Fifty potato genotypes from the breeding program were evaluated against common scab, caused by Streptomyces sp., in a greenhouse of the Balcarce Integrated Unit (INTA-FCA, UNMdP). The isolation of Streptomyces sp. was done from tubers that exhibited symptoms of the disease. The "seed" tubers of each genotype were planted in plastic pots containing a mixture of sterilized soil and sand, previously inoculated with the bacteria. Cv. Bintje was included because of its known susceptibility to the disease. Plants naturally completed their cycle and tuber´s scab symptoms were evaluated. Scab severity (%), type of damage and scab index (SI) were calculated for each tuber of each genotype. A randomized complete block design with three replications was used. It was determined that there were significant differences among genotypes in scab severity, type of damage and SI. Six genotypes with a potentially resistant behavior to common scab were identified. This characterization of infection with common scab will allow resistance genes to be incorporated against this disease in breeding programs.

Keywords:streptomyces, resistance, solanum, scab index

Potato (Solanun tuberosumssp. tuberosum L.) is cultivated in Argentina mainly in the South East of the Buenos Aires Province (SEBAP) and in Córdoba, Mendoza and Tucumán provinces. Common Scab is caused by bacteria of the genus Streptomyces and is present in all regions of the country, but given that the SEBAP is the main producing region,¹ it merits the concentration of studies concerning one of the main tuber commercial quality diseases. Although Common Scab does not affect yield, the quality reduction of the tubers is punished by both the fresh market and the processing industry. Besides, affected tubers are susceptible to dehydration during storage and the lesions can be the entrance of bacteria causing wet rots as well as insects.^2,3 Streptomyces is a filamentous heterotroph actinomyces that produces a thin pseudomicelius from which it can fragmentate or asexually subdivide and originate spores.⁴ Streptomycesscabies, StreptomycesacidiscabiesandStreptomycesturgidiscabieshave been identified in Argentina. These species not only produce the same symptoms, but they benefit from a similar pathogenic mechanism.⁵

Common scab is controlled by several methods for, e.g. chemical treatments, biological control, irrigation, soil pH management, crop rotation and resistant cultivars. The use of resistant cultivars is the most efficient method, which, besides giving a satisfactory control, is environmentally safe, economical and stable.⁶ Potato cultivars differ in their susceptibility to common scab and therefore the lesions can be superficial or very deep.⁷ For the moment, there are no commercial cultivars that show a complete resistance to common scab.^7-9 The potato breeding program at Balcarce has a wide collection of genotypes coming from diverse sources in the world. They combine other characteristics as diverse resistances and nutritional factors under short or long day length backgrounds. Thus, it represents an opportunity to evaluate this material unknown for its performance against Common Scab.

Twenty five potato clones and twenty five commercial varieties were assessed in a temperature regulated greenhouse for their performance against Streptomyces sp. (Annex 1) at the Balcarce Integrated Unit (EEA INTA-FCA, UNMdP) in July of 2014. The plants were watered according to the crop requirements. Weekly sprays of insecticides were performed to control white flies (Bremicia tabaci) from November to senescence. Plants were grown until natural senescence. Once the tubers were mature they were manually harvested by December 2014. The tubers were kept at room temperature until they were evaluated. A complete randomized block design with three replicates was used. The experimental unit was one potted plant of each genotype.

Isolates

Common scab isolates were multiplied in the lab ten days prior to inoculation. The isolates were obtained from diseased tubers which were washed carefully with distilled water. A piece of the surface of the tubers with scab symptoms was taken and subsequently scrapped off from the limit between the healthy and the diseased tissue. The tissue thus obtained was homogenized with 5ml of sterile distilled water in a mortar. One milliliter of the homogenate was placed in 9cm Petri dishes containing the Küster growing media.¹⁰ The plates were incubated during 10days at darkness and at 24°C.¹¹ Seven isolates were obtained. Pathogenicity tests in the lab and in the greenhouse were carried on these isolates.

Pathogenicity assessment

Laboratory pathogenicity test: A protocol developed by Loria et al.,¹² with some modifications was followed. 5ml of distilled water was added to each plate containing the growing media with the bacteria and then stirred. One milliliter of the resulting bacterial suspension was taken and the concentration was adjusted to1x107 conidia/ml by means of a Neubauer chamber. Flame and alcohol sterilized mini tubers of cv. Spunta were cut into 1 mm thick pieces and inoculated with 20µl of the bacterial suspension for each isolate. The mini tuber pieces were placed in trays inside an incubator during ten days at 24°C. The presence of necrosis was evaluated in the inoculation spot in order to select the more pathogenic isolates.

Greenhouse pathogenicity test: The protocol developed by Fischer et al.,^4,8 was followed. Washed cv. Spunta minitubers were planted in one liter pots containing a mixture of sterile soil and sand. The bacterial colony grown on a 9cm diameter Petri plate was poured into each pot and mixed with the first 10cm of soil. Four replications per isolate were evaluated. Harvest of tubers produced was performed after 12weeks and symptoms were evaluated, determining incidence and severity of scab and subsequently identifying the more pathogenic isolates.

Inoculum preparation and inoculation: One isolate capable of producing symptoms in both pathogenicity tests was used. The isolate was multiplied and increased in Küster media ten days before inoculation. The 320 plates thus obtained were incubated for ten days at 24°C. Five random samples of two plates each were taken to estimate inoculum concentration. Sterile water (5ml) was added to each plate and the content was scrapped with a glass rod forming a bacterial suspension. An aliquot of 5ml from each of the two plates of a sample were mixed together and completed up to 15ml with sterile water. A 6x107 conidia/ml concentration was determined in a Neubauer chamber. Inoculation was performed one day before planting, following the protocol of Bjor and Roer.¹³ The inoculum content of two plates was placed in each pot and mixed manually with a sterile mixture of soil and sand (50:50 v/v), 9x108 conidia were added per seven liter pot.

Tuber scab evaluation. The affected surface and the lesion type were determined in January 2015 for each tuber from each genotype, according to the Ministry of Agriculture, Fisheries and Food¹⁴ key. The lesion type is the visual assessment of severity in a scale of 1 to 3 [1, superficial scab; 2, intermediate scab (≈1–3 mm depth); 3, deep scab (>3mm depth)]. For each genotype, a relative scab index that includes the percentage of tubers affected by type of damage, the percentage of affected area and the percentage of tubers affected was calculated as shown below: RSI=(((% tubers with Type 1 damage+% tubers with Type 2 damage+% tubers with Type 3 damage) x % of affected tuber surface)/300) x % of total affected tubers)/10000.

Data analysis. The affected surface for each tuber and the percentage tubers affected for each genotype were normalized and an analysis of variance was performed. Means were compared with a Duncan test with α=0,05 using a SAS Institute Inc. (1990) statistical package.

Affected tuber surface

Significant differences in the percentages of affected tuber surface among genotypes were obtained (Table 1). Clones 304013.18, B 07.516.1 and B 84.617.4 LR showed the highest affected tuber surfaces, 45 %, 42 % y 40 %, respectively. Cultivar (Cv.) Bintje showed a 22% of affected surface, significantly lower than those three highly susceptible clones. Bintje performance was similar to that shown by Melegari.¹⁵ Clone B 07.577.3 presented no damage in its tubers. The genotypes with better performance were 304072.6LB and B and 03.602.4 1.67%, 2.33% respectively). Cultivars (cvs.) Frital INTA, Achirana INTA and Pampeana INTA behaved similarly as shown by Melegari.¹⁵ Cultivars Snowden and Atlantic showed severities of 32% and 34%, respectively, although Navarro et al.,¹⁶ mentioned severities higher than 60%.

Lesion type

Superficial scab (type 1 lesion) was the lesion type found in higher frequency (Figure 1). Lesion types 2 (intermediate) and 3 (deep) obtained the same genotype frequency. Clones 304013.18, B 07.516.1 and B 84.617.4 LR, with the largest affected surface, also showed the deepest scab symptoms (lesion 3 type). On the other hand, genotypes with the lowest affected surface (304072.6LB, 304152.5, B 03.04.573.1, B 03.602.4 and Innovator) showed type 1 lesion. Cv. Bintje showed type 2 lesions (intermediate) different from the deep scab symptoms reported by Melegari.¹⁵ Differences with that research were also observed in cvs. Achirana INTA, Pampeana INTA and Kennebec. Uruguayan cvs. INIA Yaguarí and INIA Iporá showed type 2 lesions, coincident with Lapaz Eugui et al.¹⁷ Cvs. Snowden and Atlantic showed type 2 lesion instead of the type 3 lesion published by Navarro et al.¹⁶

Figure 1 Superficial scab (type 1 lesion) was the lesion type found in higher frequency.

Relative scab index

Significant differences (p<0,001) for RSI among genotypes were obtained (Tables 1) (Table 2) and (Figure 2). The RSI distribution curve was not normal and was shifted towards the low values. Cv. Bintje´s RSI showed susceptibility coincidently with Pasco et al.¹⁸ Genotypes with the higher RSI were clones 304013.18 (0.180), B 78.502.5 (0.185), B 87.605.1 (0.189), Arazati (0.193), 393371.35 (0.230), Snowden (0.246), B 84.617.4 LR (0.333) and B 07.516.1 (0.338), and therefore can be classified as highly susceptible.

Figure 2 Significant differences (p<0,001) for RSI among genotypes.

B 07.577.3, 304072.6LB, B 03.602.4, 304152.5, Nicola, Alpha, B 03.04.573.1 and. Innovator showed the lowest RSI and were significantly lower (p<0,001) than the rest of the genotypes. Clone B 07.577.3 presented no damages in any of its tubers.

In this study, cv. Innovator behaved similarly as shown by Barrera et al.,⁵ who classified this cv. as moderately resistant to common scab. Spunta, Frital INTA and Pampeana INTA performed as moderately susceptible in coincidence with the results obtained by Melegari.¹⁵ According to Giménez et al.¹⁹ Yagana shows a good performance against the disease but here behaved as susceptible. INIA Yaguarí showed an intermediate RSI similarly to what was informed by Giménez et al.¹⁹ and by Lapaz Eugui et al.,¹⁷ that classified it as susceptible. Agata showed in this work an intermediate RSI and it has been classified as susceptible by Canadian Food Inspection Agency.²⁰ The same authors classify Snowden as susceptible coincidently with the high SI obtained here. Nevertheless, Navarro et al.,¹⁶ classified Snowden as highly susceptible cultivar. The intermediate RSI obtained by Asterix is coincident with the moderately resistant classification by Canadian Food Inspection Agency.²⁰ Nicola also behaved similarly to what was reported by Peeten et al.,²¹ and by Pasco et al.,¹⁸ who classified it as moderately resistant but Monalisa behaved as susceptible inversely to those authors reported. Table 3 shows the grouping (according to the national legislation) of genotypes based on their RSI in this study. Tuber affected surface was significantly correlated with RSI, r=0.85 (p<0.0001). The index also considers the number of affected tubers and type of damage. Therefore it has a more holistic approach to evaluate the disease.

Significant differences among the 50 genotypes studied were observed with respect to RSI, type of damage and tuber affected surface. The performance of the genotypes observed in this study was in most cases coincident with the data found in the literature, when available. A group of potentially resistant genotypes was identified. Among these, clone B 07.577.3 presented no scab blemishes. This low RSI group needs further evaluation in order to confirm resistance through progeny tests in the field, either with artificial or natural infection. This study resulted in the classification of a set of genotypes widely used as progenitors both in the Argentine breeding program as well as in programs in Latin America and Europe with respect to their performance against common scab.

None.

The author declares no conflict of interest.

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