VIRUS DISEASES OF RUBUS AND STRATEGIES FOR THEIR CONTROL

Over 30 virus and virus-like diseases infect Rubus species. In all instances, the number one priority for virus control is to establish plantings with virus-free material. Many aphid-borne viruses remain uncharacterized, due in part to their limited host range and in part to the limited number of people working on them. Breeding for aphid resistance has been successful in controlling the incidence and spread of aphid-borne viruses, primarily because within a geographic region, a single vector is responsible for transmitting these viruses. There are four nematodeborne viruses in Europe and three in North America that can cause serious diseases in Rubus species. Control of nematode-borne viruses was enhanced greatly with the introduction of soil fumigants to reduce nematode populations. However, as some of these chemicals are being removed from the market for environmental reasons, alternative control measures are needed. There are sources of genetic resistance to some of the nematode vectors but the use of these resistant genes has been a low priority in breeding programs. There are two pollen-borne viruses in Rubus, Raspberry bushy dwarf virus (RBDV) and Tobacco streak virus, the latter being of minor importance. RBDV has been controlled through the use of gene Bu that confers resistance to D-type isolates of RBDV. RBDV has become a serious problem in all raspberry production areas over the past 10 years due to the planting of newer cultivars that lack RBDV resistance and the presence of a new strain of RBDV that overcomes the resistance conferred by the Bu gene. With developments in molecular biology, new sources of virus resistance derived from the pathogens themselves have been developed for a large number of viruses. In Rubus, this type of resistance is being developed for RBDV in Scotland and North America and for Tomato ringspot virus in North America. With RBDV, we know that approximately 1/3 of the transgenic plants remain virus-free after grafting twice with RBDV-infected scions. These still need to be evaluated for fruit quality, yield and field resistance. At this time, it appears that genetically engineered virus resistance to the important viruses of Rubus will be developed, however, whether fruit produced from such plants will be accepted socially remains to be determined. INTRODUCTION Rather than discuss the 30 or more viruses that infect Rubus individually, the most logical and efficient way to present the information is to group the viruses based on their vectors. Control methods for viruses transmitted by the same type of vector will be quite similar despite the virus. There are three main means of transmission for viruses that infect Rubus, by nematodes, aphids and pollen. Much of the work on the viruses of Rubus has been done in Europe and North America. As a result there is little information on the viruses that occur in Rubus in other parts of the world. Several of the aphid-borne viruses of Rubus have been reported in the southern hemisphere, but not of their vector. It is important that researchers and growers in these areas not become complacent about these viruses since it is quite possible that an aphid endemic to these regions may vector one or more of the viruses. Rubus stunt and black raspberry witches broom are phytoplasmas that are transmitted by leafhoppers (Ellis et al., 1991) and will not be discussed here. The phytoplasma of greatest importance is Rubus stunt which occurs in Europe but has not Proc. 8th IS on Rubus and Ribes Eds. R.M. Brennan et al. Acta Hort. 585, ISHS 2002 266 been reported from the western hemisphere or Australasia. The vector can be controlled with insecticides and because the pathogen has a long latent period in the vector, frequent sprays are not required. For excellent photographs of the symptoms caused by viruses in Rubus readers are referred to previous publications (Converse, 1987; Ellis et al., 1991). APHID-BORNE VIRUSES Raspberry mosaic disease is caused by a complex of 2-4 viruses, each of which is transmitted by Amphorophora species. In North America, Black raspberry necrosis virus (BRNV) and Rubus yellow net virus (RYNV), vectored by A. agathonica, have been shown to cause the mosaic disease (Stace-Smith, 1956). These viruses cause the most damage in black raspberry (R. occidentalis) and only mild symptoms in red raspberry in North America, with few if any symptoms in blackberry. The Ag1 gene for resistance to Amphorophora has been very effective in controlling the aphid-borne viruses in the Pacific Northwest of North America. A new biotype able to reproduce on cultivars containing the Ag1 gene was first observed in the early 1990s but it has not emerged as a major component of the raspberry aphid populations in the area. In Europe, the disease (also referred to as raspberry veinbanding mosaic disease) is caused by a combination of RYNV and Raspberry leaf mottle virus (RLMV) (Jones, 1991). Symptoms of mosaic were intensified when BRNV or BRNV plus Raspberry leaf spot virus (RLSV) were combined with the RYNV plus RLMV combination. RLMV and RLSV, as single infections, can cause a severe chlorotic spotting, leaf distortion and stunting of plants of susceptible cultivars. The discrepancy between which viruses cause raspberry mosaic between North America and Europe may be due to environmental factors, severity of isolates used in the studies, or the presence of mixed infections in some virus cultures used in the studies. Also, in Europe there are four biotypes of the vector A. idaei that overcome different resistance genes. Until recently, plants with the A10 gene were resistant to A. idaei, but there is a new biotype that now survives on plants with this gene. To differentiate the four viruses that can be involved in the Raspberry Mosaic Complex it is necessary to transmit the viruses to indicator plants by grafting or using aphids. In black raspberry, Rubus occidentalis, RYNV shows an uneven growth of the two basal leaflets of the trifoliates, and can show a yellowing of the veins when young plants are inoculated using aphids; the symptoms are less distinctive if transmission is by grafting. Since A. idaei does not feed on R. occidentalis, the indicator R. macraei, which also develops a vein netting with RYNV infection, is used as an indicator for RYNV in Europe. BRNV, RLMV and RLSV all cause a tip necrosis in black raspberry and need to be differentiated on red raspberry cultivars (Converse, 1987; Ellis et al., 1991). RLMV causes symptoms in ‘Malling Landmark’ but not in ‘Norfolk Giant’, whereas, RLSV causes symptoms in ‘Norfolk Giant’ but not in ‘Malling Landmark’. BRNV does not cause symptoms in either of these two cultivars. Detection of these four viruses is by leaf grafting or aphid transmission. Recently, primer pairs have been developed to detect RYNV by PCR (Jones et al., 2001). Raspberry leaf curl virus (RLCV), transmitted by Aphis rubicola, is found only in North America, primarily east of the Rocky Mountains. Infected plants are severely stunted, produce very little fruit of poor quality and are susceptible to winter damage. Symptoms include distortion, some chlorosis and curling of leaves and very poor fruit set. Raspberries show symptoms of severe dwarfing and curled foliage after the initial year of infection. Detection is by leaf grafting onto wineberry (R. phoenicolasius) (Stace-Smith and Converse, 1987). Raspberry vein chlorosis virus (RVCV), transmitted by A. ideai, is apparently widespread in Europe. Symptoms include chlorosis of minor veins. The virus appears to be a rhabdovirus based on its morphology and is transmitted in a persistent and propagative manner (Jones et al., 1987). Cucumber mosaic virus has also been reported to infect Rubus but it is rare and appears to do little damage (Jones, 1987).