High soil moisture reduces common scab caused by Streptomyces turgidiscabies and Streptomyces europaeiscabiei in potato

Abstract

Common scab (Streptomyces spp.) is a worldwide economic important disease of potato (Solanum tuberosum L.) because it degrades the appearance of the tubers and thereby decreases market value. For the newly reported Streptomyces turgidiscabies and Streptomyces europaeiscabiei in Norway, little is known about their specific environmental requirements, particularly their pathogenicity under varying moisture levels. Scab development after soil inoculation of S. turgidiscabies and S. europaeiscabiei in dry, normal and wet soils was studied in three pot experiments (2009–2011) under controlled climate conditions at natural light in Tromsø, Norway (69.7 N). With increased soil moisture, statistical significant reductions in scab severity were found in two out of three experiments for S. turgidiscabies (about 50% reduction). Similarly, scab caused by S. europaeiscabiei was reduced significantly in one experiment (65% reduction), and with a tendency of reduction in the other experiments. Scab development from seed-transmitted inoculum was investigated for both species. In one trial with four scab severity levels on mother tubers (0%, 10%, 30% and 50% of surface covered), we found that seed-transmitted disease development occurred for both species although few lesions were recorded. There were no significant differences in scab severity on progeny tubers, no matter the infection levels on seed tubers. These results indicate that seed tubers might play a minor role for transmitting common scab for S. turgidiscabies and S. europaeiscabiei, but further work is warranted. Related to growers practice, the results implicate that potato common scab caused by S. europaeiscabiei and S. turgidiscabies may be reduced by irrigation during the early stages of tuber formation, similar to currently recommended control measures for Streptomyces scabies.

Keywords:

• diseases
• pathogenicity
• Solanum tuberosum
• surface lesions
• inoculation

Introduction

Common scab is a worldwide established economic important disease of potato (Solanum tuberosum L.; Loria 2001; Lehtonen et al. 2004). Streptomycetes that cause common scab on potato are mainly soil‐borne, and in the absence of host plants, they can readily survive as saprophytes on plant debris (Loria et al. 1997; Wang & Lazarovits 2004). Seed tubers can also be a source of inoculum, and infected seed tubers provide a great potential for dissemination of the pathogens over long distances (Wilson et al. 1999; Wang & Lazarovits 2005). Recently, Tegg et al. (Forthcoming) has provided both visual and DNA assessments of seed tubers to conclusively show that infection of seed tubers is correlated with progeny tuber infection.

Previously, common scab-inducing bacteria that produced melanin were designated to Streptomyces scabies, but the introduction of molecular analysis for the characterization of the strains suggested that there could be additional, melanin-producing species that cause common scab (Goyer et al. 1996). One of these recently detected species is Streptomyces europaeiscabiei, which was first described in France and is frequently found in Europe and also North America (Bouchek-Mechiche et al. 2000; Flores-Gonzalez et al. 2008; Wanner 2009; Dees et al. 2013; Leiminger et al. 2013). The species Streptomyces turgidiscabies does not produce melanin or other diffusible pigments (Miyajima et al. 1998).

Although common‐scab‐causing streptomycetes are diverse, they seem to have the same mechanism of pathogenicity, a pronounced similarity of the thaxtomin biosynthesis gene cluster. The ability to produce Thaxtomin A has a positive correlation with development of common scab lesions on the tuber (Bignell et al. 2014). Thaxtomin has an impact on cellulose biosynthesis (Scheible et al. 2003; Bischoff et al. 2009), and it can aid lesion formation on expanding plant tissue by inhibiting primary cell wall deposition. It has been shown that Thaxtomin A is synthesized by S. scabies, S. turgidiscabies, Streptomyces acidiscabies and S. europaeiscabiei (King et al. 1989; Miyajima et al. 1998; Bouchek‐Mechiche et al. 2000; Healy et al. 2000).

Common scab symptoms on the tubers may be raised, superficial or pitted lesions dependent of the Streptomyces strain and potato cultivar. The most widely studied species producing such symptoms is S. scabies (Lambert & Loria 1989). More recently, S. turgidiscabies has been found to cause common scab in the Nordic countries, Japan, North America and other parts of the world (Miyajima et al. 1998; Kim et al. 1999; Kreuze et al. 1999; Lehtonen et al. 2004; Wanner 2009; Thwaites et al. 2010; Dees et al. 2013). In a recent study, potato pathogenic Streptomyces strains originating from 130 fields in Norway were identified as S. turgidiscabies and S. europaeiscabiei (Dees et al. 2013).

Several methods have been applied or are tested for control of common scab in potato. Among these are breeding for resistance, crop rotation, organic amendments, chemical treatments, biological control using antagonistic Streptomyces species, soil pH-management and reduction of oxygen content in the soil (reviewed in Dees & Wanner 2012). However, so far no methods alone seem effective and reliable for control of common scab in the field. The most widespread method is irrigation to reduce aerobic soil conditions during the early stages of tuber formation (Lapwood et al. 1970; Adams & Lapwood 1978). Traditionally, the strategy for management of common scab in Norway has been to avoid liming before planting potatoes in order to maintain a low soil pH. Packing of light sandy soil has also been performed, whereas irrigation during stages of early tuber formation is rarely done in this country.

Most of the work aimed at controlling common scab has focused on S. scabies, but it is possible that other pathogenic species of this genus do not respond in the same way. For the newly reported S. turgidiscabies and S. europaeiscabiei in Norway, little is known about their specific environmental requirements, particularly their pathogenicity under varying moisture levels. Therefore, the objectives of the current study are to investigate (1) how soil moisture may affect the severity of common scab lesions from soil inoculum of S. turgidiscabies and S. europaeiscabiei and (2) how the severity of scab lesions from both these species on seed tubers may impact scab severity in progeny tubers.

Materials and methods

Growth conditions and material used

The work included three trials (2009–2011) with soil inoculation and different soil moisture levels and one trial (2012) with seed-transmitted inoculum. The trials were performed in controlled climate chambers at the University of Tromsø, Norway (69.7 N). The chambers had natural light conditions and a temperature of 18°C (constant) in 2009 and 18/12°C (day/night) in 2010–2012.

Isolates of S. turgidiscabies (08-13-01-1; 1B) and S. europaeiscabiei (08-05-02-1; 08-12-01-1) from our collection were used as the source of inoculum. The isolates were grown on yeast malt extract agar for two weeks. For inoculation, two isolates of S. turgidiscabies and two isolates of S. europaeiscabiei were used. Two plates of the same species were mixed together in the growth media. Seed potato material was pre-basic pathogen-free tubers (about 40 g) of the susceptible Norwegian cultivar ‘Gullauge’. In the trial with seed tuber-transmitted inoculum, infected and uninfected tubers (about 60 g) from the soil inoculation trial in 2011 were used.

Two different growth media were used in the experiments: in 2009, a 50/50 (volume) mixture of a sandy soil (Proffjord®, Grønn vekst Sør AS, Grimstad, Norway, pH 7.5) and perlite and in 2010–2012, a 50/40/10 mixture of standard fertilized peat soil (Superflora®, Tjerbo Torvfabrikk AS, Rakkestad, pH 5–6), perlite and Proffjord®. Fertilizer (Yara Fullgjødsel® 11-5-18 micro) was added to all soils at the rates of 1, 0.4 and 1.6 g (nitrogen, phosphorus and potassium, respectively) per plant.

Experimental set-up

In the pot trials with different soil moisture levels, 5 L growth medium were filled into 12 L pots. On top of this, 4 L growth medium with thoroughly mixed agar with inoculum and fertilizer were added. Similarly, agar without inoculum was incorporated into control pots. Finally, more growth medium was added on top to a desired total pot weight. The moisture level was adjusted to a standard level by equal supply of water to all pots before planting of pre-sprouted seed tubers at 5 cm depth.

The soil moisture was differentiated and maintained during four weeks from the onset of tuber formation (swelling of stolon tips as determined by destructive analyses). Three levels of soil moisture (dry, normal and wet) were based on standard practice (= normal moisture content) and then 1 kg difference in pot weights to dry and wet pots. The differences were maintained by weight control of pots and daily supply of water. Measurements using Delta-T, HH2-moisture meter, in 2009 showed soil water contents of 8%, 15% and 22%, respectively, for dry, normal and wet soil.

In the trial with seed tuber-transmitted inoculum, mother tubers propagated in inoculated soil were divided into four groups (0%, 10%, 30% and 50% of surface covered by scab; Figure 1) for both pathogens and planted in pathogen-free soil. In addition, uninfected mother tubers (propagated in pathogen-free soil) were planted in pathogen-free soil for control. Moisture conditions were kept at standard levels (see above).

Figure 1. Pre-sprouted seed tubers with different common scab severity used in trials with seed-transmitted inoculum 2012.

From left to right: Visual scores of 10%, 30%, 50% and 0% of surface covered with lesions caused by S. turgidiscabies.

At harvest, the number of tubers (>10 mm) and total tuber yield were assessed per pot. The common scab severity was scored visually as percentage cover of symptoms on the tubers surface, as previously described by Nærstad et al. (2012). The assessment was supported by a key with illustrations of various coverage percentages. It was performed by placing all tubers from each pot in a circular container (20 cm diameter) and viewing from above. This method deviates from methods with assessments on individual tubers, but a previous work (Johansen, unpublished) has shown that this less time-consuming group assessment may give comparable results.

Statistics

A complete randomized A × B factorial design was established for each trial: two Streptomyces species combined with three moisture levels (three trials) or four seed infection levels (one trial). For each treatment combination there were 4 or 5 pots (specified in each result table). Statistical analyses of differences between treatments were performed for each trial with Minitab 16, analysis of variance (ANOVA; GLM procedure).

Results

Effect of soil moisture on scab severity

Soil inoculation with S. turgidiscabies and S. europaeiscabiei were successful as severe lesions occurred on the progeny tubers in all trials, while tubers from a total of 15 control pots without inoculations showed no symptoms. The lesions were mostly superficial, but in some cases raised or pitted, for both species. Overall, the levels of tuber lesions averaged about 23% and 16% coverage of the tuber surface for S. turgidiscabies and S. europaeiscabiei, respectively. Neither common scab severity nor moisture levels had any significant effect on total yields or tuber numbers per plant (data not shown).

For S. turgidiscabies, results showed a significantly different scab severity between levels of soil moisture in two out of three experiments (2010–2011; Table 1). In these two experiments, the average scab coverage on the tubers decreased with increasing soil moisture and was about 50% lower in wet than at dry soil conditions. The third experiment (2009) showed no clear trends in the results. For S. europaeiscabiei, significant differences were found in one of the experiments (2010; Table 1). In this case, the severity was reduced by about 65% by increasing the moisture. In the two other trials, there was a weak trend of reduced scab severity with increasing soil moisture.

Table 1. Effect of soil moisture on common scab severity (% of tuber surface covered with lesions) in potato tubers at three levels of soil moisture.

	2009		2010		2011
	S. turgidiscabies	S. europaeiscabiei	S. turgidiscabies	S. europaeiscabiei	S. turgidiscabies	S. europaeiscabiei
Moisture
Dry	22.3	11.5	39.0a^a	32.4a	13.2a	17.6
Normal	42.5	6.0	22.0b	25.4ab	9.6ab	17.6
Wet	31.3	5.8	20.0b	11.2b	6.6b	12.0
P-ANOVA	0.401	0.291	0.012	0.03	0.028	0.607

Note: Controlled climate experiments with S. turgidiscabies and S. europaeiscabiei. Average results of four pots per treatment in 2009 and five in 2010 and 2011. Planting and harvest dates: 23 April and 21 July 2009, 6 May and 23 August 2010 and 27 May and 30 August 2011.

^aValues within each entry accompanied by different letters are significantly different according to Tukey's pairwise comparisons at p = 0.05.

Seed-transmitted inoculum – impact of seed tuber scab severity

In spite of severe lesions on seed tubers from both S. turgidiscabies and S. europaeiscabiei, results from one trial in 2012 showed only minor scab lesions on progeny tubers (Table 2). Nevertheless, results implicate that inoculum could be transmitted by seed tubers as no symptoms were recorded on tubers from four control pots with pathogen-free tubers (data not shown). The four different levels of scab severity on seed tubers did not seem to affect the scab severity in the progeny tubers. The results were similar for both S. turgidiscabies and S. europaeiscabiei.

Table 2. Effect of seed tuber common scab severity (% of tuber surface covered with lesions) on common scab severity on progeny potato tubers.

Seed tuber	Progeny tuber
S. turgidiscabies
0	0.06
10	1.69
30	1.38
50	0.44
P-ANOVA	0.362
S. europaeiscabiei
0	0.06
10	0.25
30	0.88
50	1.38
P-ANOVA	0.257

Note: Controlled climate experiment with two Streptomyces species. Average results of four pots per treatment. Planted on 19 April and harvested on 11 July 2012.

Discussion

In this study, scab development after soil inoculation of S. turgidiscabies and S. europaeiscabiei in dry, normal and wet soils was investigated in three pot experiments over three years. In addition, scab development from infected seed tubers was studied in one experiment. For both species, significant reductions in symptoms with increased soil moisture could be observed in some of the trials, similar to the traditional management strategy using irrigation to suppress common scab. Most studies before year 2000 designated common scab-inducing bacteria to S. scabies, while the introduction of molecular analysis has shown that several other species are involved in common scab disease. These species may have different tolerances for environmental soil conditions. Our results for soils inoculated with S. turgidiscabies and S. europaeiscabiei are yet consistent with previous work done with S. scabies (Lapwood et al. 1970; Davis et al. 1976; Loria 2001). The results also indicate a similar tolerance for elevated soil moisture levels for S. turgidiscabies and S. europaeiscabiei which is contradictory to previous assumptions of S. turgidiscabies being most tolerant (Dees & Wanner 2012).

Our results varied between trials, similar to results from other studies of the use of irrigation as a control method (Lapwood et al. 1973; Larkin et al. 2011). One explanation for the inconsistent results in our study might be the different growth media used. In 2009, a heavy sandy soil was the main ingredient while a peat soil mixture with lower bulk density was used in 2010–2011 due to easier manual handling during pot weighing. Hence, with the same water weight difference (2 kg) between dry and wet soils all years, there has been larger relative differences between the dry and wet soils in 2010–2011, with possible effect on the results. Unfortunately, the soil water content was only measured in 2009. The duration of the treatment period, in our case four weeks, may also have affected disease development. According to Khatri et al. (2011), the infection window, when inoculation leads to scab development, may last until eight weeks after tuberization. Extending this period in our experiments beyond four weeks may have increased the efficacy of the irrigation control of scab symptoms.

The most important source of common scab inoculum, tubers or soil, has been studied over years, and it has been shown that Streptomyces pathogens may be seed-transmitted (Lehtonen et al. 2004; Wang & Lazarovits 2005). The current study demonstrates a seed-transmitted disease development for S. turgidiscabies and S. europaeiscabiei, although showing a very low severity level on progeny tubers. Our results also indicate that the abundance of scab disease symptoms on seed tubers had minor relationship with the disease on progeny tubers. This is in accordance with previous results from Bjor and Roer (1980), who found no significant relation between Streptomyces scab infection on mother tubers and on progeny tubers. Another study has demonstrated that neither scabby seed tubers nor tubers treated with scab inoculum produced more scabby tubers than disinfected seed tubers (Pavlista 1996). However, contrary to our results, Wilson et al. (1999) and Tegg et al. (Forthcoming) showed that the severity of common scab in the daughter tubers was directly related to the severity on the mother tubers.

Although the effects are inconsistent between studies, seed-born inoculum may still be important for introduction of the disease to new fields and regions, especially under favourable environmental conditions (Wang & Lazarovits 2005). It may be questioned if our experimental conditions with normal soil moisture content were favourable for such seed-transmitted disease development, although similar soil media and moisture level produced high disease levels after soil inoculation. More work is therefore warranted under varying disease pressures and soil conditions to assess how seed tuber-borne inoculum of identified Streptomyces pathogens is correlated with progeny common scab disease.

We conclude that elevated levels of soil moisture can lead to a significant reduction in common scab produced by both S. turgidiscabies and S. europaeiscabiei. Seed-transmitted disease development may occur for both species, but the abundance of common scab on seed tubers seems to play a minor role for common scab incidence in progeny tubers. This study contributes to the knowledge of two recently identified Streptomyces species widespread in Norway. From a practical point of view, the results implicate that potato common scab caused by both S. turgidiscabies and S. europaeiscabiei may be reduced by the use of irrigation in combination with other management methods. However, more work is warranted to give more knowledge on the role of both soil moisture and seed tubers in disease development.

Acknowledgement

Thanks to Arild Sletten for kind contribution and to anonymous referees for suggestions that have improved this study.

Additional information

Funding

This work was supported by grants from the Research Council of Norway, the Foundation for Research Levy on Agricultural Products, the Agricultural Agreement Research Fund and Norwegian food industries [grant number 185053/I10].