[Bacterium] Spiroplasma citri
Little-leaf disease (Israel); safargali (Egypt); early descriptions in California include pink nose, acorn fruit, blue albedo and crazy top in Arizona.
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 C. haematoceps (syn. Neoaliturus haematoceps) in the Mediterranean region. The pathogen multiplies in the vector but no transovarial transmission occurs. Spatial and temporal analysis of CSD incidence indicate only primary spread occurring and no or very limited secondary spread (citrus to citrus). Scaphytopius can develop on citrus but the population remains low to negligible throughout the season. C. tenellus and C. haematoceps have a wide host range which includes many natural hosts of S. citri but citrus is a non host of these leafhoppers. Citrus becomes infected when inoculative Circulifer vectors feed temporarily on citrus during migratory flights.
S. citri is graft-transmissible with side grafts but rate of bud transmission is very low due to low pathogen titer. Seed transmission does not occur. S. citri is phloem-restricted where it multiplies 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.
Leaf - symptoms can vary with season and variety but typically include small size with upright position; some mottling resembling nutritional deficiencies; shortened stem internodes leading to bunchy-type growth. Symptoms on mildly-infected trees are often localized within a sector of a tree.
Fruit - symptoms are variable but include small size, lopsided, with immature acorn-shaped fruit and stylar-end breakdown or greening. Fruits exhibiting blue albedo have been reported. Fruit drop is common but extent depends on horticultural management. Internal fruit development can be imperfect with thinner rind on one side and thicker on the other. Severely-affected fruit can be insipid or bitter flavored. Seeds produced are often aborted.
Whole tree - symptoms and growth habits of affected trees varies. Some infected trees may not appear different from non-infected trees and remain unnoticed for several years. Field diagnosis is complicated with freeze and insect damage, poor nutrition and other diseases. Trees with obvious symptoms include: fruit drop; mildly-infected trees may be normal in size but severely-affected trees are stunted with thin canopy often with a flattened top and tip dieback. Irregular flowering is common resulting in various-sized fruit with different maturities. Fruit production can be greatly reduced.
All cultivars of citrus are susceptible to S. citri infection. Young trees appear to be more vulnerable to CSD than mature trees. The natural host range of S. citri is broad and includes ornamentals such as periwinkle, zinnia, marigold, and viola; many brassicaceous plants; potato and carrots. It’s also been reported in sesame in Turkey.
CSD occur in citrus grown in the Coachella Valley and interior valleys of central and southern California and Arizona. It is present in the Mediterranean region inducing Israel, Syria, Turkey, Morocco, etc. The disease is prevalent in temperate regions with arid or semi-arid climates where citrus is grown with irrigation. 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.