Phytoplasmas are single-celled organisms descended from bacteria, but they have lost the cell wall. They are considered mollicutes (“soft skin”), and able to change shape in response to their environment. Phytoplasmas are obligate parasites that only live in the phloem cells of plants or in their insect vectors. They cannot be grown in pure culture (many bacteria can be grown in culture). Due to the inability to grow them without a host, phytoplasmas are not given Latin names (genus and species). Instead, they are referred to as ‘Candidatus Phytoplasma’.

Phytoplasmas are transmitted by leafhoppers, planthoppers, or psyllids; they reproduce in their vectors. Over 1000 plant species are known to be infected by phytoplasmas, including: apples, cherries, potatoes, tomatoes, strawberries, and clovers. Phytoplasma diseases were assumed to be caused by viruses until the late 1960’s, when newer detection capabilities were developed.

Symptoms of phytoplasma infections include two main types: yellows and witches’ brooms. Yellows symptoms are due to phloem dysfunction and changes to resource allocation. Plants may have a general unhealthiness, foliage may appear chlorotic (yellow), wilting may occur, stunting of plants, dieback, and necrosis (death) of phloem may occur.

a forest with mostly deciduous trees

Ash yellows phytoplasma, thin crown and dieback, white ash, Joseph OBrien, USDA Forest Service, Bugwood.org

Witches’ brooms are due to growth regulator changes. Symptoms include proliferation of axillary shoots, bunchy top growth, swollen veins, asymmetrical flowers, sterile flowers, abnormal petal color breaking (dark and light colors), phyllody (leafy structures in place of normal flower parts), or virescence (green petals).

a stem with bunches of short green leaves

Ash yellows phytoplasma, witches’ broom, ash, Joseph OBrien, USDA Forest Service, Bugwood.org

stunted leaf growth nodules on stem

Aster yellows phytoplasma, phyllody, purple coneflower, Holly Thornton, University of Georgia, Bugwood.org

Management of phytoplasmas is accomplished by planting clean materials. Phytoplasmas are resistant to antibiotics that target cell walls (which they don’t have), but other antibiotics may be used for especially valuable plants. Heat therapy and meristem culture may be used to clean germlines. Monitoring of insect vectors and application of insecticides may reduce the spread of phytoplasmas.