Effects of cabbage plant residues and chemical soil fumigation on bacterial wilt caused by soil-borne ralstonia solanacearum
Ralstoniasolanacearum (RS) is the causal agent of bacterial wilt (BW) which affects over 200 species of tropical and subtropical plants. The study of these bacteria is of economic importance since it poses a big threat to the solanaceous crops. The objective of this study was to determine the efficacy of inorganic compounds treatment(s) namely: Chalim™ (Calcium hypochrolite), Metham sodium (MS) (positive control) and negative control (Co) versus organic compound such as Brassica spp. against soil-borne-inocula in the field as well as in the laboratory. Fumigation using synthetic and non-synthetic materials is one such mechanism three test crops: tomato, capsicum and potato were used. Suspected diseased plants samples were collected randomly from Thika and Kangemi areas, identified by symptoms and confirmed by 'bacteria streaming test' in the laboratory at KARI-NARL (Kenya Agricultural Research Institute National Agricultural Laboratories). The positively identified capsicum and tomato stems and potato tubers were used for isolation, purification, multiplication of RS as well as making composite bacterial inocula. Soil was sampled for initial bacteria counts and soil pH. The inocula were sprayed evenly in the. 24- field plots each (4.5x2.7 M). Three plots were used as negative controls (Co) and three positive (MS). The study established higher disease incidence in tomato and capsicum relative to potato. The study findings reported significant difference in Bacteria wilt disease incidences ranging from 0.93 % - 30.1 %,)0.65 % - 33.1 % and 7.18 % - 49.3 % for potato, capsicum and tomato respectively. Brassica treatment(BT) at 3096 treatment was the most effective compared to the control in the field though not significantly different at 95% from other treatments followed by Brassica treatment(BT) at 5292g and Chalim™ at 303.75g (22.2 %, 32.1 % and 32.1 % respectively). The least effective were Brassica treatment at 1908 and CM607.5 (43.8 % and 37.0 % respectively). Subsequently, the in vitro experiments were conducted where, Chalim ™ treatments were prepared at the rates of: 0,0.5, 1,2,3,4,5, 7.5, and 10% while Brassica crude extract were prepared at 0, 0.5, 1,2, 3,4, 5, 7.5, 10, 15,20,25, and 30%. Chloramphenical and penicillin (antibiotics) were used as positive controls and sterile distilled water as negative control. The study results indicated that Ralstonia solanacearum was more sensitive to chloramphenical in nutrient agar with an inhibitory zone of about 28 mm, followed by Casamino peptide glucose agar (CPG) with chloramphenical (24mm). The cumulative concentrations in nutrient agar and CPG media with Brassica crude extract exhibited a moderate inhibitory zone of 2.5 mm, while CPG and nutrient agar with ChalimTM had 1.5 mm and lmm respectively. Ralstonia solanacearum greatly resisted Casamino peptide glucose agar (CPG) and nutrient agar media when combined with either sterile distilled water or penicillin in which the RS colonies appeared fluidy with dense growth. The study results implies that it is advisable for farmers to use Brassica at highest level (4355.56 kg/ha) and moderately use ChalimTM(250 kg/ha) interchangeably with Metham sodium (2000Llha). These treatments lowered the disease incidences in the test crops at that rate.