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, 9 (10), e109328
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Appraisal of Artificial Screening Techniques of Tomato to Accurately Reflect Field Performance of the Late Blight Resistance

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Appraisal of Artificial Screening Techniques of Tomato to Accurately Reflect Field Performance of the Late Blight Resistance

Marzena Nowakowska et al. PLoS One.

Abstract

Late blight (LB) caused by the oomycete Phytophthora infestans continues to thwart global tomato production, while only few resistant cultivars have been introduced locally. In order to gain from the released tomato germplasm with LB resistance, we compared the 5-year field performance of LB resistance in several tomato cultigens, with the results of controlled conditions testing (i.e., detached leaflet/leaf, whole plant). In case of these artificial screening techniques, the effects of plant age and inoculum concentration were additionally considered. In the field trials, LA 1033, L 3707, L 3708 displayed the highest LB resistance, and could be used for cultivar development under Polish conditions. Of the three methods using controlled conditions, the detached leaf and the whole plant tests had the highest correlation with the field experiments. The plant age effect on LB resistance in tomato reported here, irrespective of the cultigen tested or inoculum concentration used, makes it important to standardize the test parameters when screening for resistance. Our results help show why other reports disagree on LB resistance in tomato.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. DSI from artificial P. infestans inoculations of ‘Rumba’ detached leaflets.
A total of 46 local isolates were used. Each isolate was tested on 10 leaflets, in a series of three independent assays. Inoculum concentration of 5×104 sporangia/ml was used. Assay evaluations were performed on the 7th day after inoculation. Disease assessment scale was based on the % of leaflet area being infected; 1 = 100% area infected; 9 =  lack of disease symptoms or few and small necrotic spots. Error bars indicate standard deviation (SD).
Figure 2
Figure 2. Severity of LB symptoms on tomato cultigens in the detached leaflet assay.
Detached leaflets of tomato cultigens (• ‘Rumba’; ▪ WVa 700; ♦ L 3708; ▴ LA 1033) were inoculated with the P. infestans isolate IWP 13, at indicated age (weeks), and concentration. Assay evaluations were performed on the 7th day after inoculation. Data shown are the means of ratings from at least three independent experiment sets for each combination; 25 leaflets per cultigen were inoculated per experiment. Vertical bars at each data point signify the standard deviations (SD). LSD0.05 calculated according to Tukey procedure (inset) for comparing cultigens at each plant age and inoculum concentration (C×I): 1.34, and for comparing inoculum concentration for each cultigen and plant age (C×A): 1.43.
Figure 3
Figure 3. Severity of LB symptoms on tomato cultigens in the detached leaves assay.
Third to fifth fully expanded leaves were collected for testing from the plants (• ‘Rumba’; ▪ WVa 700; ♦ L 3708; ▴ LA 1033) at indicated ages (weeks). Detached leaves of the tested tomato cultigens (12 to 24 leaves per cultigen and per developmental stage) were inoculated with suspension of the P. infestans isolate IWP 13, by spraying at indicated concentration. Presented data, for each treatment combination, are the means of ratings from three independent experiment sets. Vertical bars at each data point signify the standard deviations (SD). LSD0.05 calculated according to Tukey procedure (inset) for comparing cultigens at each plant age and inoculum concentration (C×I): 0.58, and for comparing inoculum concentration for each cultigen and plant age (C×A): 0.74.
Figure 4
Figure 4. Severity of LB symptoms on tomato cultigens in the whole plant assay.
Plants (• ‘Rumba’; ▪ WVa 700; ♦ L 3708; ▴ LA 1033) at indicated ages (3- to 8-weeks) were inoculated with suspension of the P. infestans isolate IWP 13, by spraying at indicated concentration (12 to 72 plants per cultigen and per developmental stage). Each combination was tested in three independent experiment sets. Vertical bars at each data point (means of the ratings) signify the standard deviations (SD). LSD0.05 calculated according to Tukey procedure (inset) for comparing cultigens at each plant age and inoculum concentration (C×I): 0.55 and for comparing inoculum concentration for each cultigen and plant age (C×A): 0.61.
Figure 5
Figure 5. Cross-comparison of the methods for testing the tomato LB resistance.
Data from the field experiments (F) were pairwise compared with the results of each of the controlled-conditions method used, under the conditions optimized towards the maximal LB resistance expression (LL: Detached leaflets tested on 8-week old plants, under 5×104 sporangia/ml; L: Detached leaves tested on 15-week old plants, under 5×103 sporangia/ml; P: Whole plants tested on 8-week old plants, under 5×104 sporangia/ml). Calculated trend lines, with respective determination coefficients (r2) and P-values are indicated. A: Comparison of detached leaflet assays with field assays (LL and F); B: Comparison of detached leaf assays with field assays (L and F); C: Comparison of whole plant assays with field assays (P and F).

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Grant support

This research was supported by The Polish Ministry of Agriculture and Rural Development (Grant # HOR hn-801-15/13; “Searching for new sources of resistance to tomato late blight with regard to pathogenicity changes in Phytophthora infestans populations and attempts of markers identification linked with resistance genes”), carried out in the period 2008–2013. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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