Abstract:
The circular leaf spot disease (CLSD) is a newly reported devastating disease of rubber
cultivation in Sri Lanka. The CLSD is a serious threat to Hevea brasiliensis, affecting
the both latex yield production and overall plant health. Previous investigations have
reported the possible involvement of Colletotrichum spp. and the Pestalotioid group as
fungal pathogens of CLSD. However, etiology and management of the CLSD are not
fully understood. The purpose of this study was to isolate, identify, and assess the
pathogenicity of pathogens associated with CLSD in Kalutara and Monaragala districts.
Symptomatic leaf samples were collected and the associated fungi were isolated.
Morphological analysis revealed the species diversity within these two primary
causative agents. Pathogenicity tests utilizing spore suspensions and plug inoculations
revealed dilferences in virulence among isolates. The disease severity assessments
demonstrated that Colletotrichum isolates KD/C/I and MK/C/I exhibited the highest
disease index (DI) values (3.3241 and 3.2083, respectively), indicating strong
pathogenic potential. In contrast, Pestalotioid isolates showed relatively lower
virulence, with the lowest DI values recorded for KNK/P 12 (1.6944) and MP/P/1
(1.5463). Furthermore, wounding condition significantly increased disease severity
across all isolates, confirming the role of leaf integrity in pathogen establishment.
Statistical analyses revealed substantial variations in growth rates of fungi, with \/lflp12
and MK/C/I displaying the highest conidial growth rates, exceeding 7.5 cm by day 5
and reaching 9.0 cm by day 7. Additionally, analysis of conidial morphology identified
significant length variations, with the highest mean length recorded for KN/P/2 (10,0S
cm) and the lowest for MK/C/I (5.67 cm). These findings provide critical insights into
the epidemiology of CLSD, emphasizing the importance of targeted disease
management strategies. The results underscore the need for resistant clone
development, integrated disease management approaches incorporating chemical and
biological control measures to mitigate the impact of CLSD on Sri Lanka's rubber
plantations.
