Few streptomycetes are capable of infecting plant tissues and causing disease. The most notable, in terms of their agronomic importance are the Streptomyces
species able to cause common scab of potatoes. The causative agent has been described as Streptomyces scabies (Lambert & Loria, 1989). Current knowledge of plant pathogenic streptomycetes is given in Table 1.
Common scab is an important disease in potato growing regions throughout the world (Davies & Gamer, 1978) affecting all underground parts of the potato plant. Infection results in superficial corky lesions on the tuber surface and an unpalatable tasting potato flesh as a result of geosmin production by streptomycetes.
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Table 1
Summary of current knowledge of plant pathogenic streptomycetes Species
S.scabies
Host Plant Potato, Sugar beet Carrot and others
Disease Common scab (severe) Reference Hoffman, 1958 Lapwood, 1973 S.griseus S.aureofaciens S.flaviolus
Potato Common scab (mosdy mild)
Hoffman, 1958 Corbaz, 1964
Streptomyces sp. Potato Common scab
in acid soils
Bonde & McIntyre, 1968
Streptomyces sp. Potato Russet scab Harrison, 1962
SJpomea Sweet potato Soil rot & pits Person &
Martin, 1962 (Taken from Kutzner, 1981)
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Various types of scab have been described according to the severity and type of scab lesions these include, pitted and raised scab (Jones, 1931), ordinary, superficial, deep and elevated scab (Emillisson & Gustafasson, 1953), russet scab (Harrison, 1962) and netted scab (Scholte & Labruyere, 1985).
Common scab is most prevalent in soils that promote optimal growth conditions for streptomycetes, i.e. sandy, calciferous soils that are well drained and neutral to alkaline in pH. A number of agronomic practices have been employed to minimise the incidence of the disease in scab infested soils (Labruyere, 1971). These include the planting of resistant potato varieties, ensuring the crop is well irrigated particularly during the period of tuber initiation and expansion (Lapwood & Lewis, 1967; Lapwood & Hering, 1970) and reducing the pH of the soil through the application of green manures. Tubers initiate as a swelling of an underground stem (stolon) as a tuber grows, nodes (eyes) separate from the apical bud as intemode tissue forms between them. When first formed the tuber intemodes have stomata, but as the tissue expands the stomata are transformed into lenticels (Fellows, 1926). The stoma guard cells are gradually raised above the tuber surface, by division of cells beneath them, further cell division and expansion causes the stoma to rupture and expose the inner cells with the formation of a young lenticel. If the soil is wet, cells proliferate from the lenticel opening, however, if it is dry cell division stops and suberin is deposited on the outermost cell walls (Lapwood, 1973). Initiating potato tubers pass through a phase of susceptibility to infection by scab-causing streptomycetes, corresponding to a specific developmental stage of the lenticels. Susceptible lenticels appear to be newly formed with incompletely suberised filling cells, each lenticel is thought to be susceptible for about seven days (Adams, 1975). Hooker & Page (1960)
demonstrated that suberized periderm could not be breached by pathogenic
S.scabies isolates even after autoclaving. Lenticel development appears
comparable for both scab-resistant and susceptible varieties (Adams, 1975). In wet soils prevention of infection has been attributed to the antagonistic and competitive effects of other soil microorganisms (Lewis, 1962 ; Adams & Lapwood, 1978). However, cell proliferation under wet conditions would displace freshly colonised tissue and may also be involved (Labruyere, 1971).
1.4.1 The ecology of scab-causing streptomycetes
Few attempts have been made to study the ecology of scab-causing streptomycetes and much of the early work is contradictory with some workers finding that the incidence of scab is influenced by the pattern of crops grown between the potatoes and others not. Goss & Afansiev (1938) found that common scab was most severe at sites with short rotations or those incorporating sugar beet. Most workers appear have found that the incidence of common scab was often associated with short rotations or continual cropping with potatoes. Hooker (1956) also found that the common scab infections were most severe in peat soils with short rotations and or a continuous potato crop than by the inclusion of other crops in the rotation. Hooker (1956) was unable to correlate plate counts of streptomycete populations with the incidence of scab. Counts appeared highest in plots cropped to com, followed by potatoes and lastly onions and soybean. Fallow plots exhibited the lowest counts.
Russet scab (or netted scab) caused by Streptomyces sp was identified as a disease associated with short rotations (Scholte et al., 1985).
Keniath & Loria (1989) have investigated the population dynamics of melanin
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producing Streptomyces, a group that has been identified to include many of the scab-causing strains. Population dynamics of the melanin producing group was studied in response to the growth of different potato cultivars. Streptomycete populations isolated from field plots cropped to potatoes were more diverse than those from the control fallow plots. Differences in population diversity were also seen amongst the streptomycetes colonising the potato rhizosphere and tuber surface, indicating that conditions for streptomycete growth differed in both environments. Qualitative and quantitative differences in streptomycete populations were seen with time, probably as a result of the development of plants and seasonal changes.
The authors also demonstrated differences in the relative numbers of different streptomycete species groups in the rhizosphere o f the scab susceptible potato cultivar, ‘Chippewa’ and the scab resistant cultivar, ‘Superior’.
In a further investigation Keniath & Loria (1990) studied the influence of the two potato cultivars on the melanin producing streptomycete populations. Populations were monitored in plots planted with the scab susceptible variety, ‘Chippewa’ and the scab resistant variety, ‘Superior’ and also in fallow soil, over a three year period. In general, populations of streptomycetes in soils planted with potatoes increased in the soil, the rhizosphere and on the tuber surface throughout the growing season. While populations in the fallow soil remained stable or declined. No significant differences were observed between the population levels of streptomycetes isolated from the soil and rhizosphere samples from plots planted with both potato varieties. However, the population levels on the tuber surface of the scab susceptible cultivar were significantly higher than those on the scab resistant cultivar. Melanin positive streptomycete population densities in the soil corresponded well with scab
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incidence and severity. The authors suggest that this correlation may reflect actinomycète reproduction on the tuber surfaces. As the population levels of actinomycètes as opposed to melanin positive streptomycetes also correlated well with the incidence of scab. Less than 6% of the melanin positive isolates appeared to be common scab pathogens. This finding illustrates the need for selective and distinct markers for pathogenic strains in order to study the ecology and epidemiology of pathogenic strains.