Cey. J. Sci. (Bio. Sci.) 35 (1): 79-86, 2006

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BARK CRACKING DISORDER IN THE LOWER TRUNK REGION OF SEEDLING AND IN THE ROOT STOCK OF BUD-GRAFTED TREES OF

HEVEA BRASILIENSIS MUELL.ARG

L.S.S. Pathiratna*, P. Seneviratna, M.K.P. Perera and C.K. Balasooriya Rubber Research Institute of Sri Lanka, Dartonfield, Agalawatta, Sri Lanka.

Accepted 27 April 2006

ABSTRACT A disorder causing the cracking of bark in the lower trunk region of seedlings and in the root stock

bud-grafted trees of Hevea brasiliensis was observed recently. A preliminary investigation was carried out fthe first time to study the occurrence, symptoms, its relationship to the genotype of clones or seedlings, aand growth of trees. The observations were that, this disorder appears only after about three years establishment and reaches a high percentage in about seven years and is completely absent in bud wonurseries. Bud-grafted trees with the disorder had a greater girth than those unaffected. The genotype of tscion in bud-grafted trees did not have a significant influence on the occurrence of this disorder. The natuand the symptoms of this disorder do not show the involvement of a pathogen of, but secondary infectiomay be the reason for bark decay around cracks. The information gathered so far points to an inheritphysiological disorder that appears at maturity affecting both root stocks in bud-grafted trees and the lowtrunk region in seedlings.

Key words: bark cracking, physiological disorder, root stock, graft union INTRODUCTION

Rubber plantations are established using vegetatively propagated plants raised by bud grafting scion material from selected genotypes to root stocks from illegitimate seedlings. Seedlings are not usually used as planting material and continuous breeding programmes are being carried out to select new clones mainly for growth vigour, higher latex yields and disease resistance. Yet these new selections are also susceptible to various diseases of the foliage, stem, branches and roots mostly caused by fungal pathogens (Jayasinghe, 2001). Physiological disorders in Hevea are not many, but one which initiates in the tapping cut region and leads to the drying up of latex vessels in trees under tapping. This is known as ‘Brown bast’ and an important physiological disorder. Apart from above, these trees are also subjected to physical injuries such as ones caused by the twisting of the trunk by strong winds. Such wounds are also subjected to secondary infections by saprophytic and weakly parasitic fungi (Jayaratne, 2001).

Observations were made recently on a condition where latex exudes from cracks that appear in the bark of the root stocks in many bud-

grafted trees with different scion genotypes andsome seedling trees. These symptoms have been reported earlier and in bud-grafted trees thcracks are limited to the bark of the root stock.many sites a high percentage of affected trees wfound making it an important condition investigation. The same disorder was observedthe lower trunk region of seedling trees. A simcondition reported by the Rubber Research Institof India is believed to be of pathogenic in oriand Phytophthora palmivora, Phytophthora meaor Pythium vexans are considered to be causative agents (www.rubberboard.org.in).

In Sri Lanka these symptoms were observfor the first time in rubber and the results of initial study are presented here. MATERIALS AND METHODS Sites

Bud-grafted trees of ten clones and seedltrees with known parentages were investigatedthis study. Plants in four bud wood nurseries walso inspected. Most of the bud-grafted trexamined were in the sub-station of the Rubb *Corresponding author’s e-mail: dirrri@tradenetsl.lk

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L.S.S. Pathiratna, P. Seneviratna, M.K.P. Perera and C.K. Balasooriya 80

Research Institute of Sri Lanka (RRISL) situated in the low country wet zone. The soil here is acidic (pH 4.8) and belongs to the order Ultisol. The annual rainfall was about 4000 mm and was well distributed during most part of the year. The other site was the main station of the RRISL at Dartonfield (D/F) where the soil is acidic (pH 4.3) and the annual rainfall is about 4000 mm. The third site was in the Pallegoda Estate also in the low country wet zone having similar conditions as in the RRISL main station. Bud wood nurseries in the RRISL sub station, in a commercial estate and in a government nursery in the Ratnapura district and one nursery in the RRISL main station were inspected.

Ten rubber clones inspected in the RRISL sub- station at Kuruwita and in the main station at Dartonfield were of different ages planted in monoclonal plots comprised of clones RRIC 121, 100, 102, 133, 110, RRISL 201, 217, 202, 222, and one area with mixed clones (Table 1). Thirteen years old trees belonging to ten seedling families arising from RRIC 100 x RRIM 712 (family 1), RRIC 100 x PB 255 (family 2), RRIC 100 x PR 255 (family 3), RRIC 121 x PB 255 (family 4), RRIC 102 x PB 255 (family 5), BPM 24 x RRIM 712 (family 6), RRIC 100 x PR 309 (family 7), RRIC 121 x PR 255 (family 8), RRIM 102 x PR 309 (family 9) and RRIC 121 x PR 309 (family 10) and planted as completely randomized single tree plots used in the study were also from the Dartonfield estate. The other seedling trial (Pallegoda Estate) is an experiment with five seedling progenies where the genotype of the female parents is known and planted in a randomized block experiment with three replicates and are five years old. Seeds for this trials have been collected from large plots planted with clones RRIC 100, PB 260, PB86, RRIC 121, PB 28/59 and it can be assumed that in all these progenies, the male parent has to be the same clone as these plots are large enough to avoid mixing. However, the individuals in these seedlings are not identical.

Bud wood nursery plants belonging to 16 clones in a commercial estate, three clones in the government nursery and two nurseries in the RRISL sub-station, all in the Ratnapura district were inspected for the presence of this disorder.

One nursery in the RRISL main station was also investigated. The clones present in these nurseries were RRIC 121, 102,100,133,130,117; RRISL 217,218, 203; PB 235, 217, 255; PR 305 and RRII 105. These nurseries were 5-10 years old.

Method of investigation

Tree to tree inspections were made and information on the incidence, position of the bark crack, severity of the condition and growth in girth, age and clone of trees was recorded. Severity was scored as one, two, three or four, four being most severe.

Girth data of affected and unaffected bud-grafted trees and seedlings were subjected to a paired t-test for any relationship of this condition to clones and to the growth of the trees. A similar test was performed to examine the relationship between the age and the incidence of this disorder.

RESULTS Occurrence

The disorder was first observed about two years back in an experiment established in 1998 with the clone RRIC 121. Further observations showed that this disorder was present in bud-grafted trees of different clones in all sites investigated, but with varying percentages of incidence and severity. The percentage of affected trees varied from a low 0.2 – 11.8 (Table 1).

Despite the large variation in the progeny size

in the Dartonfield seedling trial (Table 2), the absence of the disorder from four of the families (1, 5, 6 and 9) was an important observation. Families one and five had 46 and 22 trees respectively while families six and nine had only five and one respectively. However, among these families it can be noted that two families (families 1and 6) had RRIM 712 as the male parent while other two (families 5 and 9) had RRIC 102 as female parent. Among the other six families where the disorder was present, the progeny size of families 8 and 10 was low while that of others was large and the percentage of incidence of the disorder was also high. The severity of the disorder was also high particularly in families two and three (Table 2).

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Table 1. The rubber clones included in the survey, percentage of damaged trees, severity and the girth of affected and unaffected trees.

Site Clone Age (years)

No. affected % affected Severity

(1-4) Mean girth of affected

trees

Mean girth of healthy

trees 1 RRIC 121 3 4 0.5 1 21.0 22.5 2 RRIC 121 4 4 3.1 1-3 40.1 38.6 3 RRIC 121 7 44 7.4 1-4 57.4 55.5 4 RRIC 121 7 32 11.8 1-4 57.7 54.0 5 RRIC 121 15 2 0. 7 3 81.8 97.0 6 RRIC 121 13 9 2.9 1-2 76.6 78.3 7 RRIC 100 7 33 5.6 1-4 54.8 56.2 8 RRIC 100 10 12 4.5 1-2 73.6 62.8 9 RRIC 100 13 2 1.3 2-3 87.5 82.3

10 RRIC 100 18 1 0.2 1 72.0 71.0 11 RRIC 201 10 6 2.6 1-3 72.6 66.5 12 RRIC 217 10 6 2.6 1-3 57.4 52.6 13 RRIC 202 10 11 3.2 1-3 66.7 62.4 14 RRIC 222 5 2 0.85 1 61.5 56.6 15 RRIC 102 11 8 2.7 1-4 76.7 66.2 16 Mixed 5 14 2.8 1-3 57.6 54.1 17 RRIC 133 10 9 3.0 1-3 67.8 65.7 18 RRIC 110 13 1 1.0 3.0 77.2 58.2

Table 2. The percentage occurrence and severity of the disorder in 12 year old seedling families of known parentages.

Seedling family No. of trees affected % Occurrence Severity (1-4) family 2 7 10.9 1-4 family 3 4 8.4 2-3 family 4 2 6.7 1-2 family 7 1 8.3 2 family 8 1 12.5 2

family 10 1 33.0 1

The other set of data was from the trial at Pallegoda estate where the female parents were known and all families had this disorder but with varying severities. Only one tree among 170 trees of RRIC 100 family had this disorder and the severity was also low. Percentage of affected trees and the severity were low in all other families (Table 3).

Symptoms of the disorder in bud-grafted trees Bud graft union in rubber trees can either be

seen above the ground level or covered below the

soil level depending on the depth of planting. When the graft union is exposed above the ground, enlargement of the part immediately below the graft union takes place and this is called the ‘elephant foot’ and lateral roots do not initiate from this exposed part. In trees where the graft union is below the ground level at planting, such an enlargement of the part below the graft union will not take place and the origin of lateral roots is seen immediately below the point of the union.

L.S.S. Pathiratna, P. Seneviratna, M.K.P. Perera and C.K. Balasooriya 82

Table 3. Percentage occurrence and severity of the disorder in seedling progenies from seeds of RRIC 100, PB260, PB 86, RRIC 121 and PB 28/59. in the five year old trial at Pallegoda estate planted in the year 2000.

Clone of the mother trees

Number of trees affected

% trees affected

Severity (1-4)

RRIC100 1 0.6 1 PB 260 8 4.3 1-2 PB 86 7 4.1 1-2 RRIC121 6 3.5 1 PB 28/59 7 4.1 1-2

This disorder was always restricted to a

position immediately below the graft union and manifested differently in trees with and without ‘elephant foots’. In trees with the ‘elephant foot’, the cracks appeared on the exposed root region immediately below the graft union and have not spread into the bark above. One or many cracks appeared around the ‘elephant foot’ and continuous oozing of latex was seen (Fig. 1a & b). No coagulated latex pads were found in between the wood and the bark of the affected area (Fig. 1b). Under severe conditions abnormal enlargement of the ‘elephant foot’ was common compared to those trees without the disorder (Fig. 1a). There was no decay of bark in the vicinity of the crack and signs of any infection were not seen in these wounds that has been there for nearly two years (Fig. 1 a & b).

In trees where the graft union was below soil surface, the bark cracks were not seen superficially. It was manifested as some coagulated latex on soil surface near the trunk of the tree (Fig. 1c &d Fig. 2b). Latex oozed out from the cracks seemed to have seeped on to the surface in between the tree trunk and soil. In these instances the cracks were very small (Fig. 1d) and were only seen after removing the soil below the coagulated latex (Fig. 1e & f). When these cracks were exposed, rotten and smelling coagulated latex pads were seen in between the wood and the bark beneath the crack (Fig. 1f), unlike when the cracks were on ‘elephant foots’. The bark around the cracks was rotten and when the decayed parts of the bark was removed, callus formation was also seen in all wounds (Fig.

Symptoms of the disorder in seedlings

Seedlings did not have a graft union as in bud-grafted trees and the disorder was found in positions immediately below the root collar (Fig. 2b & c) and also in positions up to about 30 cm from the root collar (Fig. 2d). When the cracks were above the ground level the symptoms manifested were similar to those in trees with an ‘elephant foot’. When the cracks appeared in the root collar immediately below the ground level, the symptoms were similar to those in bud-grafted trees with the graft union below the ground level. None of the bud wood nursery plants examined had this disorder in their root stocks. Relationship with age of trees

The paired t test suggested that the age of trees had a significant relationship with the occurrence of the disorder in bud-grafted trees of all clones investigated. Young trees of age 1-2 years were free from this disorder. The incidence was as low as 0.5% in three-year old trees of clone RRIC 121 and was highest in seven years old trees. It was low in 18 years old trees of the clone RRIC100 (Table 1). Age of all seedling trees available for investigation was above 5 years and since there was no possibility of investigating younger seedling trees, any relationship could not be established. Influence of the genotype of the scion

There was no significant evidence to show any relationship between the genotype of the scion (clone) on the incidence of this disorder.

2a). Trees had one or several cracks around the root collar immediately below the graft union.

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Figure 1. (a) Bark cracks in the ‘elephant foot’ of a 7 year old RRIC 121 tree. Abnormal enlargement of the ‘elephant foot’ is seen ; (b) Bark around a crack on an ‘elephant foot’ after cut open. Crack and its vicinity looks healthy without any infection; (c) Coagulated latex on the soil surface oozed out from a three year old bud-grafted tree; (d) the same tree with the crack exposed and some coagulated latex; (e & f) A bark crack in 7 year old RRIC 121 tree-soil is removed to expose the wound with the latex pad. Note the position of the crack immediately below the graft union and the bark around the wound is rotten.

L.S.S. Pathiratna, P. Seneviratna, M.K.P. Perera and C.K. Balasooriya 84

Effect on the growth of rubber trees

The bud-grafted trees with this disorder had significantly greater girths than those without irrespective of the age as indicated in RRIC 121 when analysis was performed without the 15 year and 18 year old trees. Other clones did not have sufficient data to test this. Possible influence of herbicides

Application of herbicides is a common practice in rubber estates as a cheaper mean to remove weeds from bole of the tree. A record of the use of herbicides in the sites under investigation showed that sites 1, 2, 4 and 16 in Table 1 have been free of herbicides throughout but the disorder was present. DISCUSSION

Cracking of bark and phloem necrosis due to stock-scion incompatibility or interaction have been reported for some other crops (Simons 1987). The cause of this disorder in rubber cannot be due to such an effect because the disorder was equally present in seedling trees where bud grafting is not involved.

The symptoms and the appearance of bark cracks and exudation of latex seen here is different from those of bark rot caused by Phytophthora spp. There are no black stripes or rotting of bark as clearly seen in the bark cracks appearing in ‘elephant foots’ (Fig. 1b). There has been continuous exudation of latex for more than one year in these trees and the Fig. 1b clearly shows that the bark around the crack is healthy and without any decay. If it is a condition initiated by a pathogen, the affected tissues cannot stay healthy for such a long time. The restriction of this disorder to the root stock in all bud-grafted trees, its absence in young trees below three years of age and in bud wood nurseries also do not support the view that the disorder of pathogenic origin. Pathogens (except biotrophs) usually kill the tissue around the infection that can stop the flow of latex. The cambium around the crack does not seem to be damaged as there is good regeneration immediately under (Fig. 1b). An organism like Phytophthora spp. also needs a wound in the bark to get entry (Jayasinghe, 2001). Appearance of cracks in the

root collar region due to the twisting of the trunk by winds is most unlikely to occur because this is the strongest part of the tree with lateral roots supporting the trunk.

Rotten bark around the cracks that appear below the surface of the soil could be due to soil borne organisms entering the crack. When the cracks were below the surface, there could have been a restriction to the free flow of oozing out latex by surrounding soil and this could probably have formed thick latex pads in between the bark and the wood (Fig. 1f). It can be assumed that the necrosis onsets within the bark and cracks appear in the bark thereafter, leaking the latex. These latex pads can encourage soil borne organisms to cause rots. But this has not happened when the crack was in the ‘elephant foot’ as latex from the crack flows freely and there is no chance for the formation of latex pads and no infection with soil borne organisms was seen. These evidences clearly rule out possibility of the involvement of a pathogenic organism in this disorder.

The nature of cracks and continuous oozing of latex that prevails for several years without causing any serious destruction to the bark and abnormal enlargement of the ‘elephant foot’ as seen in such trees (Fig. 1a) suggest the physiological nature of this disorder. The absence of this disorder in young plants, and its appearance after about three years of age suggests a relationship with the maturity of the trees. An important clue to support this view comes from the absence of this disorder in bud wood nurseries where the trees are kept juvenile throughout by regular pruning to obtain bud wood (Seneviratne, 1996).

The highest number of trees with this disorder was in the age group of 5- 7 years and was lowest in mature clearings as seen in the clone RRIC 121. There are also indications that the severity of the disorder increases with the increase of the age of the trees. Few affected trees found in old clearings could possibly be due to the loss of the affected trees by wind damage or death due to the secondary infections destroying the root systems near the graft union.

Bark cracking disorder in rubber

85

A severe wound as result of secondary infection in a crack in the root collar below theround level. Note the callusing and formation of roots from the callus. The symptoms are different om those of bark rot caused by Phytophthora spp.; (b) Latex oozing from the crack below the soil

urface in a 13 year old seedling tree in the Dartonfield trial; (c) Coagulated latex on the bark cracks in seedling from the same trial; (d) Bark cracks appearing at higher position in the trunk in a seedling in e same trial.

Figure 2. (a) gfrsath

L.S.S. Pathiratna, P. Seneviratna, M.K.P. Perera and C.K. Balasooriya 86

Herbicide application does not seem to be a

ason for the occurrence also seem to be absent as is disorder was present in all clearings respective of the use of herbicides. Further, the ymptoms of the disorder discussed above, the low ercentage of affected trees (not exceeding 11.8 ercent) and the scattered nature of affected trees in ll field investigated also do not support such a iew.

this can damage the root system that makes these trees susceptible to wind damage. The presence of a few of affected trees in older clear gs is possibly due to breaking of these affe

ges and 900 seedling trees with the female arent known were investigated. 1886 bud wood

plan

ected trees in many areas is cons erable and demands further research to conf

ACKNOWLEDGEMENTS

The authors are grateful to Mrs. Wasana Wijesuriya, the Biometrician for assistance in data analyses, K. B. A. Karunasekera and Mr. K. W. Rupatunga of the Genetics and Plant Breeding Department for their valuable support. REFERENCES Jayaratne, A.H.R. (2001). Conditions caused by

Nugawela (Eds), Hand book of Rubber, Rubber

t ience department. Annual Review the Rubber

p. 18.

ock-scion teractions as related to dwarfing. In: Roy Rom &

.), Root stocks for fruit crops. Pp 79-106.

rethirsppav

When the disorder is below the surface of soil, coagulation of latex in between the wood and the bark takes place and secondary infections can destroy the bark around the crack and also the wood beneath as observed in certain trees (Fig. 1f). Under severe conditions

physical injuries. In: L.M.K. Tillekeratne and A. Nugawela (Eds.), Hand book of Rubber, Rubber Research Institute of Sri Lanka. Jayaratne, A.H.R. (2001). Disorders of non-parasitic origin. In: L.M.K.Tillekeratne and A.

incted trees by wind. Establishment of any effect

of the disorder on latex yield of rubber needs at least one year data.

In this study ten rubber clones in 18 plots in the

RRISL sub-station and the main station (Dartonfield) in different age groups (total of 6132 trees) and 251 seedling progenies with known parentap

ts belonging to 16 clones were also inspected for the presence of this disorder. The available data is insufficient to reach a conclusion but points to a physiological disorder that is carried over by the planting material in use and its recent expression in the rootstocks of bud-grafted trees is possibly due to the use of seeds carrying the genetic information causing this disorder in seedling nurseries. The use of seeds from a very few clones to raise seedling nurseries in the recent past could also be a reason for the recent occurrence of the disorder. However, the number of aff

idirm the cause and develop remedial measures.

Research Institute of Sri Lanka. Jayasinghe, C.K. (2001). Common Diseases. In; L.M.K. Tillekeratne and A. Nugawela (Eds.), Hand book of Rubber, Rubber Research Institute of Sri Lanka. Samaranayake, A.C.I. (1980). Reviews of the plant science department. Annual Review of the Rubber Research Institute of Sri Lanka, Pp. 22.

amaranayake, A.C.I. (1981). Reviews of the plan

SscResearch Institute of Sri Lanka, P Seneviratne, P. (1996). The growth phase change and rejuvenation of trees with special reference to Hevea brasiliensis (Muell.Arg.). Journal of the Rubber Research Institute of Sri Lanka 78, 1-14.

imons, R.K. (1987). Compatibility and stSinRobert Carlson (EdsWilly International. www. www rubberboard.org.in. (January, 2006).