Western Cape wheat producers who farm according to conservation agriculture principles should aim to implement zero-tillage (using disc planters) and crop rotation with broadleaves to keep outbreaks of Fusarium crown rot (FCR) to a minimum. That is the conclusion a new South African study made on the effect that crop rotation and different tillage practices have in controlling this global disease. It was completed by Mr Stephan Theron as part of his recent MSc studies in Agronomy at Stellenbosch University.

Conservation agriculture (CA) principles such as crop rotation, the retention of residue and reduced soil disturbance are widely applied throughout the Western Cape wheat belt.

FCR is caused by a fungus called Fusarium pseudograminearum and occurs globally. It is also one of the most devastating diseases of wheat in the Western Cape. It impacts the quality of wheat, and can reduce yields by as much as 40% in some cases. Experts believe it will occur more frequently and have a greater impact as droughts become more frequent and prolonged.

“The predictions are that droughts will increase in the Western Cape in future due to climate change. Therefore it is important that we learn how to manage FCR locally as best as possible, and not merely follow guidelines set for other countries that have a different climate or soils from ours,” says Mr Theron.

Crop rotation research

His experimental work was conducted as part of existing long-term soil quality studies on two research farms of the Western Cape Department of Agriculture: Tygerhoek in the southern Cape, and Langgewens in the Swartland. Funding was received from the South African Winter Cereal Industry Trust.

To study the impact of crop rotation, wheat was rotated with either canola (Brassica napus), lupin (Lupinus spp.) or annual medics (Medicago spp.) during the 2020 and 2021 seasons. The effect of these treatments were compared with wheat planted in monoculture.

Within each crop rotation, Theron also tested the impact of tillage practices like zero tillage (no prior soil disturbance, double disc planter), no tillage (no prior soil disturbance, tine planter), minimum tillage (shallow tine tillage, tine planter), and conventional tillage (shallow tine tillage followed by deep soil disturbance with a plough, tine planter).

Theron established that the use of crop rotation with broadleaf crops works well to significantly reduce the incidence and severity of FCR, compared to wheat planted in monoculture. It also improved the yield and quality of harvested wheat.

“Crop rotation with non-host crops, in our case canola, medics and lupin all reduced FCR parameters, mainly by breaking the disease cycle and build-up of disease over years.

“The incidence and severity of the disease is also reduced with zero tillage, where more crop residue is retained on the soil surface. This practice does not influence grain yield or quality,” Mr Theron added.

With zero tillage, seeds were planted shallower than treatments with more soil disturbance. This results in the sub-crown internode being shorter than when seeds were seeded more deeply.

According to Mr Theron, a shorter sub-crown internode has definite benefits, as it boils down to a smaller area that the FCR pathogen can infect. Seedling establishment was better, likely due to the protection of higher residue loads. Soil water conservation later in the season was also better in less disturbed soil.

“This contributes to FCR management, a disease that is generally more severe in drought conditions,” says Mr Theron. Plant residues decomposed faster with less soil disturbance, likely due to better microbe activity. This too helps to decrease FCR levels.

“Crop rotation determines the type of residue present in a field of wheat, whereas the type of tillage used determines the distribution and load thereof,” explains Theron.

Some of his findings are published in the journals Crop Protection and the South African Journal of Plant and Soil. His project was supervised by Prof Pieter Swanepoel, chair of the Department of Agronomy at Stellenbosch University (SU), Dr Lindy Rose of the SU Department of Plant Pathology, and production scientist Dr Gert van Coller of the Directorate of Plant Science at the Western Cape Department of Agriculture.

“Theron’s interdisciplinary study, in which he combined both agronomic and plant pathology information, sheds light on complex questions and shows what farmers can do to sustainably manage and protect wheat from FCR. His recommendations on how wheat producers can manage it have already attracted the attention of the wheat industry, and are set to be impactful,” says Prof Swanepoel.

“His findings provide good news to producers, since they show that conservation agriculture practices like crop rotation and zero tillage, while being cost effective, also reduced FCR levels,” adds Dr Van Coller.

According to Mr Theron, the impact of FCR in Australia was already estimated in 2009 to be around 79 million Australian dollars per year. “Its impact on the South African wheat industry has not yet been quantified,” he notes.

* Theron graduated in December 2022, and works as a field agronomist for Adama South Africa in Cape Town. He graduated from Huguenot High School in Wellington in 2016, and developed a love for farming thanks to numerous family visits to his grandfather’s farm in the Free State. Before pursuing his MSc degree in Agronomy, he completed a BScAgric in Plant Pathology and Agronomy, also from Stellenbosch University.