Chayote (Sechium edule) (Cucurbitaceae), also known as vegetable pear, mirliton, or mango squash, is a commercially important vegetable crop in Brazil, where it is affected by chayote witches'-broom disease. Affected plants exhibit witches'-broom growths and other symptoms characteristic of plant diseases caused by phytoplasmas. Since previous electron microscopic studies revealed the association of a phytoplasma with chayote witches'-broom, the present work was aimed at detecting and classifying the phytoplasma that may be the causal agent of the disease. Strains of a phytoplasma belonging to group 16SrIII (X-disease phytoplasma group) were discovered in chayote affected by witches'-broom disease and in diseased plants of Momordica charantia that were growing as weeds in fields of chayote in Brazil. On the basis of results from restriction fragment length polymorphism and nucleotide sequence analyses of 16S rDNA, the phytoplasma was classified in a new subgroup, designated subgroup III-J. This classification was supported by a phylogenetic tree constructed by the Neighbor-Joining method.
In a polymerase chain reaction (PCR) diagnostic test, phytoplasma (formerly
known as plant-pathogenic mycoplasma-like organism or MLO) ribosomal DNA was
detected in total DNA extracts prepared from 56 out of 63 plants collected
from geographically diverse locations across Australia. The list of
phytoplasma hosts consisted of 38 different species in 16 different families.
Restriction site analysis of the PCR-amplified DNA accessions was used to
divide the phytoplasmas into 2 groups. The majority of the tomato big bud
group and sweet potato little leaf group phytoplasmas were closely related to
a phytoplasma originally obtained from Crotalaria in
Thailand, which is a member of the faba bean phyllody strain cluster. In
contrast, phytoplasmas associated with Australian grapevine yellows and papaya
dieback were most similar to members of the aster yellows strain cluster.
Twelve phytoplasmas were compared by Southern blot hybridisation with DNA
cloned from the sweet potato little leaf phytoplasma strain V4. The
restriction fragment length polymorphism pattern of all phytoplasmas compared
was identical except for 2 sweet potato little leaf phytoplasmas.
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