RESPONSE OF LOCAL POTATO CULTIVARS TO LATE BLIGHT DISEASE (Phytophthora infestans (Mont.) de Bary) UNDER FIELD CONDITIONS

Developing host resistance is an economic and long-term approach to disease management; however, resistance reactions that differ depending on the genotypes. Potato late blight is the devastating disease caused due to Phytophthora infestans (Mont.) de Bary. In order to identify late blight resistance in potato genotypes, seven local potato cultivars (Bardiya Rato Local, Bardiya Seto Local, Cardinal, Deukhuri Rato Local, Deukhuri Seto Local, Kailali Local and Khumal Ujjowal) were evaluated in randomized complete block design (RCBD) with three replications during October 2018 to January 2019 on naturally infested soils in Deukhuri, Dang, Nepal. Results showed that significantly the highest disease incidence (99.17%), and the highest disease severity (88%) were found on Cardinal followed by Deukhuri Rato Local (64%), and Deukhuri Seto Local (60%). Potato cultivar namely Khumal Ujjowal was moderately resistant, whereas Deukhuri Rato local and Deukhuri Seto Local were susceptible to late blight disease. Significantly the highest yield (12.67 t ha) was produced by Khumal Ujjowal followed by Bardiya Rato Local (10.78 t ha) and Bardiya Seto Local (9.40 t ha). The disease incidence and Area under disease progressive curve (AUDPC) value was negatively co-related with the tuber yield. The potato cultivar Khumal Ujjowal followed by Bardiya Rato Local were found moderately resistant to late blight disease in Deukhuri conditions. This study suggests that potato cultivar Bardiya Rato Local can be grown for higher tuber production in Dang and similar topographic regions.


INTRODUCTION
Potato (Solanum tuberosum L.) is one of the most important vegetables and agronomic crops grown in Nepal. It ranks fourth important staple crop after rice, maize and wheat in Nepal (Gairhe et al., 2017). The crop is grown across the broad agro-ecological conditions from 100 m to 4000 m altitude (Dahal & Rijal, 2019). The productivity is quite low (14.7 t/ha) (MoALD, 2019) compared with other developed agricultural countries. Potato is also an important crop from a viewpoint of food security and even in increasing poverty reduction measures. Among the various constraints including biotic and abiotic causes, Late blight caused by Phytophthora infestans (Mont.) de Bary. is one of the major (Hardy et al., 1995) cause of reducing yield in almost all potato growing area among the world. In Nepal, the disease was first reported between 1883 and 1897 and has become epidemic since the mid-1990s (Shrestha, 1976). It appears occasionally in the plains (Terai) rather it occurs in epidemic conditions in high hill every year (Shrestha et al., 1998). Sometime about 50-90% loss has been reported in the terai similarly more than 75% in the high hill (Shrestha, 2000). Potato varieties grown in Nepal have low levels of general resistance to late blight disease (André et al., 2014).
The genus Phytophthora derived from Greek words: Phyton = plant and Phthora = destroyer; and in case of Phytophthora infestans this name makes sense (Mizubuti et al., 2007). The pathogen seriously infects potatoes as well as tomatoes and some other members of the family Solanaceae (Agrios, 2005). Internal inoculum sources include infected seed tubers, infected volunteer plants and cull piles around the field which play major roles in late blight epidemics in temperate regions (Powelson et al., 2002). External inoculum sources include infected potato debris from previous seasons which contribute a major role in the disease epidemics in tropical and subtropical regions (Lima et al., 2009).
Mostly the commercial potato farmers rely on fungicide applications for control of late blight (Neupane et al., 2018). Fungicide treatments for the management are not economic and the frequent and haphazard use of fungicide creates for the emergence of new virulent races which may cause havoc in potato cultivation by the breakdown of resistance even in resistant cultivars . Moreover, the use of host resistance genotype can allow a significant reduction in fungicides application along with maintaining the yield and quality of the produce (Inglis et al., 1996). Low cost and effective management of late blight requires resistant varieties. Thus, host plant resistance is the most economical, effective, efficient, and safe measure. The resistance level for each potato variety is to be screened and updated each year. Screening of the existing genotype is necessary to find out the resistant source of variety or genotype against late blight disease. So, the present study was undertaken to find out the late blight resistant potato cultivars for the efficient management of late blight disease of potato.

Experimental location
The experiment was conducted in research plot of Prithu Technical College, IAAS, Dang which is located in Province 5, Lamahi Municipality, Ward No. 1, Dang, Nepal. The experimental site was situated at 410 km west from Kathmandu, the capital of Nepal and 7 km west from Lamahi. Geographically, it is located at 27.9904' N Latitude and 82.3018' E Longitudes at the elevation of 275m masl.

Climatic observation
Experiment was conducted in winter season from October 29, 2018 to January 30, 2019. The site has winter type of climate and data of temperature ( o C), relative humidity, solar intensity, wind velocity (m s -1 ) and total rainfall (mm day -1 ) were described below Table 1. The maximum temperature was recorded on 29 th October and the minimum on 28 th January. Similarly, relative humidity was highest on 28 th January. The maximum rainfall was recorded during 28 th January, as shown in Table 1.

Experimental design, treatment and cultural practices
Seven potato varieties collected locally were tested in the experiment. The name of potato cultivars was given in Table 2. The experiment was conducted on randomized complete block design with 3 replications. Individual plot size was 7.2 m 2 (3 m× 2.4 m) and the total area of research field was 208.08 m 2 (20.4 m× 10.2 m). There were 4 rows in individual plot at 60 cm apart and each row consists of 12 plants. The spacing between treatment and replication was 60 cm.
Two manual weeding were done throughout the potato growing period. First weeding was done 20 days after sowing (DAS) and second on 45 DAS. Irrigations were done prior to sowing, 30 DAS and 45 DAS. Irrigation was applied on furrows. Water application was stopped 2 weeks prior to harvesting to enhance storage life. Earthing up was done to cover the tuber properly on 60 DAS. Plant nutrient in the form of N, P2O5 and K2O @ 100:100:60 kg NPK ha -1 , through urea, di-ammonium phosphate and murate of potash were applied on the furrows and covered with soil just prior to planting. Sprouted potato seed tubers of approximately similar physiological age were planted at 5-6 cm depth in ridges.

Scoring and data collection
Ten plants were randomly selected from the middle of the plot and were tagged for disease scoring. Data on disease scoring was recorded from all tagged plants in 0-5 scoring scale. AUDPC value, disease incidence and disease severity were recorded for determining disease infestation and total yield was also recorded for observing diseases impact in plant. From each experiment plot, total yield were taken from the net harvested area (7.2 m 2 ) at the time of harvesting. Tubers were cleaned properly and each grading was weighed separately from each plot. The weight of tuber from each net plot was recorded. The data were then converted in tones per hectare (t ha -1 ). Percentage disease severity was then calculated using the following formula; The area under the disease progress curve (AUDPC) was estimated using the following formula.
where, Yi= late blight disease severity % on the i th date ti= date from sowing up to date of disease scored n= numbers of dates on which disease was scored Harvesting was done manually after taking all the data on 20 th January 2019. From each experiment plot, total yield was recorded from the net harvested area (7.2 m 2 ). Tubers were cleaned properly and each grading was weighed separately from each plot.

Statistical analysis
Data were collected and analyzed by analysis of variance (ANOVA). Means were compared by the least significance difference (LSD) at 5% levels of significance (Gomez & Gomez, 1984;J. Shrestha, 2019). Statistical software used for data analysis was Microsoft Excel 2010 and R version.

Relationship between tuber yield and disease incidence and severity
There was negative linear relationship between yield and disease incidence. About 13% variation in grain yield was due to disease incidence and remaining portion due to other factors. Similarly, Tuber yield was also negatively correlated with mean AUDPC. About 21% variation in grain yield was due to AUDPC and remaining portion due to other factors.

Discussion
Different potato cultivars tested under natural infection showed different response to late blight. Due to different genetic background, prevailing environmental conditions etc., differential rate of disease development may have been recorded in different cultivars. The most effective approach to combat plant disease is the use of host resistance (Subedi, 2015). In the present study Khumal Ujjowal was taken as resistant check and Cardinal as susceptible check. The results clearly indicated that the resistance and susceptibility of these genotypes are still maintained as reported by earlier workers. Rate of disease progress was less in Bardiya Rato Local and Bardiya Seto Local compared to other cultivars indicating their higher level of partial resistance in these cultivars. Late blight symptoms first appeared on Bardiya Rato Local (40 DAP) followed by Cardinal (Susceptible Check, 41 DAP) and Deukhuri Seto Local (41 DAP). 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. Sharma et al. (2013) reported that late blight severity observations in the field started at 30 days after planting and continued up to 76 days until the susceptible check had 100% infection.
It was observed that late blight directly affect the yield attributing characters like vine mass and tuber yield. The highest disease severity (up to 88%) was recorded in susceptible check plot indicates suitable climate for late blight development in the tested site. The present study shows the significant effect of late blight in reducing the tuber yield. Similar findings were also reported by Shrestha et al. (2019). It clearly indicates that the higher yield under low disease condition and lower yield in higher disease severity. The results showed that selection of resistant cultivar has significant effect on late blight control and increase in tuber yield under field conditions when compared with susceptible cultivar, though it varies on yield potentiality of the cultivar. The result clearly indicated that the cultivation of the Bardiya Rato Local cultivar is effective for the management of late blight of potato as obtained by the lowest mean AUDPC (183), lowest disease severity (21.3%) and the highest yield (12.67 t ha -1 ). Addisu et al. (2013) and Baye (2002) also reported potato genotypes had higher tuber yield than the existing commercial potato varieties.

CONCLUSION
From this experiment we found that potato cultivars Bardiya Rato Local and Khumal Ujjowal appear more resistant and produced higher tuber yield than other cultivars. The identified late blight resistant potato cultivars could be used for the source parents to breeding programs to develop late blight resistant potato varieties in Dang, Deukhuri valley of Nepal.