Assessing the effectiveness of locally available fungicides in managing Cercospora leaf spot of Swiss chard under field conditions in Botswana

Abstract

Cercospora leaf-spot (CLS), caused by Cercospora beticola, is an important foliar disease of Swiss chard in Botswana. The regularity of outbreaks of the disease in Botswana is a major cause of concern and thus, have necessitated the use of fungicides under field conditions in order to control the disease and bring the crop to maturity. The objective of the study was to assess the most effective, locally available fungicides in the control of CLS of Swiss chard under field conditions. The experiment was carried out in Sebele, where a tractor-ploughed field was partitioned into 24 plots, which were split into three blocks. Seedlings were transplanted into the plots, with each plot containing a maximum of 45 plants which were left for natural infection. Disease control commenced once symptoms were observed, using Copper oxychloride; Benomyl + Mancozeb; Mancozeb + Copper oxychloride; Benomyl; Benomyl + Copper oxychloride; Mancozeb; Benomyl + Mancozeb + Copper oxychloride applied at recommended rates. Where more than one fungicide was used, the fungicides were applied at alternating intervals. Control plants were sprayed with water only. The treatments were applied for a period of twelve weeks at two week intervals. The average number of leaf spots per leaf was recorded two weeks after each fungicide application and samples were collected prior to spraying. Data were subjected to analysis of variance and means separated using LSD test at p=0.05. Cercospora leaf spot severity was significantly lower in all treatments involving Benomyl than in those treatments involving Mancozeb and Copper oxychloride used alone or in combination. This study confirms that when Benomyl is alternated with Copper oxychloride, it is much more effective in managing Cercospora Leaf spot than Mancozeb or Copper oxychloride used alone or alternated.