The Psorosis B Syndrome in Citrus Is Associated with a Sequence Variant of the Citrus Psorosis Virus RNA2
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Psorosis disease of citrus is characterized by bark scaling in the stem and main branches of trees at least 10 years old. Graft-inoculation of sweet orange (Citrus sinensis (L.) Osb.) seedlings in the greenhouse with non-scaled bark pieces from an infected tree incites the psorosis A syndrome (PsA), including a shock reaction of the first flush and transient chlorotic flecking in young leaves of the following flushes, whereas inoculation with scaled bark pieces incites the psorosis B syndrome (PsB), that in addition to PsA symptoms includes gummy pustules in old leaves and branches. Psorosis disease is presumably caused by Citrus psorosis virus (CPsV), an ophiovirus with three negative-stranded genomic RNAs. While comparison of RNA1 or RNA3 fragments by single-strand conformation polymorphism (SSCP) analysis did not show any difference between plants affected by the PsA or the PsB syndromes, a fragment of RNA2 enabled discriminating both syndromes by their SSCP profile. A plant inoculated with scaled bark did not show PsB at 6 months post-inoculation (mpi) and its RNA2 gave a PsA-type SSCP profile. At 12 mpi this plant showed pustules in the trunk but not in the leaves. SSCP analysis of RNA2 showed only PsA-type variants in the leaves, whereas the pustuled trunk areas contained both PsA and PsB variants, the latter being predominant. These results suggest that psorosis-affected trees contain the two sequence variants and that the PsB variant is associated with pustules and tends to accumulate in the trunk bark.