EFFECT OF CITRUS STUBBORN DISEASE ON NAVEL ORANGE PRODUCTION IN A COMMERCIAL ORCHARD IN CALIFORNIA

The impact of citrus stubborn disease (CSD), caused by Spiroplasma citri, on commercially cultivated citrus is not fully understood or quantified. Our objective was to measure these impacts on citrus production and assess bacterial distribution in trees having different symptom severities. S. citri-positive and adjacent healthy navel orange trees in a commercial grove in central California were evaluated. Measurements included canopy height and width, trunk diameter, fruit number and weight, and number of prematurely dropped fruit. Thirty fruit per tree were evaluated for color, size and sunburn. Juice was extracted, weighed, and total soluble solids and titratable acidity measured. Bacterial distribution in trees exhibiting mild or severe symptoms was assessed by q-PCR and spiroplasma culture. Fruit from S. citri-positive trees were smaller, and more often mis-shapen, than those from healthy trees. Significant fruit number reduction occurred only in severely symptomatic trees, in which S. citri was broadly distributed within the tree canopy. All other variables were statistically indistinguishable regardless of symptom severity or pathogen presence. The reduction in fruit weight, size and number in severely symptomatic trees validate the concern that CSD is a significant constraint to production and marketability in California.