Citrus stubborn disease (CSD)

Scientific name

[Bacterium] Spiroplasma citri

Other common names

Little-leaf disease (Israel); safargali (Egypt); early descriptions in California include pink nose, acorn fruit, blue albedo and crazy top in Arizona.

Disease cycle

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 pathogenpathogen:
an organism, usually a microorganism, which causes disease by intimate association with its host
multiplies in the vectorvector:
an organism that does not cause diseases itself but which carries the disease-causing microorganism from one host to another
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 hosthost:
an organism that is infected with or fed upon by a pathogenic or parasitic organism
range which includes many natural hosts of S. citri but citrus is a non hosthost:
an organism that is infected with or fed upon by a pathogenic or parasitic organism
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 budbud:
a.) a small lateral or terminal projection on the stem of a plant, often enclosed by protective scales, from which shoots, leaves, or flowers develop b.) an undeveloped shoot, leaf, or flower c.) an outgrowth from an organism that creates a new individual (asexual reproduction)
transmission is very low due to low pathogenpathogen:
an organism, usually a microorganism, which causes disease by intimate association with its host
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.

Symptoms

Leaf - symptoms can vary with season and varietyvariety:
a taxonomic category, a subdivision of species, consisting of naturally occurring or selectively bred populations or individuals that differ from others of the same species in certain minor but heritable traits
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 diebackdieback:
progressive death of shoots, branches, and roots that generally begins at the tip and works back to the main body of the plant
. Irregular flowering is common resulting in various-sized fruit with different maturities. Fruit production can be greatly reduced.

Host range

Citrus scab is only a serious problem on some varieties. It is severe on rootstockrootstock:
a living plant, sometimes just the stump, with a healthy, established root system, used for grafting a cutting or budding from another plant; the grafted portion is called the scion
seedlings of rough lemon, sour orange, Rangpur lime, and Carrizo citrange, and scions of Murcott tangor, Temple tangor, and other tangerine hybrids. It is occasionally found on grapefruit.

Distribution

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.