Screening Teak (Tectona grandis L.f.) clones for variation and resistance to Loranthus infestation at the Manchikere clonal seed orchard, Karnataka, India

Introduction

Teak (Tectona grandis L.f.) is one of the most valuable tropical hardwood species in the world owing to its excellent timber quality, dimensional stability, natural durability and high economic value (Tewari, 1992) [9] . The species is naturally distributed in India, Myanmar, Thailand and Laos, but has been extensively introduced into tropical regions across Asia, Africa and Latin America because of its superior wood properties and adaptability to diverse environmental conditions (Tewari, 1992) [9] . The increasing global demand for quality teak timber has accelerated the establishment of plantations, creating a need for genetically superior planting material capable of sustaining high productivity, superior wood quality and enhanced adaptability. Tree improvement programmes in teak have increasingly focused on the selection and deployment of elite genotypes through clonal forestry to maximize plantation productivity and genetic gain. Clonal seed orchards (CSOs) play a pivotal role in these programmes by producing genetically improved seed from selected parent trees while conserving superior genetic resources for future breeding (Gunaga et al., 2011) [2] . Evaluation of clonal performance under field conditions enables the identification of superior genotypes with desirable growth, reproductive potential and adaptability, thereby improving the efficiency of breeding and seed production programmes (Gunaga et al., 2011) [2] .

Despite considerable progress in teak genetic improvement, the productivity of clonal seed orchards is frequently constrained by insect pests and parasitic plants that adversely affect tree growth, health and seed production (Gunaga et al., 2011) [2] . Among these biotic constraints, mistletoes belonging to the family Loranthaceae are recognized as one of the most destructive aerial hemiparasitic plants in tropical forests and plantations (Watson, 2001) [10] . These parasites establish haustorial connections with host branches to extract water and mineral nutrients while retaining their own photosynthetic capability, while their seeds are primarily dispersed by birds, facilitating rapid colonization of susceptible host trees within plantations and seed orchards (Watson, 2001) [10] . In teak, Loranthus infestation diverts water and nutrients from infected branches, resulting in branch dieback, reduced crown development, poor reproductive performance and progressive decline in tree vigour (Gunaga et al., 2011; Watson, 2001) [2,10] . Infestation has been reported to reduce diameter increment by (37-42%), highlighting its substantial economic impact on plantation productivity (Ghosh et al., 1983) [1] . Surveys conducted in teak plantations of Dandeli Province, Karnataka, recorded Loranthus infestation ranging from (10.88 to 23.35%) across different age classes of plantations (Hanumantha et al., 2010) [3] . Similarly, a survey conducted in a 42-year-old teak clonal seed orchard at Waghai, Gujarat, revealed that (21.76%) (47 of 216) ramets were infested, whereas (78.24%) remained healthy, and only one clone was completely free from Loranthus infestation, demonstrating considerable inter-clonal variation in susceptibility (Singh et al., 2017) [8] . Evidence accumulated over the past decade indicates that susceptibility to Loranthus varies considerably among teak clones, suggesting that host genotype plays a major role in determining infestation levels (Gunaga et al., 2011) [2] . In Karnataka, Gunaga et al. (2011) [2] reported significant clonal variation in Loranthus infestation and identified clones exhibiting comparatively lower infestation, although no clone exhibited complete resistance. Likewise, Singh et al. (2017) [8] identified one resistant clone and several tolerant and susceptible clones in a teak clonal seed orchard in Gujarat. More recently, Muttaqin et al. (2025) [7] demonstrated that differences in morphological traits among teak clones were associated with variation in mistletoe infestation, indicating that resistance-related characteristics can be incorporated as additional selection criteria in teak breeding programmes. Although previous studies have confirmed the existence of clonal variation in Loranthus susceptibility, information remains limited for several established teak clonal seed orchards in southern India. Furthermore, long-term evaluations of mature seed orchards are essential because infestation intensity may vary with orchard age, environmental conditions and host-parasite interactions. Identification of resistant or tolerant clones would therefore contribute not only to improved orchard management but also to the development of genetically superior planting material for sustainable teak improvement programmes. Therefore, the present study was undertaken to assess the extent of clonal variation in Loranthus infestation among teak genotypes maintained in the Manchikere Clonal Seed Orchard, Karnataka. The study also aimed to identify comparatively resistant and tolerant clones that could be utilized in future breeding programmes and integrated management strategies for enhancing the productivity and sustainability of teak clonal seed orchards.

Materials and Methods

Study Area

The investigation was conducted in the Manchikere Clonal Seed Orchard (CSO) located in the Manchikere Forest Range of Yellapur Forest Division, Uttara Kannada District, Karnataka, India (14°51′34.1″ N, 74°50′41.0″ E). The orchard was established by the Karnataka Forest Department in 1980 for the production of genetically improved teak seed through clonal forestry. The plantation occupies approximately 4 ha and was established at a spacing of 8 × 8 m using grafted ramets of phenotypically superior teak trees selected from different forest divisions of Karnataka. The orchard represents diverse geographical origins, making it suitable for evaluating clonal variation in susceptibility to Loranthus infestation.

Experimental Material

The study was conducted using 24 teak clones maintained in the Manchikere Clonal Seed Orchard. These clones were originally selected from phenotypically superior trees representing different forest divisions of Karnataka and were established as grafted ramets in the orchard. The experimental material comprised 13 clones from the Haliyal Forest Division (Northern region), four clones from the Shivamogga Forest Division (Central region), six clones from the Hunsur Forest Division (Southern region), and one clone from the Mysore Forest Division (Southern region). Thus, the orchard represented three major geographical regions of Karnataka, providing an appropriate genetic base for evaluating clonal variation in Loranthus infestation. The clone identity, forest range, forest division and geographical origin of each genotype are presented in Table 1.

Assessment of Loranthus Infestation

Field assessment of Loranthus infestation was carried out through systematic visual inspection of all teak clones and their respective ramets maintained in the Manchikere Clonal Seed Orchard from September 2024 to September 2025. Each ramet was carefully examined from the lower branches to the upper crown for the presence of Loranthus (Dendrophthoe falcata var. pubescens). The number of mistletoe clumps attached to each ramet was recorded by direct counting. Based on the observed number of Loranthus clumps, every ramet was assigned an infestation score to quantify the severity of parasitism. The recorded data were subsequently pooled at the clonal level to compare infestation intensity among clones and to identify resistant, tolerant, susceptible and highly susceptible genotypes.

Infestation Scoring System

The severity of Loranthus infestation was assessed using the weighted scoring system described by Gunaga et al. (2011) [2] . Each ramet was classified into one of six infestation categories according to the number of mistletoe clumps observed. The weighted infestation score for each clone was calculated by multiplying the number of ramets in each category by the corresponding score and summing the values across all categories. This weighted score represented the overall infestation intensity of each genotype.

Classification of Clone Resistance

The weighted mean infestation score was computed for each clone, and the overall orchard mean (𝑥̅ ) and standard deviation (SD) of the weighted scores were calculated. Based on the procedure described by Gunaga et al. (2011) [2] and subsequently adopted by Singh et al. (2017) [8] , the clones were classified into four reaction categories as follows:

No ramets infested (Weighted score = 0) Tolerant (T)

Clonal mean < (Orchard mean -1 SD) Susceptible (S) (Orchard mean -1 SD) ≤ Clonal mean ≤ (Orchard mean + 1 SD) Highly Susceptible (HS)

Clonal mean > (Orchard mean + 1 SD)

This classification was used to categorize the evaluated clones according to their relative resistance or susceptibility to Loranthus infestation and to identify promising genotypes for future breeding and clonal seed orchard management.

Statistical Analysis

The infestation data recorded for individual ramets were compiled and analysed using Microsoft Excel. The weighted infestation score for each clone was calculated following the procedure described by Gunaga et al. (2011) [2] . Based on the weighted scores, the orchard mean and standard deviation were computed, and the clones were classified into Resistant, Tolerant, Susceptible and Highly Susceptible categories. One-way analysis of variance (ANOVA) was performed to determine significant differences in weighted infestation scores among clones, clonal source (forest range) and geographical regions. Homogeneity of variances was assessed using Bartlett's test prior to ANOVA.

Results and Discussion

Clonal variation in Loranthus infestation

A total of 525 ramets representing 24 teak clones were evaluated for Loranthus infestation in the Manchikere Clonal Seed Orchard. Considerable clonal variation was observed in the incidence of Loranthus among the evaluated clones (Table 2). The proportion of healthy ramets varied from (60.0%) in MyHuT 6 to (100.0%) in MySA1, whereas total infestation ranged from 0 to 40.0%. The weighted infestation scores also exhibited wide variation, ranging from 0.0 in MySA1 to 60.0 in MyHuT 6 (Table 3). Based on the weighted infestation score, one clone (MySA1) was classified as resistant, two clones (MySS2 and MyHaV1) as tolerant, nineteen clones as susceptible, and two clones (MyHuT 6 and MyHuT 7 ) as highly susceptible, indicating substantial genetic variation in the response of teak clones to Loranthus infestation. The present findings are in close agreement with those of Gunaga et al. (2011) [2] , who reported marked clonal variation in Loranthus infestation among 24 teak clones in the Karka Clonal Seed Orchard. In their study, 142 out of 625 ramets (22.72%) were infested with Loranthus, while clone-wise infestation ranged from (3.8%) in MySS1 to (48.1%) in MyHaV2. Similarly, the weighted infestation score varied from 3.8 to 85.2, reflecting substantial differences in susceptibility among clones. The present investigation recorded a comparable pattern of variation, although the maximum infestation (40.0%) and weighted infestation score (60.0) were lower than those reported by Gunaga et al. (2011) [2] . The occurrence of clones with no infestation together with highly infested clones in both studies clearly demonstrates that susceptibility to Loranthus is strongly influenced by genotype. Comparable levels of Loranthus infestation have also been reported under plantation conditions in Karnataka. Hanumantha et al. (2010) [3] observed infestation ranging from (10.88 to 23.35%) in teak plantations of different age classes. In the present study, infestation among individual clones ranged from (0 to 40.0%). While several clones exhibited infestation levels within the range reported by Hanumantha et al. (2010) [3] , clones such as MyHuT 6 (40.0%) showed substantially higher infestation, indicating pronounced clonal differences in susceptibility under clonal seed orchard conditions. The present observations are further supported by Singh et al.

(2017) [8] , [2] highlights the importance of considering the forest range of origin while selecting superior clones for breeding programmes and clonal seed orchard establishment.

Variation in Loranthus infestation among geographical regions

Significant differences in Loranthus infestation were observed among teak clones originating from different geographical regions of Karnataka (Table 5). The mean weighted infestation score was highest in clones originating from the Southern region (40.98 ± 13.16), followed by the Northern region (24.90 ± 18.24), whereas clones from the Central region recorded the lowest infestation (14.93 ± 12.70). Analysis of variance indicated that the differences among geographical regions were statistically significant (P = 0.003), suggesting that the geographical origin of the clones significantly influenced their susceptibility to Loranthus infestation. A similar regional trend was reported by Gunaga et al. (2011) [2] , who compared Loranthus infestation among teak clones originating from different geographical regions of Karnataka. The authors reported mean weighted infestation scores of 43.20 for the Southern region, 35.88 for the Northern region, and 29.88 for the Central region, with the Southern region exhibiting the highest infestation. The present investigation showed the same overall pattern, with Southern-origin clones recording the greatest infestation (40.98), followed by Northern (24.90) and Central (14.93) clones. Although the absolute infestation scores differed between the two studies, both consistently identified the Southern region as the most susceptible source of teak clones. The lower infestation observed in Central-region clones compared with those from the Southern region suggests that geographical origin may influence host resistance through differences in the genetic composition of source populations. The nearly 2.7-fold higher weighted infestation score recorded in Southern-origin clones (40.98) compared with Central-origin clones (14.93) indicates substantial regional variation in susceptibility. Similar regional variation reported by Gunaga et al. (2011) [2] further supports the existence of geographical differences in resistance to Loranthus among teak populations in Karnataka. The observed regional differences may reflect long-term adaptation of teak populations to their native environments, resulting in variation in morphological and physiological characteristics that influence parasite establishment. Differences in canopy architecture, branch characteristics, bark properties and host defence mechanisms among geographical populations may affect the successful establishment and development of Loranthus. From a breeding perspective, the relatively lower infestation observed in Central-region clones indicates that these populations may provide useful genetic material for selecting Loranthus-resistant planting stock, whereas highly susceptible Southern-origin clones require careful monitoring and management when incorporated into clonal seed orchards.

Classification of resistant and susceptible teak clones

The classification of teak clones based on weighted Loranthus infestation scores revealed clear differences in their response to the parasite (Table 6). Of the 24 clones evaluated, one clone (MySA1) was classified as resistant, two clones (MySS2 and MyHaV1) as tolerant, nineteen clones as susceptible, and two clones (MyHuT 6 and MyHuT 7 ) as highly susceptible. The resistant clone remained free from Loranthus infestation throughout the assessment period, whereas the highly susceptible clones recorded the highest weighted infestation scores, indicating a wide range of host responses within the clonal seed orchard.

A comparable classification pattern was reported by Gunaga et al.

(2011) [2] in the Karka Clonal Seed Orchard, where screening of 24 teak clones identified one resistant clone, one tolerant clone, nineteen susceptible clones and three highly susceptible clones based on weighted infestation scores. Likewise, Singh et al.

(2017) [8] classified 18 teak clones into one resistant, one tolerant, eleven susceptible and five highly susceptible categories. The close agreement among these independent studies, conducted under different agro-climatic conditions, demonstrates that resistance to Loranthus is consistently expressed in only a small proportion of teak clones, while the majority remain susceptible to varying degrees. Although the proportion of resistant and highly susceptible clones differed slightly among studies, the overall trend remained remarkably consistent. The present investigation identified (8.3%) of the evaluated clones (2 of 24) as resistant or tolerant, whereas (91.7%) were classified as susceptible or highly susceptible. Similarly, Gunaga et al. (2011) [2] reported (8.3%) resistant or tolerant clones (2 of 24) and (91.7%) susceptible or highly susceptible clones, while Singh et al. (2017) [8] Values in the parenthesis are arc-sine transformations Bartlett's test for the homogeneity of variances: a. Total Weighted Score: Chi-square = 8.504 (d.f. 5) NS [2] and Singh et al. (2017) [8] , which also demonstrated considerable genetic variation in resistance to Loranthus among teak clones. The identification of resistant and tolerant clones across independent studies indicates that resistance is a heritable and exploitable trait that can be incorporated into teak improvement programmes.

The resistant clone MySA1 and the tolerant clones MySS2 and MyHaV1 represent valuable genetic resources for clonal propagation, breeding and the establishment of healthier clonal seed orchards. Conversely, highly susceptible clones should be subjected to regular surveillance and timely removal of Loranthus to reduce parasite spread and maintain orchard productivity.

Incorporating resistance screening into routine evaluation programmes will contribute to the development of productive and sustainable teak plantations.

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