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Keywords:

  • inoculum level;
  • potato powdery scab;
  • root galling;
  • soil moisture;
  • soil type;
  • temperature

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The incidence and severity of root infection and root galling caused by Spongospora subterranea were assessed in potato plants (cv. Estima) grown under controlled environmental conditions. The effects of temperature, soil type, soil moisture regime and soil inoculum level on infection and root gall development were determined by molecular and visual methods at two plant growth stages. Root gall severity was scored at harvest, after which DNA was extracted from the roots and quantified in a real-time polymerase chain reaction (PCR) assay specific for S. subterranea. Root galling was severe at 17°C, with a disease score of 3·1 on a 0–4 scale, low (0·6) at 12°C, and did not occur at 9°C. The level of inoculum in soil, in the form of artificially added sporosori, had no effect on the incidence and severity of visual symptoms, with 21%, 41% and 33% incidence observed at 5, 15 and 50 sporosori g−1 soil, respectively. Incidence of infection, as detected by the real-time PCR assay, was greater with increasing soil inoculum concentrations, ranging from 48% at 5 sporosori g−1 to 59% (15 sporosori g−1) and 73% (50 sporosori g−1) of plants infected at maturity, but this effect was not statistically significant. No correlation was found between the occurrence of galls on roots and powdery scab on tubers of the same plants.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Infection of potato by the plasmodiophoromycete Spongospora subterranea is most commonly associated with powdery scab symptoms on tubers. Spongospora subterranea is also the vector of Potato mop-top virus (PMTV), one of the causes of spraing (Jones & Harrison, 1969).

Powdery scab is an important disease worldwide and causes extensive losses in both seed and ware potato crops (Wale, 2000). When mature, the tuber scabs form powdery masses of thick-walled resting structures called sporosori (also known as ‘cystosori’ or ‘sporeballs’) which contain large numbers of resting spores and are an important source of inoculum since they are very persistent and may survive in soil for decades, making powdery scab a difficult disease to control (de Boer, 2000). Under moist conditions, resting spores release primary zoospores that can infect potato and a wide range of other plant hosts (Kole, 1954; Jones & Harrison, 1969; Iftikhar & Ahmad, 2005). In addition to the tubers, zoospores of S. subterranea may also infect the roots of potato, eventually developing into either zoosporangia, which release secondary zoospores, or sporogenous plasmodia. The development of the latter is often associated with hyperplasia and hypertrophy of the root cells, resulting in the formation of galls. In common with scabs on tubers, mature root galls form powdery masses of sporosori that are released into the soil.

Low levels of soil inoculum have been shown to result in significant powdery scab symptom development under favourable environmental conditions (Burnett 1991; van de Graaf et al., 2005). However, it is reasonable to assume that, in the field under suitable conditions, higher inoculum concentrations may result in more disease because of the increased likelihood of the pathogen being in contact with the host. Root galls may therefore have an important role in terms of increasing soil inoculum levels and therefore the risk of disease. This is of particular concern where cultivars with a high susceptibility to galling in combination with a low susceptibility to tuber symptom development are successively cropped, leading to an unseen build-up of inoculum (de Boer, 2000).

Although root infection of tomato bait plants in nutrient solution is widely used as a diagnostic technique (Merz, 1989; van de Graaf et al., 2003), studies of the development of root symptoms in potato caused by S. subterranea have been scarce. Merz et al. (2004) developed a laboratory bioassay for the rapid screening of potato cultivars for resistance to powdery scab and showed that zoosporangial root infection correlated better than root galling with ranked tuber disease data. There is also apparently no direct relationship between field resistance to powdery scab and root gall symptoms (Christ, 2001; Schwärzel, 2002). Potato varieties are generally only tested for resistance to tuber symptoms and not to root infection or galling, despite indications that root infection could be an important factor in disease epidemiology (Falloon et al., 2003).

Whilst the effects of soil inoculum level and environmental factors on tuber infection, and the development of tuber symptoms, have been described extensively (de Boer et al., 1985; van de Graaf et al., 2005), this is not the case for potato root infection and the development of root galls caused by S. subterranea. Previous work on tomato showed that the optimum temperature for root hair infection by S. subterranea is about 16–17°C and that the minimum and maximum temperatures are < 11°C and 22–25°C, respectively (Kole, 1954). It has been suggested that zoospores of S. subterranea are active, and may be able to infect hosts, at temperatures substantially below 11°C (Harrison et al., 1997; Navia & García, 2005), but there are very few published data on the conditions favouring root infection in potato. Even less is known about the circumstances under which root galls are formed and the frequency with which they concur with powdery scab on the tubers of the same plant.

A real-time PCR assay, which can detect different stages in the life cycle of S. subterranea, was developed and offers a quick, reliable and specific method for the detection and quantification of DNA of the pathogen in host tissues (van de Graaf et al., 2003; de Haan & van den Bovenkamp, 2005). It was therefore used in this study as a tool to investigate the incidence and development of potato root infection by S. subterranea under different controlled conditions.

The objective of this work was to determine the effect of soil inoculum level and several environmental factors on the infection of potato roots by S. subterranea and on subsequent root gall development. The relationship between root gall and powdery scab development in the same plant was also investigated. These studies aimed to result in a better understanding of the role of root galls in the biology of S. subterranea and thus to increase the knowledge that could lead to an improvement in disease prevention and control.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Preparation of inoculum and soils

Three different types of soil, loamy sand, sandy loam and silty clay (hereafter referred to as sand, loam and clay, respectively), were collected in Aberdeenshire, Scotland, air-dried, passed through a 1-cm-mesh sieve and stored at room temperature. Soil moisture curves were established for the sieved soils using standard water-column and pressure-plate techniques.

Potato tubers with powdery scab symptoms were harvested in Aberdeenshire and stored in the dark at 4°C at SCRI. Tubers were washed with tap water and left to dry overnight at room temperature. Scabs containing sporosori were then scraped off the tuber surface with a scalpel and ground gently with a pestle and mortar to separate sporosori from plant material. The resulting powder was passed through a 53-µm-mesh sieve and suspended in sterile distilled water. The concentration of sporosori was established microscopically using a haemocytometer.

The three soils were moistened with distilled water containing different numbers of sporosori, resulting in final inoculum concentrations of 5, 15 or 50 sporosori per g soil. Control treatments were moistened with water that did not contain inoculum and the amount of water added to each soil was calculated to bring the soil moisture level to –1 bar (damp). The water was blended into the soil thoroughly by hand and the moist soil (2·5 L) distributed into plastic plant pots (3 L). One non-sprouted seed-potato tuber of the powdery scab-susceptible cv. Estima was planted in each pot and placed in a growth room with a 16-h light regime at a constant temperature of 12°C. The pots were watered regularly to maintain a constant soil moisture level.

An additional soil moisture regime was applied to the loam soil only. The soil was saturated at the start of the trial, left to dry for 10 days and then saturated again. This procedure was repeated until the end of the experiment. All treatments with loam soil were incubated at 9, 12 and 17°C.

Plants were harvested at two growth stages (tuber initiation and maturity). As the time of tuber initiation is affected by temperature, plants were harvested at different times, i.e. 2, 3 and 4 months after planting for plants grown at 17, 12 and 9°C, respectively. Mature plants were harvested after senescence, which was similarly staggered according to temperature treatment. There were five replicates per treatment for each growth stage.

An overview of all treatments and replicates is given in Table 1.

Table 1.  Overview of Spongospora subterranea treatments and number of replicates used in trials on potato cv. Estima
Soil typeTemperature (°C)Moisture regimeSoil inoculum levels (sporosori per g soil)Growth stagesReplicatesTotal no. of tubers planted
Sand12Constant0, 5, 15, 50Tuber initiation, maturity51 × 1 × 1 × 4 × 2 × 5 = 40
Clay12Constant0, 5, 15, 50Tuber initiation, maturity51 × 1 × 1 × 4 × 2 × 5 = 40
Loam 9, 12, 17Constant, fluctuating0, 5, 15, 50Tuber initiation, maturity51 × 3 × 2 × 4 × 2 × 5 = 240

Visual disease assessment

At each harvest date, the roots of each plant were removed from the soil, washed, visually assessed for symptoms of root galling, air-dried at room temperature and stored in the freezer before DNA extraction. Root gall severity was scored using the root gall scoring scale as agreed by European powdery scab researchers, where 0 = no root galls, 1 = one or two root galls, 2 = several galls, mostly small (< 2 mm in diameter), 3 = many galls, some > 2 mm in diameter, and 4 = most major roots with galls, some or all > 4 mm in diameter. (see http://www.spongospora.ethz.ch/LaFretaz/scoringtablegalls.htm.)

The tubers belonging to each plant were also harvested, washed and visually assessed for powdery scab symptoms on the basis of the percentage tuber surface area covered with powdery scab, using the scale published by Merz (2000), which ranges from 1 (no symptoms) to 7 ( > 75% covered with powdery scab). Scores were determined for each individual tuber and then averaged per plant.

DNA extraction

DNA was extracted from roots using a Nucleon® PhytoPure plant DNA extraction kit (Tepnel Life Sciences). In each case, the plant tissue samples were cut into small pieces and 0·1-g samples were taken at random for extraction. Where the total dry weight of the sample was less than 0·1 g all the available material was used. Each root sample was ground in 600 µL of reagent 1 of the kit. The DNA extraction kit protocol was then followed.

Real-time PCR amplification

The root DNA samples were amplified in a real-time PCR assay as described by van de Graaf et al. (2003). A range of standards containing known amounts of S. subterranea DNA was included in the real-time PCR assay. DNA was extracted from a known number of sporosori according to the method of Bell et al. (1999) and diluted with TE buffer to obtain a dilution series of DNA equivalent to 10 000, 10 000, 1000, 100, 25 and 10 sporosori mL−1. Using a standard curve of the threshold cycle (Ct) values against the logarithm of the amount of sporosori in the dilution series, the amount of S. subterranea DNA in the unknown samples was expressed in sporosori equivalents (hereafter referred to as ‘units’) based on their Ct values (van de Graaf et al., 2003). A non-template control containing 1 µL TE buffer instead of DNA was included in every assay and all samples were tested in duplicate and then averaged. Scores were adjusted for the amount of root tissue (mg dry weight) used for the DNA extraction. Plants were assumed to have roots infected with S. subterranea if the DNA sample tested positive ( ≥ 3 units) in the real-time PCR assay.

Data analysis

Results were analysed using Friedman two-way anova, Mann-Whitney U tests, chi-squared tests and Fisher's exact tests (α = 0·05).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Effect of soil inoculum level

The effect of soil inoculum level on root infection and root galling was assessed in a loam, a sandy and a clay soil. Inoculum level did not affect infection or disease symptoms in roots (Table 2).

Table 2.  Overall effect of the level of Spongospora subterranea inoculum in soil on root infection and root gall development in potato cv. Estima at two different growth stages. Results are averages of different treatments
Growth stageSoil inoculum concentration (sporosori g−1)nPlants with DNA detected in roots (%)aAverage amount of DNA (units mg−1 root tissue)b (±S.E.)aPlants with root galls (%)aAverage root gall severityc (±S.E.)aPlants with symptomless infected roots (%)a
  • a

    Results in the same column not sharing a common letter are significantly different (α = 0·05).

  • b

    One unit equals the amount of DNA extracted from a single sporosorus of average size.

  • c

    On a scale of 0 = ‘no root galls’ to 4 = ‘most major roots with galls’ (see Materials and methods).

Tuber initiation 03832 ab7 ± 6 ab 0 a0 ± 0 a32 ab
 53664 cd24 ± 12 c11 bc0·19 ± 0·10 bc56 cd
153479 de118 ± 47 cd18 c0·38 ± 0·17 cd65 d
502588 e185 ± 83 d16 bc0·48 ± 0·23 cde72 d
Maturity 03722 a0·3 ± 0·1 a 3 ab0·05 ± 0·05 ab22 a
 53148 bc527 ± 516 bcd19 c0·42 ± 0·17 cde32 abc
153259 cd705 ± 688 bcd41 d0·94 ± 0·23 e31 ab
502673 cde419 ± 304 d31 cd0·85 ± 0·29 de50 bcd

Detection of S. subterranea DNA in the potato root samples by real-time PCR showed that the pathogen infected the majority of plants grown in artificially infested soil, e.g. 88% and 73% of plants at the greatest inoculum concentration at tuber initiation and maturity, respectively. Variation between replicates in the amount of pathogen DNA detected in roots, particularly at maturity, was reflected by large standard errors (Table 2). Spongospora subterranea DNA was also detected in 27% of the plants grown in the unamended soil, but the amount of DNA detected was significantly less than in the plants grown in artificially infested soil (Table 2).

Visible root symptoms, in the form of galls, were not common, and at both growth stages the majority of plants remained symptomless. Symptomless root infections were detected at all inoculum levels.

At all inoculum levels, the presence of symptomless infections was less at maturity than at tuber initiation, and this effect was statistically significant at the two highest levels of soil inoculum (15 and 50 sporosori g−1) (Table 2). At tuber initiation, root galls did not occur in the control plants, and only one mature control plant showed slight root galling. The incidence and severity of root disease were always significantly less in the control than in any of the inoculated treatments (Table 2).

Since the three levels of inoculum resulted in few statistically significant differences in infection and disease levels in mature plants, the results of the treatments with added inoculum were averaged in the analysis of the effect of the environmental factors given below.

Effect of soil moisture level

The effect of soil moisture regime on root infection and root galling was investigated in loam soil only. The results for different inoculum levels and temperatures were combined.

As a result of variation between replicates, there was no statistically significant difference in the incidence and severity of root galling in the loam soil kept at constant dampness compared with that under the fluctuating moisture regime. The percentage of plants with symptomless root infections was exactly the same for both soil moisture regimes.

Fewer observations were possible under the fluctuating moisture regime than for soil at constant dampness, since some of the mother tubers planted in soil with fluctuating wetness did not sprout because of fusarium dry rot.

Effect of temperature

The effect of temperature on root infection and root galling was investigated in loam soil. Combined results from different inoculum concentrations and soil moisture levels showed a clear effect of temperature on both the infection of potato roots by S. subterranea and the formation of galls. The percentage of plants with infected roots increased with increasing temperature from 38% at 9°C to 53 and 86% at 12 and 17°C, respectively (Fig. 1a). There was also a statistically significant effect of temperature on disease incidence and severity. All plants grown at 17°C formed root galls, compared with 33% at 12°C and none at 9°C. As a result, no symptomless infected plants were present at 17°C whilst about 40% of plants grown at 9 and 12°C were infected without showing galls (Fig. 1a). In addition, the severity of root galling at 17°C was significantly greater than that observed at 12°C.

image

Figure 1. Mean incidence and severity of root infection and root galling by Spongospora subterranea, and amounts of DNA, in inoculated mature potato cv. Estima plants grown in (a) damp loam soil at three different temperatures (α = 0·05) (n = 14, 15 and 14, respectively); (b) three different soils at 12°C (α = 0·05) (n = 14, 15 and 14, respectively); and (c) artificially infested soils, measured at two different growth stages (α = 0·05) (n = 95 and 89, respectively). Results for different inoculum levels are combined. Differences between grouped columns with different letters are statistically significant.

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In light of the significant effect of temperature on root galling, the data relating to the effect of inoculum concentration on root infection and root galling at 17°C only were re-analysed (data not shown). This analysis showed that at 17°C the average percentage of plants with root galls and the average root gall severity increased when other data were excluded, but that the relative effect of inoculum level was the same as in the pooled data shown in Table 2.

Effect of soil type

The effect of soil type on root infection and galling was studied at 12°C at constant dampness. The results for different inoculum concentrations were combined.

Soil type did not affect the percentage of plants grown in artificially infested soil having infected roots, or the amount of S. subterranea DNA detected in infected root tissue. However, despite the relatively low numbers of galls seen at 12°C, the incidence and severity of root galling was significantly less in plants grown in the clay soil than in those grown in the sand or loam soils. None of the plants grown in the clay soil formed root galls despite a high incidence of root infection (79%). The level of symptomless root infection was significantly greater (approximately double) in the clay soil than in the sand and loam soils (Fig. 1b).

Disease development over time

Spongospora subterranea infected the roots of most potato plants at an early stage of development: DNA of the pathogen was detected in 76% of root samples harvested at tuber initiation. Although the average amount of S. subterranea DNA detected per mg root tissue increased during plant growth, the incidence of root infection in mature plants, as compared to plants tested at tuber initiation, decreased (Fig. 1c).

Some plants (15%) had already formed root galls at tuber initiation, but both the percentage of plants with root galls and the average disease severity were greater at maturity than at tuber initiation. In the time between tuber initiation and maturity, both disease incidence and severity of root galling almost doubled (Fig. 1c).

Symptomless root infections were especially widespread at tuber initiation, when root galling was not yet common. At maturity, 35% of infected plants did not show root gall symptoms, even though DNA of S. subterranea was detected in the roots (Fig. 1c). In contrast, in some cases root galls developed where root infection had not been previously detected, probably because of the small sample size of roots analysed. These ‘false negative’ results accounted for inconsistencies in the results, for example where the total number of plants with infected roots did not equal the sum of those with and without symptoms.

Relationship between root and tuber symptoms

At tuber initiation, the incidence of powdery scab symptoms on tubers of plants that had root infections, or galling, was not different from tuber disease levels in the group without infected roots. However, at maturity, there was a statistically significant effect of the presence of infected roots or root galls on powdery scab incidence on tubers (Fig. 2a). Nevertheless, powdery scab was observed on tubers of almost 40% of the plants in which no root infection was detected and in a similar percentage of plants that had not formed root galls (Fig. 2a).

image

Figure 2. Relationship between the absence (white columns) or presence (grey columns) of (a) root infection with Spongospora subterranea or root galling on the percentage of plants with powdery scab symptoms and (b) powdery scab on potato tubers on the percentage of plants with root infection or root gall symptoms, assessed in mature potato cv. Estima plants at two different growth stages. Results are the average of different growing conditions. Differences between twinned columns with different letters are significant (α = 0.05).

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Differences in the percentage of root infection or galling, between the group of plants that showed powdery scab tuber symptoms and the group that did not, were found in mature plants (Fig. 2b). The majority of plants with powdery scab symptoms on tubers did not form root galls (78 and 65% at tuber initiation and maturity, respectively), while some plants without powdery scab did form root galls (8 and 12%, respectively) (Fig. 2b). The average root gall severity did not differ significantly between plants having tubers with and without powdery scab symptoms (data not shown).

No correlation was found between the presence of powdery scab on tubers and root galls in the same plant, but all plants with an average tuber powdery scab severity score > 4 formed root galls.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The level of S. subterranea DNA in soil (at the same, relatively low inoculum levels as described here) is known to have no effect on tuber infection and powdery scab severity on potato tubers (van de Graaf et al., 2005). The same has now been demonstrated for root infection and galling. As is the case with tubers (van de Graaf et al., 2005), infection of potato roots by S. subterranea was found to be common, even at low soil inoculum levels. This is in agreement with the results of Burnett (1991), who found no significant difference in potato root infection levels between inoculum levels of 3, 33 and 333 sporosori g−1 soil. This is especially important in view of the fact that infected roots may form secondary zoospores, a key aspect of disease epidemiology. The release of large numbers of secondary zoospores from infected roots can greatly reduce the importance of initial inoculum level in soil (Burnett, 1991).

The fact that root gall symptoms are more common at higher temperatures (17°C) has not been previously reported, and is in contrast with the optimum temperature for the development of powdery scab, which is approximately 12°C (de Boer et al., 1985; van de Graaf et al., 2005). Whilst powdery scab can still be severe at low temperatures (9°C) (van de Graaf et al., 2005), no root galls were found to occur below 12°C in the present study. This shows that the effect of temperature on the development of root galls is much stronger than on the development of powdery scab symptoms on tubers. Root infection was detected at 9°C, but the amounts of DNA measured were very small, especially compared with those found at 17°C, although this effect was not statistically significant. This supports the work by Kole (1954), who found that the optimum temperature for the infection of tomato root hairs by S. subterranea was 16–17°C.

The effects of soil moisture regime and soil type on root infection and symptom development were similar to those reported for tuber infection and powdery scab development (van de Graaf et al., 2005). Fewer root galls were formed by plants grown in the clay soil than by those grown in the sand and loam soils, but there was no significant difference in the percentage of plants with infected roots. This contrasts with results from Australian studies in which infection of potato root hairs was more widespread in clay than sand (de Boer, 2000). Low oxygen levels in clay soil, with its small pores, could restrict germination and zoospore movement, but cannot explain the high percentage of latent root infections found in this soil type.

In the trials reported here, root infection and disease levels were both greater in plants grown in soil kept at constant dampness than in plants grown in soil with fluctuating wetness, but unlike the results for tuber symptoms (van de Graaf et al., 2005) this was not a significant effect. Jellis et al. (1987) also failed to find an effect of varying soil moisture content on the number of zoosporangia found in potato roots. The reason for this could be that the plant roots are more spread out within the soil than the tubers and thus, under dry conditions, more likely to be into contact with areas of residual moisture.

Although potato tubers become resistant to infection by S. subterranea after skin set, roots remain susceptible throughout the growing season (Hughes, 1980). Nevertheless, Jellis et al. (1987) found that the number of mature zoosporangia in root hairs decreased during the season, which could be explained by an increase in the number of zoosporangia that had released zoospores. An increase in zoospore release could account for the fact that S. subterranea DNA was detected more widely in the roots of plants at tuber initiation than at maturity. Although the percentage of plants with infected roots decreased, the amount of DNA of the pathogen detected in the positive samples increased over time. This increase could have been the result of the contribution of pathogen DNA from plasmodia, and from sporosori in developing root galls, which only occurred in a limited number of samples (mainly at 17°C). No data are available in the literature on root gall formation over time, but in the trials here, the development of root gall symptoms continued during plant growth, and the incidence and severity of root galling were both greater at maturity than at tuber initiation.

As explained above, the effect of temperature is different for root infection and gall formation than for tuber infection and powdery scab development, and the infection period for tubers is much shorter than that for roots. It is therefore not surprising that no clear link was found between the incidence of tuber and root infection and symptoms in potato cv. Estima in this trial. There was also no relationship between root gall severity and powdery scab severity, even in plants that developed both disease symptoms.

Published data suggest that powdery scab and root gall susceptibility are correlated in some potato cultivars, but not in all (Hughes, 1980; Christ, 2001; Schwärzel, 2002; Merz et al., 2004). Root infection level, measured visually in plants grown under controlled conditions, has been reported to correlate more accurately with field resistance to powdery scab than root gall severity, and almost all cultivars tested have been found to be susceptible to root infection, although they do not always form root galls (Nakayama et al., 2002; Falloon et al., 2003; Merz et al., 2004). Accurate data on infection levels in tubers (latent and symptomatic) of the same varieties are not available and whether there is a correlation between root and tuber infection in potato cultivars other than Estima remains unknown.

Differences in the ecology of infection of potato roots and tubers by S. subterranea mean that it is unlikely that root infection or galling could be used as a reliable indicator for susceptibility to powdery scab. If the control of root gall development is ignored, the level of S. subterranea inoculum in the soil could still increase during the cultivation of a potato crop, even when there is no powdery scab development (de Boer, 2000; Falloon et al., 2003). This may cause problems when cultivars susceptible to powdery scab are subsequently grown in the same field, especially since it is known that under favourable environmental conditions, the number of S. subterranea sporosori in soil does not need to be great to result in severe tuber symptoms (van de Graaf et al., 2005). Severe cases of root infection or galling can also have a direct effect on plant growth, leading to wilting and yield reduction (Wale, 2000; Falloon et al., 2005).

There was a large amount of experimental variation between replicates, which may explain the lack of statistical significance between some treatments (for example, the soil moisture treatments, where trends towards increasing root infection, DNA concentration, incidence and severity of root galling were seen under conditions of constant dampness, but were not statistically significant; data not shown). This was a particular problem of working with S. subterranea. It was not thought that this was caused by variation in the efficiency of DNA extraction from plant tissue as this has not been the case with other pathogens, where the DNA extraction and PCR amplification methods used here have proved to be consistent (Cullen et al., 2001; Lees et al., 2002). Rather, the infection of roots and the multiplication of the pathogen within roots appear to be very variable, making this a particularly difficult pathogen with which to work. Indeed, Harrison et al. (1997) noted that because of the difficulties in working with this obligate pathogen, research on the epidemiology of powdery scab has progressed slowly. In addition, disease-resistance screening for powdery scab has typically been carried out with more (6–7) replicates than other disease tests (Lees, 2000; Gans & Vaughan, 2000). It may therefore be useful to repeat some of these experiments with increased replication, to re-examine those factors that have shown trends but not statistically significant responses.

The observation of root infections and symptoms in the negative controls, albeit it at lower levels than in the treatments, was not unexpected. It is difficult, in some areas, to obtain field soils that do not contain some level of S. subterranea inoculum, and given that the inoculum threshold for causing disease symptoms is so low under conducive conditions, such as those used in the experiments, this appears almost unavoidable.

Despite these obstacles, this research provides important new information on root infection and galling caused by S. subterranea. Further studies should be conducted to investigate the incidence of root galling under normal field conditions and to examine the importance of root infection and galling in the occurrence of tuber disease caused by this pathogen. Methods for the prevention and control of powdery scab cannot be optimized without a full understanding of the relationship between the infection of potato tubers and roots by S. subterranea.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Funding of this work by the Scottish Executive Environment and Rural Affairs Department (SEERAD) and the British Potato Council (BPC) is gratefully acknowledged. Many thanks are due to Dr Jim Duncan (SCRI) for scientific advice, Dr Danny Cullen (SCRI) for designing the primers and TaqMan® probe, and to Louise Sullivan, Sandie Linton, Alison Ward and Leanne Brown for technical assistance. The authors would also like to thank Dr Alex Hilton (SAC) for the supply of soil and Dr Paul Hallett (SCRI) for assistance with soil moisture curve measurements.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • Bell KS, Roberts J, Verrall S et al ., 1999. Detection and quantification of Spongospora subterranea f.sp. subterranea in soils and on tubers using specific PCR primers. European Journal of Plant Pathology 105, 90515.
  • Burnett F, 1991. The Biology and Control of Powdery Scab (Spongospora subterranea) of Potatoes. Aberdeen, UK: University of Aberdeen, PhD thesis.
  • Christ BJ, 2001. Powdery scab: an emerging disease on potato. American Journal of Potato Research 78, 4478.
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