Storage diseases –Fusarium dry rot

Several storage diseases caused by both fungi and bacteria may cause significant yield losses during storage (Pringle et al., 2009). Fusarium dry rot is one of the most important storage diseases in potato tubers. The disease is caused by several fungal species in the genus Fusarium and can potentially cause significant yield losses with up to 60 percent of tubers affected. Fusarium spp. can infect almost all commonly grown cultivars (Leach and Webb, 1981; Secor and Salas, 2001). Fusarium species infect through wounds on tubers caused mainly by handling during planting, harvesting and grading (Secor and Salas, 2001).

2.4.1 Symptoms

The first symptoms of Fusarium dry rot are a sunken surface of the tubers with concentric circles (Kirk et al., 2013; Peters et al., 2008a). The colour of the rot is yellow-brown to dark-brown. Cavities can be seen inside the tuber. White, blue, pink or red coloured mycelium sometimes develops on the surface of the tubers or inside the cavity. In the beginning, the rot is V-formed towards the centre of the tuber and later the rot is spread to the whole tuber (Boyd, 1972; Kirk et al., 2013; Olofsson, 1976). Dry rot symptoms caused by F. coeruleum are shown in figure 4. Diagnosis of Fusarium dry rot can be complicated in the presence of soft rot bacteria, which often causes a secondary infection in the dry rot lesions. However, soft rot cause a wet rot that can very quickly encompass the entire tuber and mask the initial dry rot symptoms.

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Figure 4 Potato tuber with Fusarium dry rot symptoms caused by F. coeruleum (photo: Pia Heltoft).

2.4.2 Causal organisms

Several species of Fusarium can cause Fusarium dry rot development in the tubers (Boyd, 1972; Secor and Salas, 2001)) and most of these species have a wide host range including e.g.

cereals, legumes and beetroot (Peters et al., 2008b). In Great Britain and in the Nordic countries the most common species isolated from potato has been F. coeruleum (Bjor, 1978;

Olofsson, 1976; Peters et al., 2008a; Seppänen, 1983). Macro- and chlamydospores of F.

coeruleum are shown in figure 5. F. sambucinum is also an important species and is considered to be the most significant causal agent of Fusarium dry rot in other parts of Europe, in northern and western China and in North America (Du et al., 2012; Secor and Salas, 2001).

Other important species includes F. avenaceum (Du et al., 2012; Peters et al., 2008a), F.

graminearum (Estrada Jr et al., 2010) and F. oxysporum (Gachango et al., 2012)

Identification of Fusarium species can be done based on conidial morphology, production of chlamydospores, growth characteristics, and colony pigmentation (Gerlach and Nirenberg, 1982; Leslie and Summerell, 2006) or by using molecular methods. Real-time PCR assays providing fast identification and quantification of Fusarium spp. can be used to detect latent infections in tubers pre-storage, to validate their storability and/or suitability as seed potatoes (Cullen et al., 2005; Halstensen et al., 2006; Nicholson et al., 1998).

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Figure 5 Macro- and chlamydospores of F. coeruleum (photo: Pia Heltoft).

2.4.3 Disease cycle

Fusarium spp. are spread with contaminated seed tubers and soil (Adams and Lapwood, 1983;

Jeger et al., 1996; Secor and Salas, 2001). Adams and Lapwood (1983) investigated the transmission of inoculum in the field, and found that F. sambucinum and F. coeruleum were transmitted from seed to progeny tubers. Another study (Leach, 1985) found that seed inoculated with F. sambucinum resulted in high levels of Fusarium dry rot in progeny tubers, whilst naturally occurring low levels of F. coeruleum in soil resulted in relatively less severe dry rot symptoms. Wounds caused during harvest and by other potato tuber handling operations serve as entry points for the Fusarium spores. Once the pathogen has penetrated the tuber skin, it begins to grow in the tubers tissue causing dry rot lesions at the point of entry (Kirk et al., 2013; Secor and Salas, 2001). Fusarium dry rot develops most rapidly at high relative humidity and temperatures of 15-20 °C. There is slower growth of Fusarium at lower temperatures but it can however continue its growth at the lowest temperature safe for storing potatoes (Secor and Salas, 2001).

2.4.4 Disease management

Present control strategies for Fusarium dry rot includes use of resistant cultivars and cultural practices such as crop rotation, use of disease free seed and wound healing prior to storage.

As Fusarium spp. can only infect through wounds, avoiding injuries to tubers and providing conditions that promote wound healing are the most important management factors.

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Biological control agents and ultraviolet radiation are also used, as well as chemical control (Al-Mughrabi et al., 2013; Bojanowski et al., 2013; Bång, 1992; Gachango et al., 2012; Peters et al., 2008a; Ranganna et al., 1997; Secor and Salas, 2001). However, biological and chemical control methods are not commonly used targeted against Fusarium dry rot in Norway.

Integrated Pest Management (IPM) have received increased focus the last years. IPM is a sustainable approach to managing pests by combining biological, cultural and chemical tools in a way that minimises economic, environmental and health risks (Barzman et al., 2015).

Cultivar resistance is a key element in the IPM strategies for control of Fusarium dry rot.

Cultivars vary in their resistance to Fusarium spp. even though none of the potato cultivars have yet been found to be fully resistant to the whole Fusarium complex (Corsini and Pavek, 1986; Esfahani, 2005; Lees et al., 1998; Peters et al., 2008a; Wastie et al., 1989).

Controlling Fusarium dry rot can be challenging particularly with immature tubers, which often occurs at harvest after a short growing season. Immature tubers may be more susceptible to Fusarium dry rot. Boyd (1967) found higher infection rates in immature tubers and increased resistance with tuber maturation. He also concluded that susceptibility to F.

coeruleum in immature tubers was closely related to the higher content of sucrose. Carnegie et al. (2001) reported that harvest date was an important factor affecting dry rot development of F. coeruleum.