Viruses and phytoplasmas
Viruses are small submicroscopic particles whereas phytoplasmas are much larger and resemble bacterial cells without a cell wall or distinct nucleus. Both require a living host and cause similar appearing disease symptoms which include: a general dwarfing of the plant, lack of proper chlorophyll production resulting in a mottled appearance on foliage, yellowing and in some cases rings on leaves or fruit as well as necrotic (dead) areas. Symptoms may mimic those caused by 2,4-D damage or other disease organisms or even environmental problems. The disease can be spread by feeding insects or mites, or mechanically through hands and tools.
Other images

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Virus ring spot on leaves of a moth orchid (Phalaenopsis)
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Virus ring spot on leaves of a moth orchid (Phalaenopsis)
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Stunted, deformed hosta, possibly caused by a virus
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Possible ringspot virus on butternut squash (Cucurbita); note, spots with concentric circles on underside of leaves
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Leaf of a butternut squash (Cucurbita) possibly with a ringspot virus; note, chlorotic mottling
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Leaf of a butternut squash (Cucurbita) possibly with a ringspot virus; note, slightly chlorotic blotches
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Normal looking fruit of a butternut squash (Cucurbita) with suspected ringspot virus
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Possible virus on collards (Brassica oleracea Acephala Group); upper leaf surface
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Possible virus on collards (Brassica oleracea Acephala Group); underside of leaf
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Possible virus on collards (Brassica oleracea Acephala Group) with leaf blotch
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The yellow mottling on the inner, newer leaves of this summer squash (Cucurbita pepo 'White Bush Scallop') was caused by a virus; while the white patches on the outer, older leaves indicate powdery mildew
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A virus is suspected of stunting these bush beans (Phaseolus vulgaris) because of some of the leaves of the affected plants are crinkled and mottled.
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Some of these bush beans (Phaseolus vulgaris) are stunted and have crinkled, mottled leaves. An unknown virus is a possible culprit.
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Ringspot virus on coleus (Solenostemon)
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Ringspot virus on coleus (Solenostemon)
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Close-up of rings or circles in a coleus leaf (Solenostemon) cause by a ringspot virus
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Virus on bean (Phaseolus); note, mottled appearance
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Virus on bean (Phaseolus); note, mottling
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Possible virus on hackberry (Celtis)
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Ringspot virus on toad lity leaf (Tricyrtis)
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Ringspot virus on underside of toad lity leaf (Tricyrtis)
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Ringspot virus on toad lity leaf (Tricyrtis)
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Possible virus on ruellia (Ruellia); note, bleached patterns on leaves
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Possible virus on ruellia (Ruellia); note, bleached patterns on leaves
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Possible virus on (Phaseolus vulgaris 'Jade') leaves
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Virus on rose (Rosa)
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Virus on rose (Rosa)
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Virus on rose (Rosa)
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Possible ringspot virus on toad lily (Tricyrtis formosana 'Gilt Edge')
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Possible ringspot virus on toad lily (Tricyrtis formosana 'Gilt Edge')
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Possible ringspot virus on toad lily (Tricyrtis formosana 'Gilt Edge'); note, circular ring
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Yellow flecks on hackberry leaves (Celtis) caused by hackberry island chlorosis, a benign virus
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Yellow flecks on hackberry leaves (Celtis) caused by hackberry island chlorosis, a benign virus
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Mosaic virus on rose (Rosa)
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Virus on rose (Rosa)
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Virus on rose (Rosa)
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Virus on hybrid tea rose (Rosa 'Mr. Lincoln')
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Distorted leaves on eggplant (Solanum menongen) caused by mosaic virus
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Close-up of distorted, mottled leaves on eggplant (Solanum menongen) caused by mosaic virus
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The mottling on this rose leaf (Rosa) is typical of a virus
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The stunted, deformed and chlorotic leaves on this seed-grown avocado tree (Persea americana) are possibly the result of a virus.
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Note the lack of chlorophyl and deformity in the leaves of this seed-grown avocado (Persea americana), possibly caused by a virus
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Virus on birch (Betula)
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Close-up of virus on birch (Betula)
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Virus on birch (Betula)
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Suspected tobacco ringspot virus (TRSV) on astilbe (Astilbe); note bleached spots on foliage
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Suspected tobacco ringspot virus (TRSV) on astilbe (Astilbe); note circular bleached pattern on foliage
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Suspected tobacco ringspot virus (TRSV) on astilbe (Astilbe); note bleached ringspots on foliage
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Tobacco rattle virus (TRV) on peony (Paeonia). A. Phibbs, DATCP, Bugwood.org
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