Abstract
Phytophthora infestans 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, research recommendation on optimum planting time and fungicide application frequency for high lands area of Arsi (Kulumsa) is lacking. Thus, field experiment was carried out to evaluate the effect of planting time and fungicide application frequency on disease severity, yield and yield components of potato at Kulumsa during main cropping season of 2017. The treatments consisted of four planting time in the months of June and July and four fungicide application frequencies at the rate of 2.5 kg ha-1 of Ridomil Gold M.Z 68% W.P. Treatments were combined in 4 × 4 factorial arrangement and laid out in randomized complete block design with three replicates. Data on disease, tuber yield and yield components were analyzed using Statistical Analysis System Version 9.3. The interaction effect between treatments showed a highly significant (p<0.01) effect on disease incidence and area under disease progress curve and a significant (p<0.05) effect on percent severity index. Early planting in June produced significantly higher number of marketable and total tuber numbers per hill which were progressively reduced in subsequent plantings, whereas delay planting in July resulted in higher number of unmarketable tubers per hill. The interaction between early planting and different fungicide application frequencies gave the highest average tuber weight, marketable and total tuber yield per hectare. Therefore, the combination of early planting in June and twice fungicide application gave the optimum results in the study area. Key words: Disease incidence, fungicide, integrated disease management, potato tuber, production season.  
Highlights
Potato (S. tuberosum L.) is one of the major economically important crops worldwide
This study evaluated the interaction effect of planting time and fungicide application frequency on late blight severity, yield and yield components of potato in the study area to identify the optimum planting time and fungicide application frequency for potato production in the study area
The highest percent severity index (43.83%) was recorded from treatments planted on 15th of June and non-fungicide applied treatments and this value was statistically similar with results obtained from treatments planted on 15th and 30th of July and June and non-fungicide applied treatments
Summary
Potato (S. tuberosum L.) is one of the major economically important crops worldwide. On a global scale, potato is the most important tuber crop, ranking first in volume produced among root and tuber crops followed by cassava, sweet potato, and yam (Central Statistical Agency, 2016). Among the major factors affecting potato production, late blight (Phytophthorainfestans (Mont.) de Bary) is one of the most devastating diseases in Ethiopia mainly due to poor knowhow to properly control the disease (Guchi, 2015). It affects both potato foliage in the field and tuber in the storage which can destroy a crop, causing up to 100% crop loss (Tsedaley, 2014). Late blight disease is becoming a major limiting factor affecting potato production in highland areas of Ethiopia. This study evaluated the interaction effect of planting time and fungicide application frequency on late blight severity, yield and yield components of potato in the study area to identify the optimum planting time and fungicide application frequency for potato production in the study area
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