SUMMARYThe agent of raspberry yellows disease is transmitted by grafting but not by aphids and is resistant to thermotherapy. Further studies showed that it is transmitted by inoculation of sap through seed; it is probably transmitted to plants by pollination. Raspberry bushy dwarf virus (RBDV) shares all these attributes and is known to infect all yellows‐sensitive raspberry cultivars except Puyallup and Sumner; however, neither of these cultivars has been tested by graft inoculation with RBDV. RBDV commonly infects plants symptomiessly, even those of yellows‐sensitive cultivars, but it induced yellows when inoculated either manually to Norfolk Giant raspberry or by grafting to a yellows‐sensitive raspberry selection. The evidence suggests that RBDV is the causal agent of yellows disease but that symptom expression is greatiy dependent on genetic and environmental factors. Many red raspberry cultivars are resistant, probably immune, to the type culture of RBDV and this character was shown to be conferred by a single dominant gene designated Bu.
A yellows disease of strawberry plants was identified in propagation beds in New Zealand. Affected plants were flatter to the ground, showed purpling of older leaves, reduced leaf size, yellowing of younger leaves, and sometimes plant death. A phytoplasma was observed in the phloem of affected plants. The 16S rRNA gene of the phytoplasma was amplified by polymerase chain reaction from symptomatic plants and from one asymptomatic plant, but not from 36 other asymptomatic plants. Nucleotide sequence analysis of the 16S rRNA gene showed that the phytoplasma is closely related or identical to the phytoplasma associated with the yellow leaf disease of New Zealand flax (Phormium tenax).
As a result of field observations for virus symptoms, mechanical inoculation to herbaceous test plants, and grafting to Rubus indicators, 6 distinct viruses were identified in red raspberry, and tobacco streak virus was identified in a bramble selection. Cherry leaf roll and raspberry bushy dwarf viruses and raspberry yellows disease were found commonly in all the major raspberry producing regions. Of the aphid-borne viruses, raspberry vein chlorosis was common in some regions, but black raspberry necrosis, raspberry leaf mottle, and raspberry leaf spot viruses were detected in only 7 of 43 plants indexed. The aphid Macrosiphum euphorbiae (Thomas) (a vector of black raspberry necrosis virus) was found colonising raspberry. As the incidence of infection with most of the viruses appears to be due to propagation from infected stock, the virus health status of New Zealand raspberries could readily be improved by ensuring propagation from virus-free material. However, raspberry bushy dwarf virus is spread rapidly by pollination and this virus can be controlled adequately only by growing immune cultivars.
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