The information in this document is believed to be reliable, but neither the IRRDB nor its Member Institutes accept any responsibility for the safety or efficacy of any cited method of treatment.

Rigidoporus lignosus (Imazeki) Klotzsch

Distribution

The results of the 1998 IRRDB Disease Survey show that the disease is severe in Cameroon, Côte d'Ivoire, Indonesia, Malaysia, Sri Lanka, Thailand. Earlier surveys had indicated that this disease is described as severe in Côte d'Ivoire, Nigeria and Sri Lanka., and as a significant, endemic problem in Gabon, Indonesia, Malaysia and Thailand. In these countries regular control measures are essential.

Symptoms

R. lignosus infects and destroys the lateral and tap roots. The first visible symptom on the foliage is the change of colour of the leaves to off-green and yellowish. Initially, this phenomenon is observed only on one or a few branches but eventually the whole canopy will be affected. The leaves lack a bright waxy shine, appear leathery and curve downwards unlike boat-shaped healthy leaves. Some of the trees flower and fruit during the off-season. The entire canopies of trees with advanced disease turn yellowish-brown and eventually abscise.

The branches die back and the tree dies. White root disease is differentiated from other root diseases by the fungal rhizomorphs on the surface of the roots. These are white especially at the growing ends, possess many branches and are firmly attached to the surface of the roots. Fungal fructifications develop at the boles of trees, with advanced disease on tree stumps during wet weather. The fructifications, sometimes multilayered, are large with corrugated yellowish orange upper surfaces, white-coloured margins and light orange lower surfaces.

Spread

R. lignosus has a wide host range which includes many jungle and horticultural plants. The incidence of white root disease is normally more severe in replanted areas which were previously cropped with rubber than in plantings in former jungle areas. The main source of inoculum is the remnants of infected wood, either pieces of roots or stumps left buried in the soil. Roots of young rubber plants are infected when they come into contact with the source of inoculum and the disease spreads to the tree collar and out to the other lateral roots through the fungal rhizomorphs which grow ahead of the rot. As the trees mature and their lateral roots intermingle, white root disease is transmitted from tree to tree especially along the planting rows. Spores produced by the fungal fructifications also spread the disease. Spores deposited onto wounds on the roots or on cut surfaces of stumps initiate new source of infection.

Control

All clones of Hevea are infected by white root disease. The disease is being controlled by an integration of cultural and chemical methods. The important control strategies are to reduce the source of inoculum during clearing of the land of old rubber trees and for several months after planting, as well as to inhibit disease spread in the immature rubber areas with fungicides and in the mature rubber by constructing trenches. Clearing land of old rubber trees The incidence of white root disease can be reduced by a thorough mechanical clearing whereby the trees or stumps are bulldozed, the roots which remain buried in the soil are cut using root cutting tines, the land ploughed and raked to collect the roots for disposal normally by burning along with the other timber. Any timber left on the ground will increase the incidence or white root disease as it can be infected by fungal spores. Full mechanical clearing is expensive and the disturbed soil is exposed to erosion.

For smallholdings, the old rubber trees are cut using chain saws and the timber is sold or burned. The tree stumps, which can be infected by spores, are poisoned by brushing an arboricide on the sides of the stumps as early as possible after felling. The arboricide, 2, 4, 5-trichlorophenoxy acetic acid (5%) was used previously, but this is toxic to mammals and Garlon (triclopyr) at 5% is recommended. Poisoning of the stumps promotes faster wood decay and the stumps normally disappear within two years.

The incidence of white root disease is also reduced if the cut surface of the stumps is covered with creosote to inhibit spore colonization. Planting of creeping legume cover The incidence of root diseases in areas planted with creeping legume covers is lower compared with that on bare land and/or areas covered with grasses. The legume covers, apart from promoting faster decay of inoculum of root disease, also create soil conditions not conducive to the growth of R. lignosus. Application of sulphur The application of sulphur powder to the soil reduces soil pH and encourages the growth in population of fungi such as Trichoderma, Penicillium and Aspergillus.

These two phenomena assist in lowering the incidence of white root disease. Sulphur is best applied at the time of planting rubber. Normally, about 150-200g of sulphur powder is mixed with the soil to be used to fill the planting hole or the sulphur can be sprinkled around the plant and immediately forked into the soil. As a precaution to avoid phytotoxic effects, excessive quantities or lumps of sulphur should be avoided from coming into contact with the young plants. The cost of treatment is reduced if sulphur is only applied to areas previously infected with white root disease.

Chemical control

Application of collar protectant dressing

Detection of diseased trees

Several methods have been used to detect trees which have been infected by R. lignosus. Tree-to- tree collar inspection by slightly exposing the collar of the tree using a sharpened wooden spade to check for the presence of fungal mycelium was previously recommended and was commonly practised in Malaysia. A faster technique is to carry out quarterly rounds of foliage inspection commencing when the trees are about one year old, to detect trees with foliage symptoms of root diseases.

In Africa, diseased trees are detected by mulching the base of the trees with grasses or leaves to induce growth of fungal mycelium. Trees with advanced foliage symptoms are normally beyond saving and should be removed. The neighbouring trees should be treated with fungicides. Surgical removal of roots Infected trees with mild foliage symptoms may be treated along with the immediate neighbours especially those along the same row.

The soil is carefully dug to expose about 30-50cm of the lateral roots and also part of the tap root, preferably using a spade and a wooden stick to avoid injuring the roots. Infected lateral roots are cut to the healthy portion and those running along the tree rows are removed while those growing towards the interrow are cut at the edge of the leguminous cover. Rots on the collar and tap roots should be removed until healthy tissues are exposed.

The infected wood chips and roots should be removed and burnt. Painting with collar protectant Collar protectant fungicide formulation containing pentachloronitrobenzene (PCNB) at 20% a.i. is then liberally painted onto the tree collar, tap roots and the lateral roots. The exposed roots are then covered with soil taking care not to introduce the infected wood debris. The collar protectant remains effective for about two years.

The treated trees should be re-inspected two years after treatment and therefore the treated trees have to be clearly marked to facilitate the inspection. Drenching with fungicides The main advantage of treating white root disease with fungicides which are applied by drenching around the base of the infected tree is that the process is less laborious as compared to the treatment using collar protectant dressing because no extensive digging is required. The drawback is that drenching using the currently recommended fungicides are only effective for curing young trees which are mildly infected.

Detection of infected trees

Infected trees are identified during quarterly rounds of foliage inspection. Trees with foliage symptoms should be treated with the collar protectant dressing. The collars of the neighbouring trees should be inspected for the presence of rhizomorphs of R. lignosus. If the infection is mild, ie the fungus has not penetrated into the tree collar, the trees are drenched with suitable fungicides. Diseased trees can also be identified by carrying out regular tree-to-tree collar inspection and can be singled out with wooden pegs.

Treatment

The trees are treated by pouring an aqueous mixture of the fungicide into a shallow furrow dug around the base of the tree. Triadimefon or propiconazole at 2.5% a.i. are recommended. One litre per plant of the chemical is used to drench young trees (less than two years old) and two litres for older trees. If repeat application is necessary, especially for more severe outbreaks, the treatment is repeated six months later.

Isolation trenches

For economic reasons, chemical treatments are not recommended for mature rubber trees, but the infection should be stopped from spreading to other trees. It is recommended that trenches are dug around a root disease patch. All the trees neighbouring those infected with root disease should be inspected to make sure that no diseased trees are left outside the borders. These trenches should be deep and wide enough to inhibit the lateral roots from growing across. When necessary they need to be redug.

Sheet first issued: April 1994