Brevipalpus-transmitted plant virus and virus-like diseases: cytopathology and some recent cases

Abstract

An increasing number of diseases transmitted by Brevipalpus mite species (Acari: Tenuipalpidae) is being identified that affect economically important plants such as citrus, coffee, passion fruit, orchids, and several ornamentals. All of these diseases are characterized by localized lesions (chlorotic, green spots, or ringspots) on leaves, stems, and fruits. Virus or virus-like agents are considered to be the causal agents, possibly transmitted in a circulative-propagative manner by Brevipalpus mites. The virus or virus-like particles are short, rod-like, or bacilliform, that induce two characteristic types of cell alteration: (1) 'Nuclear type'--nuclei of parenchyma and epidermal cells in the lesions often contain a large electron lucent inclusion. Short, naked, rod-like (40-50 nm x 100-110 nm) particles may be seen in the viroplasm or nucleoplasm and in the cytoplasm. These particles are commonly arranged perpendicularly on the membranes of the nuclear envelope or endoplasmic reticulum (ER). In a very few instances, they were found to be membrane-bound, within the ER cavities. (2) 'Cytoplasmic type'--short bacilliform particles (60-70 nm x 110-120 nm) are present within the cisternae of the ER and often have electron dense viroplasm of varied shapes present in the cytoplasm. Bacilliform particles may be seen budding into the ER lumen near the viroplasm. These particles resemble those of members of the Rhabdoviridae, but are shorter. The only sequenced virus of this group, orchid fleck virus (OFV), has a negative sense (bipartite) type ssRNA genome, but its organization is similar to known rhabdoviruses, which are monopartite. Both types of cytopathological effects have been found associated with citrus leprosis. In orchids, OFV has a 'nuclear type' of cytopathology, but in some species the 'cytoplasmic type' has been found associated with ringspot symptoms. In Hibiscus and Clerodendron, green spot symptoms have been associated with the cytoplasmic type of cell alteration, while chlorotic spots, in the same species, are associated with the nuclear type. In a few cases, both types of cytopathological effects have been found in the same tissue and cell.