EVALUATION OF POTATO (Solanum tuberosum L.) GENOTYPES FOR RESISTANCE TO LATE BLIGHT AT SINANA SOUTHEASTERN ETHIOPIA

Late blight caused by Phytophthora infestans (Mont.) de Bary, is one of the most significant constraints to potato production in Bale high lands and other regions of Ethiopia. Hence, this requires to develop high yielding and late blight resistant varieties. Therefore, this study was conducted to determine host resistance of potato against Phytophthora infestans in order to develop late blight resistant/tolerant genotypes. The experiment was laid out in randomized complete block design with three replications at Sinana Agricultural Research Center during 2014 main cropping season. The genotypes showed highly significant (P<0.01) differences for all the characters studied. Genotypes exhibited wide ranges of mean values for all characters. The highest total tuber yield (46.1 t ha-1) was obtained from the advanced clone, CIP-392640.524 followed by Belete (41 t ha-1). Late blight appeared early on farmer's cultivar Kellacho (48 days after planting) and late on advanced clones CIP399062.102(74 days after planting). Percent severity index (PSI) and area under disease progress curve (AUDPC) ranged from 33.91 to 91.67% and 105 to 2370, respectively. Eleven newly introduced clones and the released variety "Belete" recorded PSI 33 to 39.7% and AUDPC 105 to 264.1. Hence, these genotypes can be categorized as moderately resistant. The study revealed the presence of considerable variability in tested genotypes for economically important traits and the higher chance of selecting genotypes with high yield and moderately resistant to late blight. However, it appears that further evaluation of genotypes across seasons and locations to identify genotypes that could be released as varieties seem to be quite pertinent.


Introduction
Potato late blight, caused by the Phytophthora infestans (Mont.)de Bary, is one of the primary problems faced by small-scale potato producers in high lands of Ethiopia.Late blight can destroy a potato field within a few days (Razukas and Jundulas, 2005).If not controlled, losses may reach 100% (Rubio-Covarrubias et al., 2005) and even lower infection levels may make the crop unfit for storage (Heinfnings, 1987).In the highlands of Ethiopia, late blight and bacterial wilt (Ralstonia solanacearum) are the most important economic diseases that cause an estimated yield loss of up to 70% (Shiferaw et al., 2009).Late blight is especially important in the traditional potato growing areas.
The search for genetic resistance to late blight has intensified, since it is a practical and economic way of controlling the disease.Initially, the search concentrated on specific resistance (Forbes and Landeo, 2006).The resistant variety development, which involves evaluation, selection, release and registration procedures pass through several stages.Some countries such as Ethiopia, the potato breeding programs depend entirely on CIP materials.In the absence of creating variation through crossing in the country, it is necessary to introduce potato genotypes every time from the source.The introduced genotypes need to be evaluated for target areas or for wide adaptability across regions in the country.Accordingly, this study was conducted to determine host resistance of potato against Phytophthora infestans and to develop late blight resistant/tolerant genotypes.

Experimental site
The experiment was conducted in Southeastern Ethiopia, Bale Zone, at Sinana Agricultural Research Center during 2014 main cropping season.Sinana is located at 07 0 N and 40 0 10 ' E at an altitude of 2400 (m.a.s.l).The area possesses a bimodal rainfall type.This bimodal rainfall system has created conducive opportunities to produce crops twice annually or double production in a season.Average annual maximum and minimum temperatures are 21 and 9 o C, respectively.The dominant soil type is pellic vertisol and slightly acidic (Nefo et al., 2008).

Experimental materials and design
A total of 24 potato genotypes which consists of 20 advanced clones, three released varieties as standard checks and one farmers' cultivar in Bale were planted during the main cropping season of 2014.The farmers' cultivar Kellacho that is susceptible to late blight was used as local check.
Belete is known as resistant variety to late blight while Ararsa and Guddane are moderately resistant varieties.
The experiment was arranged in randomized complete block design (RCBD) with three replications.The spacing between rows and plants was 0.75 m and 0.30 m, respectively.The spacing between plots and adjacent replications was 1 m and 1.5 m, respectively.At both end of each row, tubers of known late blight susceptible (Kellecho) was planted and were used as inoculum sources or "spreader rows".Thus, each genotype or plant in each plot had a chance to receive ample amount of inoculum sources under natural inoculum distribution.The middle rows were used for data collection.

Data collection
Days to late blight appearance (DLA) was recorded by counting days from planting to the first appearance of late blight symptoms in each plot (genotype).
Disease severity was taken on the basis of the percentage of leaf area affected by late blight.The reading was started on the onset of late blight (1-5% infection).After onset of the disease scoring was continued at an interval of seven days until nearly 74 days after planting.The 1-9 disease score scale described by Heinfnings (1987) was used.
Percent severity index (PSI) was calculated from disease severity.
Area under disease progress curve: was calculated according to Campbell and Madden (1990) formula and it was interpreted directly without transformation as the higher the AUDPC, the more susceptible genotype (CIP, 2006).
Where "t" is the time of each reading, "y" is the percent of affected foliage at each reading and "n" is the number of readings.The variable "t" can represent Julian days, days after planting.

Data analysis
Collected data was subjected to analysis of variance (ANOVA) for RCBD following the procedure outlined by Gomez and Gomez (1984) using SAS ver.9.1.3computer software.Means that are significantly different were compared using Duncan Multiple Range Test (DMRT) of probability at 1% and 5% probability of significance.

Analysis of Variance
Analysis of variance indicated the presence of highly significant (P<0.01)differences among genotype for all traits.This may allow breeders to select resistant genotypes to enhance the productivity of the crop.This is in line with Jaime et al. (2014) who indicated significant different among 10 potato cultivars under studies.

Mean performance of genotypes
Potato genotypes had a wide range of variations.Total tuber yield ranged from 0.8 to 46.1 t ha -1 with mean performance of 19.4 t ha -1 (Table 1).Mean total tuber yield of released varieties (Belete, Gudanie and Ararsa) was within the range of 15.9 to 41 t ha -1 .The four advanced clones gave total tuber yield higher than the mean tuber yield of the two released varieties (Gudanie and Ararsa) while one clone (CIP-392640.524)had statistically the same mean total tuber yield with the best performing released variety Belete.Addisu et al. (2013) and Baye (2002) also reported some of the newly introduced potato genotypes had higher tuber yield than the existing commercial potato varieties.This indicated the presence of variation in genotypes under study for total tuber yield that can be exploited in improving the crop.
Late blight appeared early at 48 days after planting on farmers cultivar Kellacho and other five advanced clones (CIP-399053.15,CIP-397079.26,CIP-396039.103,CIP-391930.1 and CIP-391381.9)followed by three advanced clones ) which appeared at 53 days after planting.Late blight appeared very late on CIP-399062.102 at 74 days after planting.The disease appeared at 70 days on Belete and at 63 days on Ararsa and Guddanie.There was wide range difference of 26 days between the first disease symptom appearance on susceptible and relatively resistant genotypes.Jaime et al. (2014) also reported that potato genotypes which developed late blight symptom early are susceptible and genotypes that developed late blight in the crop cycle are resistant.
The calculated percent severity index ranged from 33.00 for CIP-391058.175,CIP-395096.2, CIP-395114.5, CIP-396031.201 and Belete to 91.67% for advanced clone (CIP-391930.1).From all 24 genotypes, there was no genotype with percent severity index of <20 at nearly 80 days after planting.Hence, there were no highly resistant and resistant genotypes from the newly introduced materials and released varieties.However, eleven advanced clones and Belete had percent severity index that ranged from 33.00 to 39.70 and can be categorized as moderately resistance while others including released varieties Guddine and Ararsa recorded above 40% and categorized as susceptible.
The calculated AUDPC ranged from as low as 105 for CIP-391058.175,CIP-399062.102,CIP-395096.2, Bellete and as high as 2370 for CIP-397079.26.Most of the genotypes including the released variety Ararsa and Gudenie had high AUDPC.According to Campbell and Madden (1990), from 24 potato genotypes-CIP-395096.2, CIP-395077.12, CIP-99062.102, Belete, CIP-395114.5, CIP395017.242, CIP-396240.23, CIP391058.175, CIP396029.205, CIP396031.201, CIP-3920524 and CIP-396244.12 categorized as tolerant.However, other genotypes including released varieties Ararsa and Gudaine were considered as susceptible.The AUDPC is very convenient summary of plant disease epidemics that incorporates initial intensity, the rate parameter and duration of epidemic, which determine final disease intensity (Andre et al., 2014).Hence, the effect of disease resistance on crop can be evaluated by using area under disease progress curve (Boiteux et al., 1995).This is in agreement with the finding of Jaime et al. (2014) who reported that high susceptible cultivar Shepody had the highest AUDPC value among 10 potato cultivars in Argentina.If an incompatible potato genotype is released for use by farmers in most cases there will be selection of compatible pathogen population and a corresponding "loss" of resistance (Forbes, 2012).For this reason, some researchers have recommended selection of those phenotypes, which demonstrate resistance, but are still infected (Forbes and Landeo, 2006).

Conclusions
In conclusion, the present study revealed the existence of variability for tuber yield and resistance to late blight.This suggested the higher chance of selecting genotypes with moderate resistance/ tolerance to improve the productivity of the crop.In addition, the future potato improvement program in the study area should include genotypes highly resistant to late blight.The identified promising genotypes in this study showed only moderate resistance to late blight which can be surpassed by the disease, as the pathogen is known with high mutable characteristics that make resistant varieties susceptible soon after they are deployed.

Table 1 .
Mean performance of 24 potato genotypes for yield and late blight disease parameters at Sinana in 2014 bona cropping season Means followed by the same letter within a column are not significantly different at the prescribed level of probability.** = significant and ns = non-significant (at 1% and 5%).MTY marketable tuber yield, unMTY = unmarketable tuber yield, TTY = total tuber yield t ha -1, DLA= days to late blight appearance, PSI = percent severity index, AUDPC= area under disease progress curve, CV= coefficient of variation In 2014 main cropping season, there was high severity of late blight on potato in Bale high lands (personal observation).At 48 days after planting, late blight appeared on some genotypes.This indicated that in the process of improving potato variety through selection attention should be given to assess the disease onset as early as possible.Similar results were reported by Sharma