2019 ASHS Annual Conference

Huanglongbing (HLB) is one of the most devastating diseases in citrus (Citrus spp.) with roots being the first and foremost site of pathogen colonization. Field observation suggest that the HLB-affected root system debilitates quickly in response to high soil pH. Soil pH of the root zone directly affects nutrient availability and influences root health indicating possibility of interaction with HLB. Therefore, the objectives of this experiment were to evaluate the performance of healthy and HLB-affected citrus plants irrigated with water at different pH ranges and to investigate the molecular regulation of such response. Healthy and HLB-affected ‘Midsweet’ sweet orange (Citrus sinensis) grafted on Kuharske citrange (C. sinensis X P. trifoliata) were used in this study. Plants were arranged in completely randomized design with 4 replicates and were irrigated every 2-3 days with sodium phosphate buffers at pH 5.8, 7.0 and 8.0. Plant performance was monitored for a period of 60 days. HLB-affected plants at pH 8 had the highest death rate of about 40% compared to the 100% survival rate at pH 5.8. HLB-affected plants at pH 5.8 showed about 6.6% increase in plant height while at pH 8.0 the increase was 0.78%. At pH 8.0, HLB-affected plants showed leaf drop of more than 80% and leaf fresh weight of about 0.25 g, while healthy plants showed leaf drop of about 60% and leaf fresh weight of about 14.75 g. Compared to the healthy plants, soil-media of HLB-affected plants had higher electrical conductivity (EC) at pH 8.0, thus indicating poor uptake of nutrients.There was the highest uptake of Zn and the least uptake of Ca at pH 8.0 among the HLB-affected plants. Leaf chlorophyll content decreased with increase in pH. Severity of HLB symptoms varied in relation to different irrigation pH and symptoms were more prominently expressed at pH 8.0 than at pH 5.8. Among all treatments, HLB-affected plants at pH 8.0 had the most limited root growth and development pattern, leading to the lowest root dry weight. In contrast, pH 5.8 treatment did not affect HLB-affected plants when compared to healthy ones. Transcriptomic analysis of leaf and root tissue is underway to understand the interaction between pH and HLB. The physiological results of this study suggest there is an interaction between HLB and soil pH, where HLB symptoms exacerbates under high pH conditions. Therefore, regulation of irrigation water pH can potentially minimize the symptom and effects of HLB.