By Prof Lucy Moleleki, Department of Biochemistry, Genetics and Microbiology, University of Pretoria. This article was first published in the Nov/Dec issue of CHIPS magazine – “Mouthpiece of the South African potato industry”. Read the original article here. Republished with permission.
The United Nations announced on 15 November 2022 that the world’s population had reached a record of eight billion people. This translates into a high demand for food security, which is already in jeopardy due to the ripple effects of the Covid-19 pandemic and the war in Ukraine. As a result, the cost of food, fuel and fertiliser have skyrocketed, especially in the global south.
The potato has been hailed by some as the ‘saviour’ of the planet. However, a decline in production of this crop has been recorded. The South African potato industry aims to increase potato production by six million tons in the next five years.
To achieve this, cheap, safe, environmentally friendly and effective farming practices should be embraced to not only control notorious pathogens of the potato plant, but also promote plant growth. Regenerative agriculture minimises carbon emissions, promotes biodiversity, protects the soil and ensures agricultural sustainability. Crop rotation is an important method used in regenerative agriculture (Figure 1).
Figure 1: The five arms of regenerative agriculture
Crop rotation is an ‘old school’ method of crop protection, yet it is still very powerful. Practicing crop rotation, for example planting potato, maize, legumes such as soya bean and chickpea, and sunflower (especially where potato soft-rot bacteria is a problem) and after two or three years planting potatoes again, has proven to be an effective method to control pathogens that affect high-quality potato yields. Thus, it is not advisable to plant potatoes in the same field year after year as this causes a build-up of diseases.
Soil-borne potato diseases such as soft rot are increasing. To alleviate this problem, farmers use large amounts of chemicals to control soilborne diseases while applying more and more chemical fertilisers to improve potato yields, which may cause potential environmental pollution. Crop rotation is an environmentally friendly practice that could potentially solve the mentioned problems.
How it works
A crop rotation system influences the microbial community in the soil as well as in the roots of plants. In most cases, this practice retains soil microbes (good bacteria) that can protect plants from soilborne pathogens and improve plant growth by acting as biocontrol agents.
In an ongoing study conducted in our laboratory, we have isolated various plant growth-promoting bacteria (PGPBs) that were collected from soils. Root samples were collected after two years of crop rotation. As the name suggests, PGPBs are known to enhance plant growth.
PGPBs promote plant growth indirectly by decreasing the inhibitory effects of various pathogenic agents on plant growth and development. For instance, the majority of PGPBs such as Bacillus spp. and Pseudomonas spp. could produce molecules called antibiotics/antimicrobials, enzymes that can either inhibit the growth or kill plant pathogens.
These molecules can be either narrow-range (affect few or specific plant pathogens) or broad-range (targets various plant pathogens). Other than producing antibiotics, PGPB produces specialised molecules called siderophores that enable them to get important and scarce nutrients like iron from the soil, thus depriving plant pathogens of these nutrients. This makes it difficult for plant pathogens to grow. PGPB also improves plant growth by reinforcing the immunity of plants.
PGPBs promote plant health directly by either facilitating resource acquisition or modulating plant hormone levels as summarised in Table 1.
Table 1: Direct plant health benefits of plant growth promoting bacteria (PGPBs)
|Facilitating resource acquisition
|Nitrogen fixation, phosphate solubilisation.
|Since nitrogen and phosphorus are the main components of chemical fertilisers used, it is therefore likely that PGPB can be used to substitute chemical fertilisers
|Modulating phytohormone levels
|Cytokinins, gibberellins, indoleacetic acid, ethylene.
|By altering phytohormone levels, PGPBs can affect how plants react to stress. This is important, especially since plants are being exposed to harsh weather conditions due to climate change.
The future of PGPBs
The crop rotation system promotes the growth of PGPB, which in turn suppresses the growth of ‘bad’ bacteria resulting in improved plant health. Some PGPBs have been isolated, identified, and purified and are available commercially.
Despite this, there are still massive opportunities to isolate and identify PGPBs that are well-adapted to South African conditions. Thus, exploring the microbial diversity in soils under a crop rotation system provides a new opportunity towards innovative biocontrol.
Isolation and identification of PGPBs, as we are already doing in our laboratory, is followed by screening for the ability to inhibit plant pathogens, especially those associated with potato diseases. Those that show significant killing/inhibition of microbes can form part of candidates for commercialisation.