Effects of abiotic factors on occurrence and development of soil-borne potato diseases
Soil abiotic components such as texture, organic matter content, pH as well as temperature and moisture greatly affect the behaviour of the pathogens and determine disease incidence or severity.
Temperature and moisture of the soil are obviously greatly dependent on the climatic conditions, but also of some cultural practices such as irrigation. Temperature is of major importance in disease development since it determines pathogen growth rate (Baljeet et al., 2005), kind of symptoms (Bouchek-Mechiche et al., 2000b) and geographical distribution of the diseases. Most of the potato pathogens can grow at soil temperatures between 10°C and 25 °C, the optim al potato growth temperatures (Table 1.2). However, gangrene, black scurf and powdery scab are favoured by mean temperatures below 15°C (Baker, 1970; Gindrat, 1984; Harrison, 1997); on the contrary, black dot, black leg, stem rot and charcoal rot are favoured by temperatures above 27°C. Similarly, sting and root-knot nematode s reproduce better between 25°C and 30°C to 35°C depending on the origin of th e populations.
Soil moisture which depends on the climate and cultural practise is also determined by the soil texture (see below). In the literature dealing with interactions between soil moisture and potato diseases, many different terms are used to characterize the water soil content.
Soil moisture content, moisture-weight percentage and water holding capacity (whc) are used to evaluate the volume of water contained in soil. It is generally expressed as a percentage of the soil’s dry weight. Other publications refer to water activity which is a dimensionless quantity (between 0 and 1) describing the amount of free water in soil for biochemical reactions. Water activity, which depends on soil texture, is related to moisture content in a non-linear relationship known as a moisture sorption isotherm curve.
High soil moisture due to abundant rainfalls, poor drainage, heavy soils or irrigation, influences disease development and the opening of the lenticels which are further entry points for soil-borne pathogens into the tuber (Helias, 2008). Several diseases, especially bacterial diseases, are enhanced by high moisture content (Table 1.2), but few diseases are favoured by low levels of moisture. This is the case for black dot, some dry rots induced by Fusarium spp., stem rot, wart, common scab, and sting and root-knot nematodes. High soil moisture generally has indirect effects which might favour disease severity. This is the case of flooding that provokes oxygen depletion and CO2 enrichment resulting in an increase of Spongospora subterranea (powdery scab) development (Harrison, 1997). In some cases, the influence of soil moisture on disease severity is not clearly demonstrated. Depending on the studies, black scurf, stem canker, silver scurf (Helminthosporium solani) and Thecaphora smut (T. solani) are either positively or negatively correlated with soil moisture (Adams et al., 1987; Hide and Firmager, 1989; Sepulveda et al., 2000; El Bakali and Martin, 2006; Wale et al., 2008). Conversely, high relative humidity during storage of tubers has always a negative impact (Table 1.2).
The soil texture described the relative percentage of sand, loam and clay contents. Most of fungal diseases are enhanced in light sandy soils (Table 1.3). Conversely, it is generally accepted that clay soils favour bacterial activity (Marshall, 1975; Alabouvette et al., 1996) explaining that clay or heavy soils are conducive to bacterial soil-borne diseases (ring rot, soft rot, brown rot and netted scab). Concerning nematodes, no general rule can be drawn up as some species are more prevalent in heavy soils (root-knot nematodes) and other species in light soils (sting nematodes). Soil texture also influences soil structure, through the distribution of different pore sizes, determining the actual living space for bacteria, fungi and predators. It also influences the water activity; water retained in pores of narrow diameter being less available for organisms that water present in big pores.
Disease development is also influenced by the soil pH linked to soil nutrient availability (Table 1.3). Soils with extreme pH values are often highly suppressive to several plant diseases (Höper and Alabouvette, 1996 ). However, pH fluctuations resulting from amendments influence pathogens and disease development. Decreasing pH increases the availability of phosphorus, nitrogen and aluminium ions and decreases potato cyst nematode, brown rot and common scab damages respectively (Mulder et al., 1997; Michel and Mew, 1998; Ruijter and Haverkort, 1999; Mizuno et al., 2003). On the contrary, addition of urea in soil induces a very large increase in pH and a good control of Synchitrium endobioticum, the fungal pathogen causing wart (Hampson, 1985).
Soil organic matter is both the substrate for and the result of microbial activity. In addition, together with clay, organic matter affects soil structure and thus moisture content and aeration. The quantity of organic matter in a soil has an effect on the appearance and the development of diseases but its quality is also an important point which has been too poorly addressed (Alabouvette et al., 1996).
Soil organic matter
Soil organic matter is both the substrate for and the result of microbial activity. In addition, together with clay, organic matter affects soil structure and thus moisture content and aeration. The quantity of organic matter in a soil has an effect on the appearance and the development of diseases but its quality is also an important point which has been too poorly addressed (Alabouvette et al., 1996). Most physico-chemical factors are not independent one from the others, which makes experiments and data interpretation very difficult. Soil texture can affect humidity, soil amendments impact on pH and all those factors influence availability of chemical elements. Thus, the pathogenic inoculum present either in the soil or on the tuber surface has to find the optimal climatic and edaphic conditions to develop.
Effects of biotic factors on the occurrence and development of soil-borne potato diseases
Autecology of pathogens
Inoculum sources, survival and dissemination pathways
The survival of soil-borne pathogens during periods without potato crop depends on their ability to resist to unfavourable conditions. Most of them survive in soil under the form of resistant structures able to directly infect the new host crop. Some pathogens can also survive as saprophytes on host crop residues or on alternative hosts during winter. Finally, inoculum can also be introduced in the field by the seeds; it is called seed-borne or tuber-borne inoculum. Inoculum sources are diverse and for any disease several inoculum sources can play a role (Table 1.4). Soil-borne fungi produce different conservation structures. Fusarium spp. forms chlamydospores resistant to adverse conditions, Rhizoctonia solani, Verticillium spp., Sclerotinia sclerotinium overwinter as sclerotia. Bacteria can survive over winter with favourable moisture, temperature and soil type (Ficke et al., 1973; Bradbury, 1977; Loria et al., 2008). Nematodes can survive and persist in soil as protective cysts surrounding the eggs (Globodera spp.) or as juveniles in host roots (Meloidogyne spp.) (Qian et al., 1996; Wharton and Worland, 2001).
In absence of resistant structures and of efficient saprophytic abilities, some pathogens need alternative hosts to survive in absence potatoes. These alternative hosts frequently belong to the Solanaceous family and act as a long term reservoir of the pathogen (Chang et al., 1992; Tomlinson et al., 2005).
Fungal dissemination occurs frequently as spores (conidiospores, chlamydospores, pycnidiospores, sporangiospores, oospores and zoospores) or mycelium transported by water (rain, irrigation, and flow in soil), by soil adhering to farm equipment or introduced by contaminated seed tubers (Zambolim et al., 1995; Stevenson et al., 2001; Bae et al., 2007). Moreover, some pathogens liberate mobile dissemination forms such as zoosporanges. Zoospores of P. erythroseptica, S. subterranea and S. endobioticum are responsible for short distance dissemination of these pathogens (Wharton et al., 2007; Merz and Falloon, 2009). Adult nematodes such as P. penetrans are able to migrate on quite long distances better than do larvae (Pudasaini et al., 2007).
Relationship between inoculum density and disease severity
Although there is not always a clear and linear relationship, the severity of the disease generally increases with an increasing level of inoculum (Table 1.4). Sometimes a minimum inoculum threshold is needed to initiate the disease development. This is the case for potato cyst nematode (Samaliev et al., 1998). Conversely, the disease severity of black dot does not increase any more beyond a maximum threshold of inoculum density (Nitzan et al., 2008). In fact as stated above, the relationship between inoculum density and disease severity greatly depends on the environmental factors which determine the level of soil suppressiveness.
Mechanisms of infection
Potato plants are essentially composed of cellulose, a very solid polymer and tubers are enveloped in a protective covering called periderm made of a suberin biopolymer providing the primary barrier against disease, insects, dehydratation, and physical intrusions for the potato tuber (Lulai, 2001). Soil-borne pathogens of potato have various ways to penetrate the host plant and break physical barriers. They enter the roots, young sprouts, underground stem, stolons or tubers. Some pathogens cannot infect intact tuber periderm or lenticels and penetrate through wounds (Stevenson et al., 2001; Taylor et al., 2004) whereas other pathogens can penetrate either directly by mechanical and/or enzymatic degradation of the host’s cells or through natural openings (stomata, lenticels, eyes) (Table 1.5).
Once they have penetrated the host, pathogens colonize plant tissues. Fungi grow through the parenchyma of the cortex and often reach the vascular vessels. T. solani, S. endobioticum and Streptomyces spp. penetration provokes hypertrophy of the colonized tissues resulting in galls. They grow in the plant, induce cell death and feed on them saprophytically. They secrete phytotoxins – for example thaxtomin produced by Streptomyces spp. – inducing the formation of several layers of suberized corky cells, creating a large lesion firmly integrated within the tuber skin (Stevenson et al., 2001; Mulder et al., 2008; Perez and Torres, 2008). Compared to common scab, powdery scab pustules formation is a relatively short process, at the end of which a single wound-cork layer remains that covers the entire lesion. After hardening off, this layer can be easily removed from the lesion without any damage of the underlying tissues (Delleman et al., 2005). C. coccodes, H. solani, P. pustulans, R. solani, S. subterranea and Streptomyces spp. are responsible for several superficial alterations called blemishes. Colonization by those pathogens is usually limited to superficial layers of tuber periderm (Harrison, 1997; Stevenson et al., 2001; Cunha and Rizzo, 2004; Lehtonen et al., 2008a; Loria et al., 2008) but they can colonize other parts of the plant until they reach vascular system. Streptomyces spp. responsible for netted scab blemishes have pathogenic mechanisms that are assumed to not implicate thaxtomin but rather a necrotic protein (Bouchek-Mechiche et al., 2006).
Table of contents :
Chapter 1 Potato soil-borne diseases – A review
Chapter 2 Review – Nomenclature and classification of potato tuber blemishes
Chapter 3 Diversity of microorganisms associated with atypical superficial blemishes of potato tubers and pathogenicity assessment
Chapter 4 Differential evolution of the structures of fungal and bacterial communities in the geocaulosphere of healthy and blemished potato tubers
Chapter 5 Genetic diversity of Rhizoctonia solani associated with potato tubers in France
Chapter 6 Effect of environmental conditions and cultural practices on the occurrence of blemishes on potato tubers