Phytophthora spp. produce resting oospores that can remain viable in soil for many years. Thus, replanting raspberries into contaminated soil is very likely to result in a recurrence of the disease. Oospores germinate to release motile zoospores that are readily carried in soil water and thus spread the disease within the crop. The prevention of new outbreaks can be achieved by ensuring that the planting material is free of disease, combined with avoiding fields with known or recent root rot problems, or planting in soil where raspberries have never been grown previously and the fungus is not likely to exist.
However, there is a continuing need for additional control measures. Field trials of integrated control of raspberry root rot (Dolan & Duncan 1997) drew the following conclusions.
Treatments involving ridging and mulching in a root rot trial at SCRI.
Since this work in the 1990s, a large proportion of the UK raspberry crop is now grown in a protected environment in which crop agronomy and management has changed greatly from traditional outside cropping. The move to protected cropping has not lessened the need to seek solutions to raspberry root rot, indeed it has increased the importance of the use of healthy planting stock, particularly for 'long cane' production. For more details of the high health scheme, see FruitHealth.
Because of the limited options and difficulties of chemical control for raspberry root rot, host resistance offers the most promising means of controlling the disease. Although few sources of resistance are known, a strong quantitative form of resistance is known to exist in some cultivars (e.g. Latham) and the genetics of this are being investigated at SCRI (see below). There are other potential sources of root rot resistance and if durable resistance is to be developed and effectively deployed some fundamental characteristics of the interaction need to be investigated. For example, it is not known if host resistance to P. fragariae var. rubi is also effective against other Phytophthora species, whether the resistance is durable across a variety of environmental conditions (e.g. in protected crops) and to the range of Phytophthora strains that exist both in the UK and internationally. Research in this area is intended to take place in the next few years at SCRI.
Gene mapping in raspberry is at an early stage, but preliminary work has already shown the potential of the approach to map genes underlying a number of commercially important traits. Further mapping of disease resistance genes is underway, including identifying the gene(s) responsible for resistance to raspberry root rot (Graham & Smith 2002). Four regions across 4 linkage groups have been identified to be linked to resistance to root rot, and further research aimed at confirming these in a second population in glasshouse and field trials is underway (see the Rubus genomics section) in the FruitBreeding site.