Temporal Variation of Leaf Spot Disease Incidence in Cinnamon (Cinnamomum verum) under Field Conditions in Sri Lanka

Leaf spot disease (LSD) caused by fungus Colletotrichum gloeosporioides (Penz.) is a major threat to cinnamon (Cinnamomum verum), yet its temporal variation remains poorly understood. This study assessed LSD incidence (LSDI) and severity (LSDS) in 40 cinnamon accessions collected from different regions of the country and grown in the field (WL2, Agro-ecological region, Sri Lanka). Correlations between LSDI and selected morphological traits were also examined. LSDS was determined using a disease assessment key, while tree height (TH), leaf length (LL), leaf width (LW), leaf length-width ratio (LL/LW), petiole length (PL), bark thickness (BT), trunk circumference (TC), and twig diameter (TD) were recorded using the Descriptors for Cinnamon.
LSDI ranged from 50% to 100% across all accessions in both years. The mean LSDI was significantly lower in 2018 (76.97) with an average monthly rainfall (MRF) of 95.62 mm, compared to 2019 (87.42) with 189.4 mm MRF (p<0.05, Paired T-test). In 2019, LSDS varied among accessions grown in the field collected from different regions, Matara (4.08), Galle (3.30), Kalutara (3.74), Kurunegala (2.00), Ratnapura (2.67), and Hambantota (3.50), comparatively to the LSDS obtained in 2014–2015 (4.50, 4.82, 4.74, 4.60, 3.65, and 3.91, respectively) when the plants were grown in the original regions. Cluster analysis grouped accessions into three distinct clusters at a distance of 9. No significant correlations were found between LSDI and morphological traits (p>0.05). Findings indicate that LSDS varies among accessions, and LSDI fluctuates over time, likely influenced by environmental factors. Further research is needed to investigate genetic basis of resistance and environmental determinants of LSD in cinnamon.