Although P. fragariae var. rubi is generally considered as the most important pathogen causing raspberry root rot, pathogens such as Phytophthora idaei, P. cambivora, P. citricola, P. cryptogea and P. megasperma have also been found to infect red raspberry in Scotland. Further research is required in order to understand the origins and significance of the threat posed to the industry by these other 'minor' Phytophthora species, and indeed, other non-indigenous Phytophthora species that could be imported on Rubus material.
Of the above 'minor' Phytophthora pathogens, P. idaei is already perceived to pose a threat because there is some anecdotal evidence of losses caused by P. idaei. This has never been proven scientifically as yet hence SCRI plans specific studies in order to understand the nature of any threat it may pose, and whether or not it should be regarded as an important pathogen of raspberry. P. idaei has seldom been reported to be infecting raspberry, perhaps because it is difficult to isolate, can be misidentified as P. cactorum and because there is a lack of understanding of its potential impact on commercial raspberry production. A series of developments has, however, placed the pathogen under the spotlight. A sensitive, species-specific polymerase chain reaction (PCR)-based detection assay capable of detecting P. idaei was developed at SCRI and the application of this assay in a SEERAD-funded survey of fruiting plantations in 2001-3 demonstrated that the pathogen is widespread in Scotland. Just under 50% of fruiting raspberry plantings sampled (n=115) were infected with P. idaei (Cooke & Duncan 2004). Such a high incidence suggests that the pathogen is well adapted to the crop and able to colonise and maintain its populations under field grown conditions in Scotland.
Dramatic progress in the raspberry industry towards more intensive and high value protected cropping has focussed attention onto maximising crop yield. A pathogen such as P. idaei that has the potential to damage the root systems, which may result in significant yield loss, therefore causes serious concern. Many production systems targeting extended season production rely on pot-grown plants and any damage to their severely constrained root systems is clearly even more of a concern.
Typical raspberry cultivation in a polythene tunnel