Efficacy of phosphoric acid and stinging nettle extract in the management of late blight of potato
Late blight of potato caused by P. infestans is the major disease constraint to potato production in Kenya. Knowledge on sensitivity of late blight pathogen (Phytopthora infestans) to metalaxyl and different fungicide and evaluation of alternative fungicides is important for effective management of late blight. Studies were carried out at two sites in Kenya to evaluate the efficacy of phosphoric acid and stinging nettle compared to other commonly used fungicides, Ridomil and Dithane M45 in the management of potato late blight. lnvitro studies were also carried out to evaluate the sensitivity of P. infestans from the major potato growing regions to different levels of metalaxyl, Ridomil, Dithane M45, phosphite and stinging nettle extract. The experiments in both locations were conducted .in 4.5x 3 m. plot with 6 rows and 10 tubers per row in a randomized complete block design laid down as a split plot and replicated three times. The effects of these fungicides alone or in combination on epidemic development, lesion growth rate and number of lesions were measured. 0 fungicide sprayed alone or in combination completely arrested epidemic development under the environmental conditions of these experiments. However, alternating Ridomil with Phosphite fungicide had the most suppressive effect with relative area under disease progress curve (R.AUDPC) and percentage disease severity of 4.8 and 5.5 in Tigoni and Marimba locations respectively. The mechanism of effect included suppression of disease progress and lesion expansion. Phosphoric acid when alternated with Ridomil resulted into the least numbers and size of lesions. It was also observed that phosphoric acid alone resulted to significantly higher yield compared to Ridomil in both locations. Plot treated with stinging nettle extract differed significantly with respect to (R.AUDPC) and percentage disease severity with other fungicide tested apart from Dithane M45, however both stinging nettle and Dithane M45 differed significantly compared with control in terms of disease control and increase in yields. Sensitivity to metalaxyl and other different fungicides was determined by culturing P. infestans isolates on 15% V8 medium amended with 0, 5 and 100 ppm metalaxyl and 20ppm of Ridomil, 25ppm of Dithane M45, 20ppm of Phosphite and 1ml of stinging nettle extract in 1ml of water. Sensitivity was determined by measuring the colony diameter at 14 and 21 days, counting the numbers of spores and determining the dry weight of the mycelium. The colony diameter reduction of isolates which were collected from Tigoni research station of 41.7 and 80.3mm in 5 and 100ppm respectively did not differ signifi-cantly with the colony diameter of isolates from Njambini which was 42.2 and 80.2mm in 5 and 100ppm respectively, however both Tigoni and Njambini location differed significantly from colony diameter of those isolates which were collected from Marimba of 4l.7 and 74.1mm in 5 and 100 ppm metalaxyl respectively. The result of the experiment indicated that there was intermediate resistance of P. infestans metalaxyl in the entire region All the fungicides tested differed significantly in their effect on pathogen colony diameter, number of spores and weight of mycelium. The pathogen was more sensitive to phosphite and least sensitive to stinging nettle extract. These studies demonstrate that incorporating alternative fungicides like phosphate may be beneficial in the management of potato late blight as well as avoiding development of metalaxyl resistance. Further studies should be carried out to determine the chemical responsible in stinging nettle, application rate and frequency of phosphate and stinging nettle extract in the management of late blight.