Studies on the efficacy and rate of application of some chemical fungicides, their mixtures and water extract of Tagetes minuta in the control of late blight of potato

Late blight of potato ( Solanum tuberosum) caused by Phytophthora infestans is the most destructive disease to the crop which is cultivated by large and small scale farmers mostly in Central, Rift Valley and Eastern provinces. At national level, about 3% of the total agricultural labour force is involved in potato production. A large amount of fungicide is used in the management of late blight as fanners commence fungicide application immediately after the emergence of the crop. Very few fanners use Ridomil alone in their spray programs. Noblite (Fenamidone + Mancozeb) and Tattoo (Propamocarb-HCL + Mancozeb) fungicide mixtures, Dithane M45, Ridomil (as controls) and water extract of Tagetes minuta were evaluated for their efficacy to control late blight of potato caused by P. infestans (Mont.) de. Bary and their effects on total and marketable yield for three seasons under rainfed weather conditions at National Potato Research Centre- Tigoni. Six applications of Noblite at 1.0 kg/ha at seven days interval gave better control (34.9% severity) of the disease and marketable yield (9.3 tons/ha) that compared well with Dithane M45 (38.9% severity) and marketable yield of (9.6tons/ha) under moderate to severe disease conditions of 1999/2000. Six applications of Tattoo (4.0Llha) gave significantly lower disease severity (23.2%) and high marketable yields (16.4tons/ha) compared to Dithane M45 under medium to severe disease conditions. Evaluation of water extract of Tagetes minuta as a potential source of botanical fungicide against late blight showed no significant difference in late blight severity and yields among the rates used (6.7, 3.9 and 4.0 tons/ha for 92.6, 138.9 and 185.2 Llha respectively) and from untreated plots(5.5 tons/ha) for 1999/2000 season. Similar results were obtained in 2000/2001 season. Application of Ridomil (Metalaxyl + Mancozeb) throughout the season was shown to reduce tuber size (75.2 grams/tuber), compared to Dithane M45 (95.2 grams/tuber), Noblite (84.2 grams/tuber) and Tattoo (114.6 grams/tuber). Different spray programs consisting of Noblite, Tattoo and Dithane M45 were also evaluated as implemented in three key potato growing areas in Kenya. The program consisting of two applications of Dithane M45 (2.5Kg/ha) followed by one of Noblite (1.0kg/ha) and one of Tattoo (4L1ha) respectively each applied at 7 days interval gave the best late blight control (5.6% severity) and increased marketable yield (15.4 tons/ha) in Molo region compared to the farmers practice (6.2% severity and marketable yield of 9.9tons/ha) where two applications of Dithane M45 (2.5kg/ha) at 7 days interval and two applications of Ridomil (2.5kg/ha) at 14 days interval are done respectively. The other program with one application of Tattoo (4L/ha) followed by one of Dithane M45 (2.5kg/ha), one of Noblite (1.0kg/ha) and one of Tattoo (4L1ha) respectively at 7 days interval gave good late blight control (4.5% severity) and led to increased yield (14.2 tons/ha) compared to the existing farmers practice where Ridomil is applied 4 times at 14 days interval. In Kinangop region, one application each of Tattoo(41/ha), Dithane M45(2.5kg/ha), Noblite(1.0kg/ha) and Tattoo (4.Llha) respectively applied at 7 days interval gave the best late blight control (6.3%severity) and the highest marketable yield (11.7tons/ha) compared to the farmer's practice which gave 10.7 tons/ha. In Mt. Kenya region where Kerr's pink is widely grown, farmer's practice six applications of a premix of Ridomil (0.54kg/ha) and Dithane M45 (1.4kg/ha) at 7 days interval gave a better blight control (5.2% severity) and higher marketable yield (12.7 tons/ha) compared to the trial programs. The two new fungicides (Noblite and Tattoo) were recommended for inclusion in the list of fungicides for use in the control of potato late blight. It is also recommended that extension staff from the ministry of agriculture explain to potato farmers the cause of late blight and when to control the disease in order to make potato farming a profitable enterprise