Described are investigations, carried out in 1963 to 1971 in Hevea brasiliensis
at the Firestone Plantation at Harbel in Liberia. Studied was the tapping panel disease, black thread, caused by the fungus Phytophthora palmivora.
The emphasis of the investigations was on control of the disease with the fungicide captafol (Difolatan). Another line of investigation was yield stimulation with 2,4-D (salts and esters of 2,4-dichlorophenoxyacetic acid) and ethephon (2-chloroethyl-phosphonic acid; commercial formulation named Ethrel), to combine application of yield-stimulants above the cut with application of captafol for black thread control. A new method of yield stimulation was tested: application of ethephon to scraped portions of the vertical guide lines.
General information on rubber in Liberia is given in sections 1, 2 and 3. Rubber is the most important cash crop grown in Liberia and accounted for 20 % of total exports in 1969. Liberian rubber production amounted in 1970 to 2.2 % of world production of natural rubber. Particulars on rubber cultivation at the Firestone Plantation at Harbel are given in Section 2. On this plantation about 25,000 ha of rubber are in production. There are 2 distinct seasons, a dry season from November to April and a rainy season from May to October. Production is lowest in the middle of the dry season when trees are wintering and refoliating.
The economic importance of diseases in Hevea
is difficult to assess as the damage depends on local conditions, which also may change with time. Moreover, very different types of damage must be compared. Within these limits it was concluded that diseases are likely to be of less economic importance in Liberia than in many other rubber producing countries. The most important diseases probably are: black thread, root rot diseases (Fomes
and leaf diseases (Helminthosporium
Brown bast - the result of a physiological disorder - is another very damaging disease. Pests are generally of minor importance (1).
A review is given of all known diseases, pests and other causes of damage of Hevea
in Liberia (3). It is remarkable that bird's eye leaf spot (caused by Helminthosporium heveae)
was such an important leaf disease in young plantings of 4 to 8 years old; in the Far East, the damage is mainly confined to the nurseries. Wide-spread damage by patch canker (caused by Pythium vexans),
done to the root collar, is also rather uncommon in most Hevea
growing countries. Mouldy rot, a known tapping panel disease in the Far East and caused by the fungus Ceratocystis fimbriata,
was never encountered in Liberia.Black thread disease
Literature on different aspects of this disease is reviewed, viz. on symptoms and damage (4.1.1) and on the causal agent, sources of infection, dissemination and predisposing factors (4.2.1.). In Liberia, black thread is often the most important disease in plantings of above average susceptibility. Highly susceptible Hevea
clones, planted on a large scale, are BD 5 and Har 1. These clones are also highly susceptible to patch canker (4.2.3).
Black thread causes rotting of the renewing bark. The recently tapped portion of the panel is subject to infection during the rainy season. The diseased bark deteriorates, the cambium is killed and, eventually the wood is exposed. In severe cases, large parts of the tapping panel are affected, rendering subsequent tapping impossible. Secondary infections may increase damage (pinhole borers; the fungi Ustulina zonata
and Pythium vexans).
Older trees with a severe black thread history are probably also more liable to wind damage (trunk snap); sections through stems of such trees showed irregular wood formation with necrotic parts (4.1.3.). Generally, the wounds start healing with the onset of the dry season. However, healed wounds have irregular bark, which is difficult to tap and possibly of lower yield potential. Progress of healing tissue was generally between 0.75 and 1.5 mm. per month (4.1.2. and 22.214.171.124). Healing was slowest during the driest months. Application of 2,4-D to artificially made wounds promoted healing (126.96.36.199). Black thread is most predominant in August and September at Harbel, the months with little sunshine, relatively low temperature and highest rainfall (2 and 4.2.2).
It is known that infection takes place by means of sporangia or their zoospores. Severe infections built up on panels protected against rain with polyethylene sheets, thus visibly dry panels can also be infected; a higher than normal relative humidity is maintained under such aprons (4.2.2).
There is a distinct positive correlation between the incidence of new and old black thread damage on the same tree (4.2.4). It is not clear whether the old wounds are a source of infection to the underlying portion of the panel, tapped during the next rainy season; disinfection of the old wounds had no effect on later disease incidence (4.2.8). At any rate, as the disease in general builds up each year on the same trees - also in clonal plantings - it might be advisable to take trees with a severe black thread history out of tapping for the entire rainy season; such a measure will also prevent spread of the disease to neighbouring trees (4.2.7). The disease is generally more predominant in low-lying areas close to swamps, probably because of the more humid microclimate (4.2.5). The tapper is probably an important factor in the spread of the disease. He spreads the disease presumably with his hands; the tapping knife is probably of lesser importance (4.2.6).
A rather unusual case of black thread damage on panel marks is also reported. It was concluded that when new panels are laid during the rainy season, the drawn channels should be protected with a fungicide (4.1.5).Control of black thread
Various fungicides were tested because the product used in practice (Treseal, a petroleum jelly) gave unsatisfactory disease control in Hevea
clones of above average susceptibility. In bio-assays (5.3), captafol proved to be a stronger fungicide against P.palmivora
than the related compound captan, which latter chemical was of promise in CARPENTER's experiments (1954). Next, these and other fungicides were tested in field trials.
Most field experiments had a randomized block or a replicated tree plot design. The trials were laid out in such a way that the damage done in the previous rainy season was of the same level in all treatments, in order to level the chances of infection. The damage was evaluated according to a scale: '0' (no damage) to '6' (maximum damage possible). The panels were exposed to natural infections; no use was made of artificial inoculation (188.8.131.52. and 184.108.40.206.).
Very satisfactory disease control was obtained with 1 % captafol suspension in water, applied weekly with a brush on a 2-2.5 cm wide strip immediately above the cut (5.4 and 5.5.). Antimucin, an organic mercury compound - much used in the Far East - gave very little protection when applied weekly in 0.8-1.0 % concentration (5.5.1.). Under the described conditions the optimum concentration for captafol. was probably about 1 % (5.5.2.). In Malaya, Difolatan is recommended in 2 % concentration, to be applied after every tapping. In most experiments, 0.1 % Ortho spray sticker was added to the suspension, although it has not yet been proved that this gives better disease control. Results indicate that products with very effective sticking properties (such as high concentrations of wax emulsions) might even lower the effectiveness of captafol (5.5.3.). A suitable colouring agent for captafol suspensions in water is yellow iron oxide, added to make captafol application to the panel visible (supervision). However, some Sterox NJ (a synthetic detergent) should be added to keep the powders better in suspension. Very satisfactory results were obtained with the following mixture in experiments and commercial practice: 1.25 % Difolatan 80 WP (containing 1% captafol) + 1 % yellow iron oxide (grade YO-2087) + 0.1 Ortho sticker + 0.01 % Sterox NJ in water (5.5.4.).
It should be emphasized that a homogeneous suspension must be prepared or otherwise disease control is less effective. Therefore, a slurry of the Difolatan powder should first be prepared. If too much water is added, the aggregated Difolatan particles do not separate and an unstable suspension is immediately obtained. After this stage, more water should be added whilst stirring, and then the other ingredients added. The whole procedure can be simplified and speeded up with the aid of strong electric agitators. The quantity of water added at one time to the Difolatan powder is then less critical. It is practical, however, to prepare first a concentrate of 20 times the strength applied to the trees and to dilute with water on the day the applications have to be made. The concentrate should be made up no earlier than I month in advance because of decomposition of captafol during prolonged storage (5.7.).
The residue of captafol on treated panels is rather persistent; however, most of it had disappeared 5-6 months after application (disintegration, dissolution, particles dropped off, particles washed off by rain). This fungicide does not penetrate into the living tissues of the bark (5.6.). It is likely that the protective action of captafol is based on the traces of it which dissolve in the water film on wet panels (7).
Side effects of captafol applications were also studied (5.5.8.). Chances are extremely remote that normal captafol applications have adverse effects on the physical properties of the rubber (220.127.116.11.). Latex yield and yield potential of treated renewing bark were also normal (18.104.22.168. and 22.214.171.124.). However, captafol treatments have a temporarily retarding effect on the activity of the cork cambium, initially reducing hard bast and cork formation; soft bast renewal is about normal from the beginning. It is to be expected that the initial differences in bark thickness between treated and untreated panels will have disappeared when the bark has fully matured and is to be tapped again at the age of 8 to 10 years (126.96.36.199.). Bark renewal can be improved when low concentrations of 2,4-D are added to the captafol mixture.Yield stimulation
This subject is introduced with a brief review of literature. Illustrations are given of 4 stimulation techniques (Fig. 15), which are:
a. weekly above-cut stimulation.
b. stimulation of a scraped band of bark parallel to and below the tapping cut.
c. stimulation of lightly scraped portions of the vertical guide lines.
d. stimulation of lightly scraped, vertical narrow strips of bark below the cut.
The experiments had a replicated tree plot design and layout was based on pretreatment yield data. So, trees were allocated to treatments in such a way that the mean production level of each treament was about the same. Layout can also be based on pretreatment girth measurements as yield and girth of the stem are positively correlated in clonal plantings. However, the correlation between later yield and pretreatment yield appeared to be significantly higher than between later yield and girth (6.2.2. and 6.2.3.). For experiments of relatively short duration, taken in older plantings, pretreatment yield data are definitely to be preferred.
Important yield increase was obtained with weekly above-cut application of 2,4-D in aqueous media (with and without captafol and other admixtures), although lower than with 2,4-D in the petrolatum Treseal. However, 2,4-D in Treseal is only applied during the last tapping cycle in commercial practice, because the treated bark becomes greatly proliferated. There is much less bark proliferation with 2,4-D in aqueous media. This method is fairly promising for younger plantings and for older rubber with thin bark renewal; in concentrations up to 0.25 % the thickness of the renewing bark can be increased without much bark proliferation (6.3.1. and 188.8.131.52.).
With Ethrel in aqueous media (with and without captafol. and other admixtures), also applied weekly above the cut, still higher yield increase was obtained, viz. in a concentration of 1 % ethephon. Yield increase was very low in 0.25 % concentration, probably because ethephon was disintegrated (the higher the pH the faster is disintegration). Yield increase was very high in 2.5 % concentration (6.3.2.).
The novel stimulation technique, i.e. application of Ethrel in palm oil on scraped portions of the vertical guide lines, showed much promise. In a concentration of 5 % ethephon, applied to 4 inches (10 cm) of each of the 2 guide lines, yield increase was about of the same level as with 1 % ethephon in water, applied to a 2-inch-wide scraped band below the cut (6.3.3.). A detailed description of this technique was given in Section 6.2. . The preliminary results indicated thefollowing:
1. A portion of each of the 2 guide lines should be treated.
2. It is of little importance which portion of each guide line is treated (above or below the cut). However, it is impractical to treat the front channel below the cut because the spout is positioned there.
3. An effective length is 10 cm per guide line. Treatment of longer portions gave relatively low extra yield increase. It is worth trying to treat shorter portions, e.g. 5 cm per guide line.
4. The same portion can be scored and treated for a second time within a few months. When a portion of 10 cm per guide line is treated then the trees can be stimulated every 2 months, provided bark consumption is at least 2.5 cm per month, the condition or the bark allows for a second treatment and continuous stimulation is wanted.
5. Scraping of the guide lines is essential to obtain effective penetration of ethephon. Only the dead bark layers are removed.
6. Ethrel should be applied in palm oil rather than in aqueous media (higher yield response and easier to apply). 7. The optimum concentration is presumably 5 % ethephon or higher.
The conclusion was drawn that this method merits further testing since high yield response was obtained at low Ethrel consumption. Moreover, the tapping panel itself is not treated, so that the risk of undesirable side-effects on bark renewal should be lower.
Results of measurements of bark thickness suggest that ethephon applications (above-cut, below-cut or on guide lines) have no or little effect on bark renewal (184.108.40.206.). However, it is still unknown whether bark renewal will continue to be normal when trees are stimulated intensively with high concentrations of ethephon for many years, as ultimate exhaustion of the tree - because of excessively high withdrawal of latex - may have adverse effects on growth. In younger plantings, girthing of the trees should be closely watched.
This chapter on yield stimulation is concluded with some considerations on effectiveness and profitability of different stimulation methods with Ethrel (6.4.). The calculations showed that relatively small increases in yield already compensate for the costs of rather expensive stimulation methods under the described conditions when the normal yield level is at least 900 kg dry rubber per ha and per year. Therefore, it is probably more important to put the emphasis in stimulation trials on safe methods, which are likely to have little effect on future yields, rather than on cheaper methods (7).