Submitted to: Review Article
Publication Type: Other
Publication Acceptance Date: 6/10/2011
Publication Date: 3/1/2013
Citation: Yokomi, R.K. 2013. Citrus stubborn disease (CSD). Review Article. available: http://www.idtools.org/id/citrus/diseases Interpretive Summary: Citrus stubborn disease (CSD) is a caused by Spiroplasma citri, a leafhopper-transmitted prokaryote. All cultivars of citrus are susceptible and symptoms of infected trees can be confused with nutritional deficiency and other diseases. S. citri has a wide host including many weed hosts of the leafhopper vectors. Citrus is an erratic host of S. citri and supports a low titer of the pathogen. However, during hot summer months the titer increases and detection is most accurate. Infected trees exhibit symptoms ranging from normal appearing to severe stunting and fruit deformation. Foliar symptoms include bunchy-type growth and small-upturned leaves, variable levels of leaf chlorosis from none to severe, off-season flowering and premature fruit drop. These symptoms can appear throughout the tree or in one sector of the tree or branch. Fruit symptoms of irregular exterior coloration, lopsided growth and aborted seeds can be confused with those caused by huanglongbing (greening).
Technical Abstract: CSD is caused by Spiroplasma citri, a phloem-limited, cell-wall-less bacterium. S. citri is transmitted in a propagative, circulative manner by several leafhoppers including Circulifer tenellus and Scaphytopius nitridus in citrus-growing regions of California and Arizona and by C. haematoceps (syn. Neoaliturus haematoceps) in the Mediterranean region. The pathogen multiples in the vector but no transovarial transmission occurs. Citrus is a non-host of the Circulifer vectors but becomes infected as inoculative leafhoppers feed temporarily on citrus while searching for host plants during dispersal flights. Seed transmission does not occur. S. citri is phloem-restricted and moves slowly through the tree. CSD has a long latent period of months to years after inoculation. Detection varies with season with highest titer being in hot summer months, concomitant with most pronounced symptom expression. Symptoms can be variable but typically include small leaves with upright position; some mottling resembling nutritional deficiencies; and bunchy-type growth. Symptoms can occur on one branch, sector or entire tree. Fruit symptoms include small size, lopsided, with immature acorn-shaped fruit and stylar-end greening. Seeds formed are often aborted. Fruit production can be greatly reduced. Whole tree symptoms vary from none to stunted, thin canopy with a flattened top and tip dieback. Off-season flowering is common which results in fruit of variable size and maturity. Field diagnosis can be confused with freeze and insect damage, poor nutrition and other diseases. All cultivars of citrus are susceptible. Young trees are more vulnerable to CSD. The natural host range of S. citri is broad and includes ornamentals including periwinkle, zinnia, marigold, viola; many brassicaceous plants; potato and carrots. S. citri has been reported to infect sesame in Turkey. CSD can be prevalent in temperate regions with arid or semi-arid climates where citrus is grown with irrigation (e.g., Coachella Valley and interior valleys of central and southern California, Arizona and Mediterranean regions). These areas have limited seasonal rainfall but enough to support rapid germination and growth of natural weed hosts of S. citri and competent leafhopper vectors. Because infected leafhoppers remain infectious for life, this pathosystem presumably sustains S. citri and only limited primary spread occurs in citrus. CSD is not known to occur in tropical or sub-tropical regions.