A small replicating RNA, encapsidated with and dependent on, but not part of the viral genome, modifies disease expression depending on the host. In tomato plants, it causes a lethal necrotic disease which is probably the same as that which, in 1972, destroyed most of the field tomato crop in large regions of the French Alsace.

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CARNA 5, the small cucummber mosaic virus-dependent replicating RNA which is the causal agent of lethal tomato necrosis disease, causes a drastic reduction of disease symptoms in at least two other plant species. Satellite-like RNA's associated with plant viruses have a disease-regulating function.

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The efficacy of seedlings of Prunus persica cv. GF 305, P. persica cv. Siberian C, and P. tomentosa (Nanking Cherry) as diagnostic indicators of plum pox infection, and of P. tomentosa for other Prunus viruses was evaluated by graft-inoculation with eight different strains or isolates of plum pox virus (PPV) representative of the Marcus (M) and Dideron (D) serogroups; and one isolate each of Prunus necrotic ringspot virus (PNRSV), prune dwarf virus (PDV), and sour cherry green ring mottle virus (GRMV). The initial PPV symptoms that developed in P. tomentosa within 30 days after inoculation were chlorotic banding along the midrib spreading to lateral veins from the leaf base upward, giving the appearance of a chlorotic oak-leaf pattern. Symptoms caused by PPV-M could be distinguished from those caused by PPV-D. Virus titers in infected P. tomentosa and GF 305 were higher than those in Siberian C when measured by triple antibody sandwich enzyme-linked immunosorbent assay. Infections by PNRSV, PDV, and GRMV were evident with the first flush of vegetative growth.

Nested polymerase chain reaction (PCR) assays were evaluated for use on a routine basis in a quarantine program to detect phytoplasmas in dormant fruit tree scionwood collected during the winter season. Phytoplasmas associated with peach yellow leaf roll, Western X, apricot chlorotic leaf roll, plum leptonecrosis, and apple proliferation diseases were detected in all known infected sources. Phytoplasmas in Prunus spp. were readily detected in both dormant bud and internodal tissues. Use of nested PCR versus a single primer pair resulted in electrophoresed PCR products that were easier to interpret. The nested PCR procedure has replaced 3-year tests with grafts on sensitive indicators to detect this group of pathogens.

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