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Potato early dying (PED) has been observed in various fields in Limpopo.
The disease manifests as the premature senescence and ultimate death of a potato plant. The cause of PED in Limpopo was determined to be the result of a combination of soilborne pathogens and plant stress. The best long-term and environmentally friendly option for the control of soilborne diseases is through the improvement of soil health.
Soil biology is a major component of soil health and contributes significantly to soil quality and productivity. There is a great need to ensure that the introduced soil management practices improve soil quality and will also result in and maintain healthy soil.
Objective A: Establishing a PED screening system
Greenhouse trial
Soil was collected from potato fields with a history of PED problems. Certified potato tubers (Mondial) were planted in 25-litre plastic buckets filled to 70% capacity. The plastic buckets contained soil either from Tom Burke or Vivo.
For the first month, greenhouse day/night temperatures were 28/13°C. In the second month, day/night temperatures were increased to 30/17°C, and further increased for the third and fourth months to day/night temperatures of 38/20°C. Pots were arranged in a randomised block design on the greenhouse benches.
The plants grew vigorously in the early stages without any signs of abnormality or disease. Plants showed yellowing and wilting from 12 weeks onwards.
Enzymatic analysis
Fluorescein diacetate hydrolysis (FDA) is an accepted technique and an accurate and simple method for measuring total microbial activity in a range of environmental samples, including soils. Compost materials can stimulate microbial activity and nutrient mineralisation due to their high organic matter content. An increase in total microbial activity and microbial biomass may lead to an increase in competition for beneficial organisms, which will suppress the pathogenic organisms in the soil.
The soil enzyme activity was high at the start of the greenhouse trial. At the end of the trial, the soil enzyme activity was significantly reduced in both Vivo and Tom Burke soils. The enzyme activity was also significantly lower in the Vivo soil that was not inoculated compared to inoculated soil. Thereby, the active biomass declined over time.
Tuber evaluations
Tubers harvested showed severe symptom development of black dot, silver scurf and scab, whereas only a few tubers showed soft rot symptoms (Figure 3). The high incidence of black dot on the tubers correlates with results from the fungal analysis of the plant material as several Colletotrichum isolates were obtained, as well as previous results from isolates obtained from diseased PED samples.
Pratylenchus sp. was isolated in very low numbers (only three individuals) from the soil. This result is similar to results obtained from samples previously collected in commercial fields with a history of PED in Limpopo. Based on these results, we do not believe that Pratylenchus spp. is a major role-player in PEDs in South Africa. Similar results were obtained for Meloidogyne spp., as only eight juveniles were extracted.
In this study, there was a high ratio of bacterial to hyphal-feeding nematodes. A high ratio of bacterial to hyphal-feeding nematodes indicates that nutrient cycling is occurring rapidly through bacterial decomposition, and it is an indication of disturbed soil. The soil used in the study was transported from Limpopo to Gauteng.
A predominance of fungal-feeding nematodes indicates that the food web is dominated by fungi and that biological nutrient cycling will be relatively slow, and soils are relatively undisturbed. The results of this study showed low numbers of fungal-feeding nematodes, therefore indicating that the soils are disturbed.
The number and biodiversity of the omnivorous nematodes were extremely low. Low populations of omnivorous nematodes indicate that the soil biology is likely to be affected by excessive fertiliser inputs, but also by disturbance through practices such as tillage. High populations of omnivorous and predatory nematodes can suppress soilborne pathogens.
Objective B: Determining the effect of soil health on the severity of PED
The goal was to determine the effect of soil health on the severity of PED. Tubers harvested had symptoms of black dot, silver scurf and scab, while only a few tubers showed soft rot symptoms (Figure 4). The effect of the different treatments such as compost and biological control agents on tuber diseases was also evaluated.
However, none of the treatments significantly reduced the disease incidence.
Figure 4 indicates the effect of the different treatments on disease severity. Treatments include control (unamended soil), inoculated (soil amended with PED pathogens), amended soil with compost, amended soil with Trichoderma species, amended soil with Bacillus amyloliquefaciens, amended soil with chemical products that enhance plant resistance (ISR), and amended soil with Trichoderma sp., B. amyloliquefaciens and compost, Trichoderma sp. and Bacillus amyloliquefaciens (CTB).
The soil enzyme activity was relatively high at the start of the greenhouse trial. The addition of compost and CTB significantly increased the soil enzyme activity. Higher soil enzyme activity may result in an increased active biomass in the soil leading to increased competition with the pathogen complex. At the end of the greenhouse trial, the soil enzyme activity was significantly reduced in the control (uninoculated and inoculated).
The depletion of nutrients in the soil through bacterial uptake can negatively influence the microbial biomass in the soil, thereby lowering the enzyme activity. There was no significant difference in the soil enzyme activity after the trial in the following treatments:
- Soil inoculated with Bacillus.
- Soil inoculated with Trichoderma.
- Amended soil with chemical products that enhance plant resistance.
Conclusions
Certified potato tubers (Mondial) were planted in soil that was collected from sites with a history of problems with PED. The trial was conducted under greenhouse conditions with cooler nights at the beginning of the trial and extreme heat stress in the latter part of the study. Plants showed typical PED symptoms after 12 weeks.
Several fungal pathogens were isolated from root and stem tissues and included Verticillium sp., Colletotrichum sp., Fusarium sp., Rhizoctonia sp. and Alternaria sp. These isolates correlated with the pathogens isolated from diseased plant material samples previously collected in commercial fields with a history of PED in Limpopo. Thus, a screening system against PEDs was established under greenhouse conditions.
The addition of compost and two biological control agents did not significantly reduce the disease severity in the greenhouse trial. Commercially available biocontrol agents may not be able to cope with the high heat stress conditions that lead to PED.
Further studies should be conducted to identify strains that may be better adapted to the conditions in Limpopo and may be better able to compete with the PED pathogen complex. Global warming is a reality and will increase stress conditions in plants.
Likely, PED will dramatically increase in the future as it is linked to plant stress. Using greener technologies to increase plant resilience and reduce disease severity and incidence is vital. We suggest that field trials should be performed to assess the effectiveness of various biocontrol agents under field conditions. – Dr Rene Sutherland, Dr Elsie Cruywagen and Dr Mariette Truter, ARC-Vegetable, Industrial and Medicinal Plants, and Dr Mariette Marais and Dr Chantelle Girgan, ARC-Plant Health and Protection
For more information, contact Dr Sutherland at sutherlandr@arc.agric.za, Dr Truter at TruterM@arc.agric.za, Dr Marais at MaraisM@arc.agric.za or Dr Girgan at JansenC@arc.agric.za.