Spatial–Temporal Colonisation of Cacao Stem Tissues by Lasiodiplodia laeliocattleyae

Cacao (Theobroma cacao L.) is a key tropical crop, but dieback has become an emerging threat, with Lasiodiplodia species identified as causal agents. In Ecuador, L. laeliocattleyae has been reported as responsible for dieback and pod rot in the CCN-51 cacao clone. However, its colonisation dynamics in stem tissues remain unclear. This study aimed to investigate the spatial–temporal colonisation of L. laeliocattleyae in CCN-51 cacao stems. Seedlings were inoculated with virulent pathogen strains LT1A and LT2A, and vascular necrosis was evaluated at 1, 25, and 50 days after inoculation (DAI). Dieback incidence was assessed at 50 DAI. Our results showed that both isolates caused 100% seedling mortality, with LT2A inducing more intense symptoms. Vascular necrosis increased over time and was significantly more intense in the basipetal region than in the acropetal region, suggesting a downward colonisation pattern. The LT2A strain caused more significant necrosis in vascular tissues than LT1A, particularly at 50 DAI. A significant interaction between strain and stem region was observed only on the first day after inoculation. Both strains exhibited similar colonisation rates in stem tissues over time. These findings provide new insights into the colonisation dynamics of L. laeliocattleyae, suggesting the possible involvement of the phloem or a migration strategy toward the soil. Understanding this process is critical for developing management strategies against Lasiodiplodia-induced dieback in cacao plants. Future studies should employ biochemical, microscopic, and molecular techniques to elucidate further the mechanisms underlying the pathogen's colonisation patterns.