Identification of soft rot species found in the Harare area of Zimbabwe and an evaluation of the susceptibility of selected potato cultivars of the pathogens

Abstract

Experiments were conducted at the University of Zimbabwe, Crop Science Department from 2012- 2014 with the overall aim of identifying soft rot pathogens in the peri-urban Harare area and to screen some of the locally grown potato cultivars for resistance to these pathogens. In the first experiment, potato tubers and stems showing soft rot and blackleg symptoms respectively were purposely collected from eight commercial potato farms within a 60 km radius of Harare. Using standard biochemical tests, a polymerase chain reaction and sequencing, the predominant soft rot pathogen in the area was found to be Dickeya species. In experiment II, five potato cultivars namely Amethyst, BP1, Jasper, Montclaire and KY20 were evaluated for their response to blackleg caused by Pectobacterium carotovorum subsp. brasiliensis (Pcb) in a potted experiment. A randomised complete block design with four blocks was used. Six-week old plants were inoculated in the stem with 6x108 cfuml-1Pcb suspension. Data on disease incidence was collected after one week. Analysis of variance on log10 transformed disease incidence data showed no significant differences among cultivars (P>0.05). All the cultivars showed typical blackleg symptoms and were susceptible to blackleg caused by Pcb. The objective of experiment III was to evaluate the biochemical defence mechanisms of the five cultivars by analysing the activities of two defence enzymes, polyphenol oxidase (PPO) and peroxidase (POD) in response to pathogen infection and mechanical damage. The experiment was arranged in a Completely Randomised Design with a 5*6 factorial treatment structure replicated twice. The moderately resistant cultivar Montclaire was used as the control. Six week old plants were stem inoculated with 6 x 108 cfuml- 1Pcb or wounded by cutting off the plant’s apical bud. Enzyme assays were conducted on leaf samples taken at 0, 12, 24, 48, 84 h and one week after treatments. Four leaves from the middle of the plant were excised using the destructive sampling method. Results showed there was no significant difference on PPO and POD activity in inoculated and wounded plants for both seasons (P>0.05). The interaction between time and cultivar had a significant effect on enzyme activity (P<0.05) with differences in POD activity observed at 12, 24 and 84 h at which Montclaire showed the highest enzyme activity, Amethyst, BP1 and Jasper were intermediate with KY20 exhibiting the lowest enzyme activity. For PPO, the interaction between cultivar and time was significant in 2012 only with Montclaire showing a higher activity than the other cultivars at 12, 24 and 84 h. These results suggest that both pathogen infection and mechanical damage can induce PPO and POD although the level of induction depends on other factors such as cultivar and time after treatment. Experiment IV was carried out to find out if foliar sprays of salicylic acid (SA) and acibenzolar-s-methyl (ASM) can cause an induction of PPO and POD. The above mentioned five cultivars were used. The experiment was a Completely Randomised Block Design with a 5*3*3 factorial treatment structure. Replication was done across blocks. In the three treatment groups, four week old plants were sprayed with 1.5 mM SA; 100 mg active ingredient/litre (a.i/l) ASM and water (control) till run-off. Sampling was carried out as in experiment III and assays were done at 0, 3 and 6 days after treatments. Results showed that cultivars had a significant effect on PPO and POD activity (P<0.05). The interaction between treatment and time showed a significant effect on enzyme activity (P<0.05). The control treatment exhibited enzyme activities higher or equal to SA and ASM treatments. Results suggest that 1.5 mM SA and 100 mg a.i/l ASM applied as foliar sprays were not effective in enhancing PPO and POD activity. Control of soft rot pathogens using on-farm cultural practices still remains an effective management strategy to reduce losses from soft rots. Defence enzymes alone were not enough to protect the plants as all the cultivars proved susceptible to blackleg caused by Pcb.With Dickeya species now the predominant soft rot pathogen in the area under study, an understanding of these plant-pathogen systems enables the implementation of appropriate control measures in order to minimise losses.