Pest status of citrus psyllids in relation to citrus greening disease (Huanglongbing) in major citrus growing areas of Tanzania

MSc Dissertation

The citrus industry is a significant contributor to the global economy, with East African countries like Tanzania playing a crucial role. However, the emergence of Asian citrus greening disease, facilitated by insect vectors such as Diaphorina citri and Trioza erytreae, poses a substantial threat to this sector. This study aims to document the impact of citrus greening and psyllid infestations on Tanzania‘s citrus industry, which results in diminished fruit yield, compromised quality, and economic losses. Field surveys were conducted in major citrus-growing regions in Tanzania, including Ruvuma, Arusha, Morogoro, and Tanga. Data was collected using molecular detection techniques, and the prevalence and severity of Huanglongbing, distribution and abundance of Diaphorina citri and Trioza erytreae were recorded. These factors were then correlated with altitude, temperature variations, and citrus varieties. DNA samples were extracted and analysed using polymerase chain reaction (PCR) to confirm the presence of Huanglongbing in selected samples. The study found regional variations in the distribution and abundance of Diaphorina citri and Trioza erytreae. Altitude significantly influenced psyllid species abundance, with Diaphorina citri decreasing and Trioza erytreae increasing with altitude. The temperature had a weak positive correlation with Diaphorina citri abundance and a moderately negative correlation with Trioza erytreae abundance. Different citrus varieties significantly affected Diaphorina citri abundance, with the ―Late Valencia (Kitenesi)‖ variety having the highest average Diaphorina citri counts per tree. The study also found a negative correlation between altitude and the Disease Severity Index (DSI), suggesting that as altitude increases, the DSI decreases. However, a weak positive correlation was observed between the number of citrus trees in an orchard or backyard and the DSI. Significant variations in Huanglongbing prevalence were observed at both the regional and ward levels. The study concluded that the Mediterranean variety was particularly susceptible to Huanglongbing, while the Valencia, Local variety, Washington navel, and Bitijuma varieties exhibited more resistance. The study also confirmed the presence of Ca. L. asiaticus in symptomatic leaves and the absence of bacteria in asymptomatic leaves using PCR Analysis for pathogen detection. The findings of this study have significant implications for developing region-specific management strategies to protect Tanzania‘s citrus industry, which is vital for food security and rural livelihoods. It is recommended to combine molecular and visual methods to enhance our understanding of the dynamics of Tanzania‘s two crucial citrus psyllid species, Diaphorina citri and Trioza erytreae, reflecting the complex ecological interactions involved. This will aid in the development of effective strategies to mitigate the impact of citrus greening disease.

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