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Plantwise Technical Factsheet

stubborn disease of citrus (Spiroplasma citri)

Host plants / species affected
Apium graveolens (celery)
Armoracia rusticana (horseradish)
Brassica nigra (black mustard)
Brassica oleracea (cabbages, cauliflowers)
Brassica rapa subsp. chinensis (Chinese cabbage)
Brassica rapa subsp. rapa (turnip)
Brassicaceae (cruciferous crops)
Capsella bursa-pastoris (shepherd's purse)
Catharanthus roseus (Madagascar periwinkle)
Citroncirus webberi (citrange)
Citrus aurantium (sour orange)
Citrus jambhiri (rough lemon)
Citrus limettioides (palestine sweet lime)
Citrus limon (lemon)
Citrus limonia (mandarin lime)
Citrus madurensis (calamondin)
Citrus maxima (pummelo)
Citrus reticulata (mandarin)
Citrus reticulata x paradisi (tangelo)
Citrus sinensis (navel orange)
Citrus unshiu (satsuma)
Citrus x paradisi (grapefruit)
Daucus carota (carrot)
Digitalis purpurea (foxglove)
Fortunella (kumquats)
Oryza sativa (rice)
Plantago ovata (spogel plantain)
Poncirus trifoliata (Trifoliate orange)
Prunus avium (sweet cherry)
Prunus persica (peach)
Pyrus communis (European pear)
Raphanus raphanistrum (wild radish)
Raphanus sativus (radish)
Sesamum indicum (sesame)
Sisymbrium irio
Sorghum halepense (Johnson grass)
Tagetes erecta (African marigold)
Viola cornuta (horned violet)
Zinnia elegans (zinnia)
List of symptoms/signs
Fruit  -  abnormal shape
Fruit  -  discoloration
Fruit  -  reduced size
Leaves  -  abnormal colours
Leaves  -  abnormal forms
Leaves  -  abnormal patterns
Seeds  -  discolorations
Seeds  -  shrivelled
Stems  -  stunting or rosetting
Stems  -  witches broom
Whole plant  -  distortion; rosetting
The name 'stubborn' arises from the persistence of the original characters of an infected citrus tree when it is 'top-worked' with healthy budwood. Affected trees are more or less stunted. Leaves are shorter and broader (hence the common name 'little leaf'), cupped, abnormally upright, sometimes mottled or chlorotic. Under very hot conditions, leaves on some shoots may have misshaped, blunted or heart-shaped yellow tips (an important diagnostic character). Shoots may be abnormally bunched and the development of multiple axillary buds may give rise to witches' brooms. Infected trees may show off-season flowering, and fruits at all stages of development. Fruiting tends to be suppressed in infected plants. Fruits may be stunted, lopsided or acorn-shaped (i.e. with thick rind at the base and thin rind at the tip), and may show colour inversion (i.e. the peduncular end discolours while the stylar end remains green). Seeds may be partially aborted. For more details see Bové (1984, 1988). The vectors, as such, cause no particular symptoms.

Other hosts show symptoms of stunting and deformation, and may be killed at higher temperatures.
Prevention and control
Production of healthy budwood is the only practical means of control, but it must be accompanied by suitable siting of orchards to avoid reinfection during the first years of development. In practice, this means that S. citri should be covered by disease-free certification schemes for imported or nationally-produced citrus (OEPP/EPPO, 1995).

Trees showing symptoms should be rogued and replaced, not so much because they constitute a risk to neighbouring trees, but because they will never bear fruit satisfactorily. Insecticide treatments against the vectors are not effective, because S. citri can be transmitted very rapidly after arrival of infective vectors in an orchard. It has been suggested (Gumpf, 1988) that trap plants (attractive to the vector, but not hosts of S. citri, such as sugarbeet) should be planted in the vicinity of orchards. Gumpf (1988) summarizes the main elements recommended for stubborn control in North America.
S. citri causes the serious stubborn disease of citrus which under hot, dry conditions can greatly reduce the quality and quantity of the yield. The fact that it is vector-transmitted, unlike most other graft-transmissible pathogens of citrus, makes it more difficult to control by use of healthy planting material. In California, USA, the main economic hosts are orange, grapefruit and tangelo, of which 5-10% of trees are estimated to be affected. In the Mediterranean area, stubborn is very serious in some countries, especially in Syria where the vector Neoaliturus haematoceps is common and introduced healthy budwood was rapidly reinfected (Bové, 1986). The Syrian practice of rebudding imported healthy stock has also favoured reinfection (UNDP/FAO, 1988). The disease is also reported to be widespread and important in Iraq and Turkey. In other Mediterranean countries (Cyprus, Egypt, Jordan, Morocco) the disease is present, but only rarely on certain cultivars. In others (Algeria, Libya, Tunisia), stubborn was reported to be common in the past but the survey of UNDP/FAO (1988) found few or no trees with characteristic symptoms. Importance no doubt depends largely on the presence or abundance of vectors. N. haematoceps occurs throughout the Mediterranean area, while Circulifer tenellus occurs mainly in its southern and eastern parts. Newly planted citrus orchards are readily invaded by leafhoppers from surrounding crops, and can suffer serious attacks, which then decline as the trees become older and less attractive to the insects (Bové, 1986). Stubborn also tends to occur only in certain years, when the vectors are abundant.

Although S. citri naturally infects many other hosts and some of these are crop plants, it is not reported to have any economic impact on these. Their main significance would be as reservoirs of S. citri for infection of citrus. Horseradish brittle root is of purely anecdotal interest.
Related treatment support
External factsheets
DPI NSW factsheets, New South Wales Government, Department of Primary Industries, Australia, 2014, English language
Plant Health Australia Factsheets, Plant Health Australia, English language
PlantVillage disease guide, PlantVillage, English language
PlantVillage disease guide, PlantVillage, English language
University of California IPM Pest Management Guidelines, University of California, 2008, English language
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