Efficacy of Reduced Dose of Fungicide Sprays in the Management of Late Blight (Phytophthora infestans) Disease on Selected Potato (Solanum tuberosum L.) Varieties Haramaya, Eastern Ethiopia

Potato is the most worldwide important tuber crop. Potato late blight disease is the major bottleneck in potato production in Ethiopia. The Field experiments were carried at Haramaya University during 2010 to determine the efficacy of reduced dose of fungicide sprays in the management of late blight disease on selected potato varieties. Combinations of five potato varieties with different levels of resistance and five rates of Ridomil applications were used. The experiment was laid out in a factorial arrangement in randomized complete block design with three replications. The combination of moderately resistant, moderately susceptible and susceptible varieties with different rates of Ridomil applications varied significantly (P < 0.001) in disease severity, disease progress rate and area under disease progress curve. Onset of the disease was delayed by almost 20 days on the moderately resistant varieties as compared to the moderately susceptible and susceptible varieties. Up to 80, 76, 75.5, 69 and 68% severity reduction was obtained from the varieties Bedassa, Gabissa, Chiro, Harchassa and Zemen plots treated with Ridomil at 3 kg ha-1 rate of application, respectively. The highest disease severity and lowest yield were recorded from the control plots, especially on moderately susceptible and susceptible varieties. The best management of late blight and high marginal rate of return was obtained on plots treated with combinations of all tested potato varieties and 0.75 kg ha-1 Ridomil applications followed by 1.5 kg ha-1 Ridomil. The lowest rate of marginal return was obtained at combinations of these varieties with the 3 kg ha-1 Ridomil. This study revealed that reduced rates of Ridomil resulted in better management of potato late blight. However, further studies at different agro-ecological zones of the country are important for preference of potato varieties and specific rates of Ridomil as opposed to approval of the blanket recommendation of the manufacturer. Keywords: Late blight, Potato, Area under disease progress curve, Ridomil, Disease severity