Pathogens in Raspberry and Other Rubus spp.

Abstract

Raspberry plants are susceptible to infection by a wide range of pathogens, however, whether any of these will cause significant disease in a plant at a specific location depends on many factors. Firstly, infection depends on the plant being exposed to the pathogen so that using only pathogen-tested plants to establish raspberry plantations will significantly reduce disease incidence. Some raspberry varieties carry natural resistance (or reduced susceptibility) to particular pathogens or their vectors. For example, resistance to aphid colonisation has been bred into many modern varieties. This reduces both damage due to aphid feeding and also reduces the transmission of certain viruses to the plants by the aphids. It should be noted that populations of aphids that overcome this resistance have emerged, and that the identification and deployment of sources of natural resistance needs to be an ongoing process. The application of chemical treatments to combat insects or fungi is common, though these treatments often have limited efficacy, may be overcome by development of resistance in the target organism, and are subject to environmental and consumer pressure to reduce their usage. Additionally, the introduction of new agronomic practices for the cultivation of raspberry plants can result in both an increase and a decline of infection by different pathogens. Raspberries are now widely grown under cover, in plastic tunnels, which increase the ambient temperature of the crop and extend the growing season. This can increase the multiplication rate and population size of insects in the crop, potentially increasing the prevalence of insect-transmitted viruses. Also, some raspberry crops are now planted in artificial substrate rather than into field soil. This helps to prevent their exposure to soil-borne diseases such as root rot or some viruses that are vectored by soil-borne nematodes. There is a movement towards reducing the period over which individual plants are maintained in commercial production. This reduces the time over which plants are exposed to pathogens, preventing the development of disease in the crop and also reducing the potential for the crop to become a reservoir of disease that can be spread to other plants. Lastly, changes to the environment, such as increased temperature or rainfall, and introduction of plants from other geographies or as new varieties, can lead to the emergence of new (or at least previously unrecognised) diseases. An example of this is the recent rise to prominence of raspberry leaf blotch disease caused by a complex of raspberry leaf and bud mite and raspberry leaf blotch virus.