EFFECT OF SEEDLING AGE AT INOCULATION ON MECHANICAL TRANSMISSIBILITY AND PATHOGENICITY OF TOMATO YELLOW LEAF CURL VIRUS ON CELOSIA ARGENTEA

This study investigated the critical relationship between seedling age at inoculation and the progression of TYLCV infection in Celosia. argentea, focusing on mechanical transmissibility, disease pathogenesis, and subsequent impacts on plant growth and productivity. The research was conducted under controlled screenhouse conditions at the University of Ilorin, Nigeria, employing a completely randomized design with five treatments: an uninoculated control (T0) and mechanical inoculations performed at 1 (T1), 2 (T2), 3 (T3), and 4 (T4) weeks post-emergence, each replicated six times. The experimental protocol involved extracting viral inoculum from TYLCV-infected tomato plants and applying it to Celosia. argentea seedlings using a standardized mechanical inoculation technique. Disease progression was monitored weekly through comprehensive assessments of plant height, leaf number, and disease severity (scored on a 0-10 scale). Fresh and dry biomass measurements were recorded at termination to evaluate yield impacts. Statistical analysis was performed using ANOVA with Duncan's Multiple Range Test (DMRT) at p < 0.05 to determine treatment significance. Results revealed striking age-dependent responses to TYLCV infection. Seedlings inoculated at one week (T1) exhibited severe growth inhibition, achieving only 42% of control plant height (13.3 cm vs. 31.6 cm) and 33% of leaf production (7.2 leaves vs. 21.8 leaves) by week 7. Disease severity peaked at 39.9% in T1 plants, manifesting as pronounced leaf curling, yellow mosaic patterns, and stunting. In contrast, plants inoculated at four weeks (T4) demonstrated remarkable resilience, maintaining 84% of control height (26.5 cm) and 38% of leaf count, with significantly lower disease severity (18.1%). Intermediate treatments (T2-T3) showed graded responses, confirming a clear correlation between seedling maturity and disease tolerance. The study identified two critical infection phases: an early susceptible window (1-2 weeks post-emergence) where plants showed acute vulnerability, and a later resistant phase (3-4 weeks) characterized by reduced symptom expression. This pattern suggests that developmental factors, particularly vascular system maturation and phytohormone regulation, may underlie age-related resistance. Biomass analysis further confirmed these findings, with T1 plants producing only 28% of control fresh weight compared