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Hit the refresh button of your crop system

May 2020

Mahlane Godfrey Kgatle, 
Post-doctoral Research Fellow, 
University of Pretoria. Send an email
to godfrey.kgatle@fabi.up.ac.za

Plant diseases have the ability to destroy an entire yield of various crops in the field. Abundance of infectious plant pathogens (germs) is one of the factors that leads to severe damage of plants by these pathogens. The abundance of these pathogens may be caused by growing the same crop on thesame field over many consecutive years. Farmers are encouraged to practice crop rotation as it has many benefits including replenishing soil nutrients and reducing the amount of pathogens in the field.

South Africa’s population increases at the rate of 1,2% annually; while land available for agriculture remains stable but soil health constantly deteriorates. In an ideal world, we hope that every planted seed (e.g. maize) fully develops into a mature plant that produces its maximum yield in order to maintain food security for the ever-growing population.

However, this is not possible due to influences such as weed, plant diseases and insect pests. Plant diseases have been previously reported to destroy an entire yield of various crops in the field (e.g. Sclerotinia head rot of sunflower (Photo 1a). 

Some of the diseases that affect sunflower production; Sclerotinia head rot (Photo 1a)

Alternaria leaf blight (Photo 1b), and Phoma black stem (Photo 1c)Alternaria leaf blight (Photo 1b), and Phoma black stem (Photo 1c)

Some of the factors that influence the greater damage of plant disease on crops include (i) a defenceless host crop, (ii) infectious pathogen (plant germs), (iii) and a suitable environment for the plant disease to develop. A defenceless host crop may be regarded as a crop cultivar that is not genetically resistant to the pathogens (germs) or one that is injured either by wind-blown soil particles thus making infection by the pathogen easier. An infectious pathogen normally causes the most severe crop damage when it is abundant in the field. One of the causes that lead to the abundance of these pathogens in the field is lack of crop rotation or planting the same crop on the same field over many consecutive seasons.

Lack of crop rotation may later affect crop yield since many pathogens can survive in the soil and on plant debris after the crop has been harvested. The University of Pretoria conducted a study to investigate how long Alternaria leaf blight (ALB), Photo 1b and Phoma black stem (PBS), Photo 1c, were able to survive on infected sunflower debris from a commercial farm. PBS was able to survive for eight months whereas ALB could survive for more than a year on plant debris. Other issues associated with growing one type of crop on the same field year after year may include nutrient depletion.

Crop rotation can therefore be seen as a refresh button in a crop farming system. While crop rotation may reduce the level of disease in the soil; it also improves soil moisture, soil carbon, macro-fauna, crop productivity and is good for conservation purposes. The choice in which crop to rotate would be based on the environment, soil and climate. The crop should still have financial benefits to the farmer or should have economic gains (feed for livestock).

Crop rotation is done by growing plants with different types of crops in succession. For instance, if a farmer plants a field of maize in 2019/2020 growing season; it would be recommendable that he might plant beans in the 2020/2021 growing season. This is because maize consumes a lot of nitrogen and beans return nitrogen to the soil.

Year 2020 has been declared as the International Year of Plant Health (IYPH). IYPH aims to create awareness on the importance of plant health. Plant health is important for developing and implementing strategies that can be used to control and prevent pests, weeds and disease. Plant health therefore plays a key role in combating hunger, protecting biodiversity and boosting economic development. Let us therefore ensure we keep our plants healthy.

Publication: May 2020

Section: Pula/Imvula