Phytophthora infestans responses to stinging nettle extract, phosphoric acid and fungicides combination, in Kenya

Abstract

Late blight is a significant disease of solanaceous crops worldwide. Knowledge on sensitivity of late blight pathogen ( Phytopthora infestans ) to metalaxyl and different fungicides is important for effective management of late blight. In vitro studies were carried out to evaluate the sensitivity of P. infestans to different levels of Metalaxyl Ridomil, Dithane M45, phosphate and stinging nettle . Sensitivity to metalaxyl and other different fungic ides was determined by growing P. infestans isolates on 15% V8 medium amended with 0, 5 and 100 ppm metalaxyl and 20ppm of Ridomil, 25 ppm of Dithane M45, 20ppm of Phosphate and 1ml of stinging nettle extract in 1ml of water. The pathogen isolates were from the leaves c ollected from different potato growing areas of Kenya. Sensi tivity was determined by measuring the colony diameter at 14 and 21 days, counting the numbers of spores and determining the weight of the mycelium.The result o f the experiment indicated that there was intermediate resistance of P. infestans to metalaxyl in all the region as the growth was recorded as 57.8% in 5 ppm and 19.8 in 1 00 ppm in Tigoni station and 58.3 in 5 ppm and 19.7 in 100 ppm in Njambini while in Meru region the growth was 52.9 in 5 ppm and 25.9 in 100 ppm metalaxyl relative to the control. Njambini and Tigoni regions did not differ significantly in term of resistance. However both r egions differed significantly with Meru region in terms of weight of mycelium and number of spores, Meru location having the highest number of spores and weight of mycelium compared to both Tigoni and Njambini locations. All the fungicides tested differed significantly in their e ffect on pathogen colony diameter, number of spores and weig ht of mycelium. The reduction of colony diameters of isol ates collected from Tigoni of 41.7 and 80.3 in 5 and 100 ppm metalaxyl did not differ significantly with that fr om Njambini significantly of 42.2 mm and 80.2 mm in 5 and 100 ppm respectively. The pathogen was more sensiti ve to phosphoric acid and least sensitive to stinging net tle extract