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Pathogen Biology

The viruses that cause BYD are typically 25-28nm in diameter and hexagonal in outline (Figure 8). They are composed of two proteins (a major coat protein and a minor "readthrough" protein) that encapsulate the single-stranded ribonucleic acid (ssRNA) genome. This RNA genome serves as a messenger RNA and has five to six genes or open reading frames (ORFs) (Figure 9). Some proteins are produced directly from the ORFs of the genomic RNA, while others are expressed from shorter RNAs, called subgenomic RNAs (sgRNAs).

Figure 8

Figure 9

The viruses that cause BYD are restricted to the phloem of host plants, where they are seen via electron microscopy in the cytoplasm, nuclei and vacuoles of infected sieve elements, companion and parenchyma cells (Figure 10). Vesicles containing filaments and inclusions containing virus particles are common cytopathological effects of virus infection. The infection and subsequent death of phloem cells inhibits translocation, slows plant growth, and induces loss of chlorophyll, resulting in typical symptoms (see Symptoms).

Figure 10

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