Raspberry root rot, caused by Phytophthora fragariae var. rubi, continues to be one of the most serious diseases of raspberry (Harrison et al., 1998). Root rot diseases have always been a problem in North America but were not regarded as a problem in Europe until the 1980s when Phytophthora root rot emerged as a major problem of raspberry with outbreaks in the UK (Duncan et al., 1987), Scandinavia and Germany (Seemuller et al., 1986).

Raspberry root rot became a serious problem throughout temperate Australia during the unusually wet years of 1994–1996 with Phytophthora fragariae var. rubi (Wilcox et al., 1993) identified as the major causal agent. This disease is now the most destructive disease of raspberries. Although named as root rot, all parts of the plant below or at ground level can be infected, including roots, root buds before emergence, crowns and the bases of canes (primocane and fruiting cane). On primocanes, the lesions can spread above soil level for up to 20 cm. Affected canes die in the first year of growth or their buds fail to emerge at the start of the second growing season. Alternatively, emerged laterals wilt and die at any time from emergence until late in fruiting.

The almost simultaneous outbreaks of a new disease across Europe in traditional raspberry-growing areas (raspberries have been grown in Tayside, Scotland for more than a century) suggested that the disease had spread through the propagation network and had been distributed to farms in new planting material. Introduction of new and highly susceptible cultivars was a major factor in disease spread. Local spread of the pathogen in water running down slopes from one field into another also contributed to the problem.

Figure 1: Aerial photograph of a raspberry field affected by Phytophthora fragariae var. rubi. Plant death due to the disease is clear.

Figure 1: Aerial photograph of a raspberry field affected by Phytophthora fragariae var. rubi. Plant death due to the disease is clear.

Chemical control and management

The negligible area of raspberry crop production within the overall agro industry means that no fungicide would be developed by the agrochemical industry on the basis of its specific potential to control raspberry root rot. Fungicides developed for similar diseases of other more important crops can be tested with ‘off-label’ approval for use on raspberries. Since 1980, the search for new and better fungicides has never ceased. Metalaxy1 + copper nitrate (Ridomil Plus) was introduced after the first surge of root rot outbreaks in the mid-1980s.

Trials at the James Hutton Institute led to off-label registration promoted by the East of Scotland College of Agriculture (now the Scottish Agricultural College) (Duncan and Kennedy 1987). However, almost from the time it received approval (Heiberg 1995; Maloney et al., 1993; Wilcox et al., 1999), growers complained that the level of control was not consistent and attempts to find a more effective replacement chemical continued uninterrupted.

Oxadixyl+mancozeb (Recoil) emerged as the replacement for metalaxyl+copper nitrate. Small and large-scale field trials in the early 1990s proved its efficacy. It was quickly adopted as the industry’s fungicide of choice. Despite some complaints about lack of control when disease pressure is high, it remained in wide use throughout the UK until 2002 when Recoil was withdrawn.

Fluazinam (Shirlan) has emerged through the usual route as the best of a newer generation of materials with anti-oomycete activity. Its efficacy was proved in a large-scale field trial held on a commercial site in Scotland. A specific off-label approval (SOLA) for control of raspberry root rot using metalaxyl-M was given in the UK (2002). Phosphonate anion salts have been used in Australia since 1985 to control phytophthora root rot (Guest et al., 1995). The complex mode of action of the phosphonate anion mitigates against selection for resistant pathogens and the product is environmentally benign (Guest and Grant 1991).

Planting raspberries on ridges (hilling) is now standard practice worldwide to improve drainage and aeration, and in some soils, improves plant growth and fruit yields (Heiberg 1995; Heiberg 1999; Maloney et al., 1993; Wilcox et al., 1999). Gypsum (calcium sulphate) is used as a soil amendment to improve calcium availability and thereby improve soil structure without altering pH. Calcium has also been implicated in the regulation of the life cycle of several Phytophthora species (Jackson and Hardham 1996; von Broembsen and Deacon 1996; Xu and Morris 1998) and has improved field resistance of avocados to Phytophthora (Duvenhage et al., 1992).

An integrated control programme involving clean planting material, fungicides and host resistance are undoubtedly the most effective ways of preventing new outbreaks and controlling the severity of existing outbreaks of raspberry root rot. Controlling the health of planting material is beyond the scope of individual growers but they should ensure that their material is sound and comes from certification schemes that meet EPPO plant propagation standards.

The other elements such as choice of cultivar (in discussion with end users), ridging, mulching polythene, probably in combination with trickle irrigation, are options for growers. Other elements of the control strategy remain to be included in management regimes (for example, biocontrol agents and chemicals for improving soil structure or aeration) but it seems doubtful if any of these would give long-term and significant improvements throughout the normal life of a raspberry plantation. It seems likely that this potent disease will be managed most effectively in the future by enhanced host resistance.


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