluorescens may also be used in nurseries. Application of FYM, neem cake encourages growth of bioagents present in the soil.
Two theories are advanced to explain the mechanism of wilt:
(i) Occlusion theory, which assumes plugging of vessels by the mycelium of the wilt fungus interfering with translocation of sap.
(ii) Toxin theory, which postulates production of plasma poison by the wilt fungus and these toxins destroy the osmotic function of the living cell.
II. Root Rots:
Root rot fungi generally possess a wide host range. Once the infection has established in the roots, its further spread depends on the host resistance. In the susceptible hosts, the pathogen may spread quickly into roots and causes disease. The symptoms of root rot attack, however, do not manifest till a large percentage of roots is attacked.
In the less susceptible hosts, the fungi may colonise roots, but then the progress inside the roots is slow because a large mass of healthy roots react strongly against spread of infection. Such hosts, therefore, generally do not show visible symptoms of infection till most of the roots become attacked.
In the natural forests, therefore, where resistant and susceptible species are in mixture, the fungi remain in an endemic state. When natural forests are clear felled for raising plantations, the host resistance is lost and the fungi on roots of susceptible hosts are able to spread quickly.
When trees are felled, the stumps and their roots remain alive for a considerable period. Gradual lowering of host resistance paves the way for colonization of roots first for specialized parasites like root rot fungi followed by weak secondary parasites and finally for saprophytes. Armillaria mellea which is able to spread quickly on roots of felled trees become greatly restricted or is absent if the roots are killed more quickly by ring barking the trees when green.
While the root rot fungi are considered to be specialized parasites, two groups may be distinguished: in one, the fungi cause typical root rot and do not attack the heartwood as in Ganoderma lucidum; in the other, the fungi may cause root and butt rot as in Heterobafrehi annosum, Armillaria mellea and Polyporus spp.
Control:
The control of root diseases generally is more difficult because of the difficulty of combating the disease on underground plant parts which are not easily accessible as compared to diseases attacking aerial plant parts. Chemical control of root diseases with fungicides applied to soil may not be effective as these chemicals may not reach the affected parts and also they may soon become inactivated due to reaction with other chemicals present in the soil or through leaching.
Difficulty in the control of root diseases of forest crop arises because of the long rotation age of such crops. In mixed plantations of susceptible and resistant species, root disease remains very much in check. However, in pure plantations, once the root disease breaks out, the spread is very rapid from tree to tree and therefore, within a short time disease becomes alarming.
Measures for Root Disease Control:
The important effective methods for controlling root diseases are described below:
i. Suitable Site:
For devising successful control measures for root diseases, a thorough knowledge of silviculture of the species is essential. Tree vigour is important in determining the susceptibility. It is also advisable to have the knowledge of root growth of a particular tree species on the site conditions and species resistance to root diseases. Raising forest species on sites favourable for their healthy growth will prevent root diseases.
ii. Control Burning:
Control burning, a necessary silvicultural measure keeps the root rot in check. Burning results in the increase of Trichoderma in the soil.
iii. Resistant Species and Mixed Stands:
In case, a species susceptible to root rot is raised in the first rotation, it should be planted in intimate mixture with resistant species which will serve as barriers to rapid spread of the disease.
iv. Isolation Trenches:
Root diseases generally spread in plantations from root to root by contact or root graft resulting in diseased plants occurring in groups. Isolation trenches may prove effective in containing the disease in between the trenches thus preventing its spread beyond the trenches.
v. Chemical Control:
An ideal stump treatment should be effective in preventing infection at the stump. Among chemicals tested and recommended for use are creosote, ammonium fluoride, sodium nitrite, borax, urea and ammonium sulphamate. Root feeding with Tridemorph @ 0.1% is also effective.
vi. Biological Control:
Root rot due to Fomes annosus is severe in plantations soon after thinning because the pathogen is able to colonise freshly cut stumps. Peniophora gigantea is a vigorous competitor of Fomes annosus for initial colonisation of stumps and may also replace Fomes annosus in stumps.
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