Spiroplasma citri is a bacterium species and the causative agent of Citrus stubborn disease.
The Citrus stubborn disease is a plant disease affecting species in the genus Citrus. It is present in the phloem of the affected plant and transmitted by several leafhoppers including Circulifer tenellus (beet leafhopper) and Scaphytopius nitridus in citrus-growing regions of California and Arizona and Circulifer haematoceps in the Mediterranean region.
The host most notably affected is sweet orange but the bacterium can also infect weeds such as periwinkle (Vinca rosea) and London rocket (Sisymbrium irio). Yellowed plants of Chinese cabbage and pak-choi (Brassica rapa) can be infected by S. citri. In the wild, shortpod mustard (Hirschfeldia incana) infested by the beet leafhopper, Circulifer tenellus, can prove to be an important reservoir of infection. S. citri can also be transmitted to China aster (Callistephus chinensis), Shasta daisy (Leucanthemum × superbum), red clover (Trifolium pratense) and radish (Raphanus sativus) by the leafhopper Scaphytopius nitridus. The bacterium has also been shown to experimentally infect white clover (Trifolium repens) using Euscelis plebejus as a vector.
Symptoms on citrus trees are variable but typically include small size with upright position. Fruits harvested from citrus trees with severe symptoms of citrus stubborn disease can be acorn-shaped or lopsided.