Effect of Planting Time and Fungicide Application Frequency on Severity of Late Blight (Phytophthora infestans (Mont.) de Bary), Yield, Yield Components and Quality of Potato (Solanum tuberosum L.) at Kulumsa, Southeast Ethiopia

Potato (Solanum tuberosum L.) is one of the major economically important crops worldwide. Potato late blight disease (Phytophthora infestans (Mont.) de Bary) is the major bottleneck in potato production in Ethiopia. Selection of appropriate planting time is highly location specific and the most important subject in rain-fed potato production. However, there is no research recommendation pertaining to appropriate planting time with minimum fungicide application frequency due consideration of the environmental condition of high land area of Arsi (Kulumsa). Thus, field experiment was carried out to evaluate the interaction effect of planting time and fungicide application frequency on disease severity, yield and qualities of potato at Kulumsa during main cropping season of 2017. The treatments consisted of four planting time (15 th and 30 th of June and July) and four fungicide application frequencies (none, once, twice and three times) applied in the rate of 2.5 kg ha -1 using 400 litters of water (0.00625 g ml -1 ) which were combined in 4 x 4 factorial arrangement and laid out in randomized complete block design with three replicates. Data were collected on growth, disease severity, yield, yield components and quality parameters and analyzed using SAS Version 9.3 statistical software. The result showed that early planting (15 th of June) delayed emergence, flowering and physiological maturity by about 5, 6 and 10 days, respectively and increased plant height by 10.68 cm compared to the last planting on 30 th July. The interaction effect between treatments showed a highly significant (p<0.01) difference on disease incidence and area under disease progress curve and a significant (p<0.05) difference on percent severity index. Early planting in June produced significantly heavier tuber weight (83.80 g) which progressively reduced in subsequent plantings, whereas delay planting in July resulted in higher unmarketable yield. Tuber specific gravity (1.092) and dry matter content (23.02) showed the highest value in early planting on 15 th of June and decreased with late planting dates of July. Correlation analysis also indicated that the relationship between marketable tuber yield with plant height and average tuber weight was positive and highly significant (P<0.01). On the other hand, negative and highly significant correlations were determined between plant height, leaf area index and marketable tuber yield with unmarketable tuber yield. Partial budget analysis shows that early planting on 15 th of June and twice fungicide applied treatments gave the highest marginal net benefit (139, 408.89 ETB ha -1 ) which is 36.8% more benefit than control. Therefore, based on the findings of the current study, early planting in June at the on-set of rain fall and twice fungicide application has been identified as the best time of planting and fungicide application frequency in terms of yield and partial budget analysis to produce potato efficiently and effectively in the study area. So it would be advisable conducting the experiment with different improved potato cultivars, planting time and fungicide application frequency to assess their response to late blight severity.