Resistance of potato genotypes to late blight

ABSTRACT Late blight (Phytophthora infestans) is a serious problem in potato crops in southern Brazil due to favorable climatic conditions for the development of the pathogen and the lack of resistant cultivars available in the market. Thus, this study aimed to evaluate the reaction of eight cultivars (Asterix, Catucha, BRS Clara, BRS F63 - Camila, Cristal, BRS F183 - Potira, Markies, and BRS Pérola) and five advanced clones (F05-11-03, F21-07-09, Odone 80-02, F50-08-01, and F63-10-07) to late bight under field conditions. The Agata and the clone CIP392.617-54 were used as susceptible and resistant controls, respectively. The experiments were conducted in the municipality of Pelotas, RS, in the autumn season of 2018 and 2019. Sixty days after planting, the plants were inoculated with the pathogen; and, after one week, the genotypes were evaluated for disease severity and the area under the disease progress curve. ‘Catucha’ was classified as resistant to moderately resistant to P. infestans; ‘Cristal,’ F50-08-01 and ‘BRS Perola’, moderately susceptible to resistant; and ‘BRS F63’ - Camila, F63-10-07, F05-11-03, Asterix, F21-07-09, BRS F183 - Potira, and Odone 80-02, moderately susceptible to susceptible. BRS Clara and Markies showed the greatest variations in both experiments, showing to be susceptible in the autumn of 2018 experiment and moderately resistant in the autumn of 2019 experiment.


INTRODUCTION
Potato (Solanum tuberosum L.) is historically one of the most important crops for human consumption, ranking fourth among the world's most-produced foods (FAO, 2022).In 2020, Brazil produced 3,767,769 potato tons with an average yield of around 32 t ha -1 , with the southern region responsible for 32% of national production (IBGE, 2022).However, one of the main challenges of the production system is phytosanitary management.Among the diseases, late blight, caused by the oomycete Phytophthora infestans (Mont.)de Bary, remains the most important in potato crop worldwide (SAVILLE; RISTAINO, 2021).Under favorable environmental conditions, the pathogen can complete its life cycle within five days in susceptible cultivars, decimating entire crops (FRY; GOODWIN, 1997).The symptoms can be observed mainly on the leaves and they are characterized by spots of variable size, light or dark green in color and moist appearance which evolve into dark brown to black, necrotic and irregular spots.A circle of sporulation can be observed on the underside of the leaves, with a velvety appearance and grayish-white color, around the lesions.As the lesions coalesce, the leaf tissue exhibits a generalized burning appearance.In the shoots, late blight causes the death of the apical buds.On the petioles and stems the lesions are dark, and elongated, and on tubercles, the lesions are deep, reddishbrown, and poorly defined (TÖFOLI; DOMINGUES, 2021).
In the southern region of the country, late blight is the main disease that affects potato cultivation, which often requires intensive fungicide applications (BOSCO et al., 2010).The use of cultivars with durable resistance (horizontal) is one of the most desired strategies due to lower costs with fungicides, and reduced risks of contamination of the environment and to human health (PEREIRA et al., 2012).However, few cultivars have this characteristic for use in regions where late blight is a serious limiting factor for the crop.In this sense, the stability of resistance to P. infestans has been one of the targets of Embrapa's potato breeding program.Therefore, the objective of this work was to evaluate the reaction of commercial cultivars and advanced potato clones to P. infestans under field conditions.
The Agata cultivar and the CIP392.617-54clone were used as susceptible and resistant controls, respectively (GOMES et al., 2009).An isolate of P. infestans belonging to compatibility group A2 from the collection of phytopathogenic microorganisms of agronomic interest (COFIA) of Embrapa Clima Temperado was used as inoculum.
Sixty days after emergence, the plants were sprayed with a 10 5 sporangia/mL suspension of P. infestans using a manual costal sprayer.Seven days after inoculation, five assessments of disease severity (%) were carried out at intervals of two to three days, based on the Reifschneider scale (1987) adapted (0 to 100%), in which the last assessment was conducted when the susceptible control Agata reached 100% severity.
From the severity data, the area under the disease progress curve (AUDPC) was calculated for each genotype, using the GW Basic program (MAFFIA, 1986).Subsequently, the AUDPC values and the last severity assessment [transformed into arc sine root √(x/100)] of each experiment were submitted to ANOVA in a factorial scheme (15 x 2).If there was a significant interaction between the factors, the means of both variables for the genotype factor were submitted to the Scott and Knott cluster test (P <0.05); and for the year factor by the t-test (P <0.05).According to the results of the AUDPC and severity analyses, the genotypes were classified as resistant, moderately resistant, moderately susceptible, and susceptible (GOMES et al., 2009).

RESULTS AND DISCUSSION
The statistical analyses showed significant interaction between the genotype and year factors (P <0.05), showing higher values of AUDPC and severity of P. infestans in 2018.Considering the results of the analyzes of both experiments, the genotypes were separated into four groups ( By analyzing the final severity values (Table 2), the resistant genotype 'Cristal',  showed low severity values in at least one of the experiments (3.67 < sev <6.63) compared to the resistant control CIP392.617-54.However, for moderately resistant and moderately susceptible genotypes, sev values were higher, ranging on average from 41.34 to 70.34% in 2019, and from 46.00 to 96.34% in 2018.In susceptible genotypes (F21-07-09, 'Asterix' and 'Potira'), in both experiments, sev values ranged from 66.00 to 99% in 2019, andfrom 87.71 to 100% in 2018, compared to 'Agata' control (87.71-99%).
Among the cultivars that showed moderate resistance, the reaction of 'BRS Clara' has varied a lot since its release (PEREIRA et al., 2013), when it was shown to be resistant to P. infestans.Although 'BRS Clara' has 'Catucha' as one of the parents in its genealogy, its resistance pattern has oscillated Rev. Caatinga, Mossoró, v. 36, n. 3, p. 585 -589, jul. -set., 2023 RESISTANCE OF POTATO GENOTYPES TO LATE BLIGHT H. N. OXLEY et al. between moderately resistant and susceptible.This same oscillation performance has also been noticed for 'BRS Pérola' and Odone 80-02 (WOLTER et al., 2016), and for 'Camila', whose field reaction ranged from highly susceptible to resistant (CASA-COILA et al., 2019), being correlated with the race of P. infestans.According to studies conducted by Rocha (2015) specifically with 'BRS Clara', a high level of quantitative resistance to P. infestans associated with qualitative resistance was observed, showing that the R genes present in this cultivar did not provide full protection to the isolate under study.According to the same author, depending on the race composition of the pathogen present in a given location, the same genotype may still present an immunity reaction to P. infestans.Despite showing variation in reaction to late blight over the years, 'BRS Clara' has agronomic characteristics (elongated oval shape, yellow and smooth skin, with moderate resistance to greening, shallow buds, cream flesh, medium-short dormancy, and moderate resistance to early blight) which allow its maintenance in the national market as one of the most important cultivars in the organic crop system for our conditions.The late blight susceptibility of 'Agata' and 'Asterix' reported in this study has already been recorded in several studies conducted under different conditions and potato production systems (TÖFOLI; DOMINGUES; ZANOTTA, 2017;WOLTER, 2018;BUSNELLO et al., 2019).For the 'Potira' cultivar, similar results were described by Pereira et al. (2022), who reported high susceptibility under field conditions.Although 'Agata' and 'Asterix' occupy a large space in the market due to their culinary qualities or ease of seed potato production (PEREIRA, 2011), both, as well as 'Camila', require a greater number of sprays with chemical fungicides (BRASIL, 2022) or efficient alternative products for P. infestans management due to their high values of severity and AUDPC under favorable conditions.

Table 1 .
Potato genotypes evaluated for reaction to Phytophthora infestans under field conditions in autumn seasons of 2018 and 2019.

Table 2 .
Area under the disease progress curve (AUDPC), severity percentage (SEV%), and the reaction of 15 potato genotypes to Phytophthora infestans in the autumn seasons of 2018 and 2019.
*Means followed by equal letters, in the column, belong to the same group by the Scott & Knott test (P <0.05); and, on the line, by the t-test ** (P <0,05); CV: coefficient of variation; 1-original values transformed into arcsine √(x/100); R= resistant, MR= moderately resistant, MS= moderately susceptible, S= susceptible.