crops – soilborne diseases

Getting to the root of healthy maize
The management of soilborne plant diseases is one of the greatest
challenges facing modern agriculture, as Lloyd Phillips discovered during
an information day of the KwaZulu-Natal No-Till Club in Bergville.

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Getting to the root of healthy maize

The management of soilborne plant diseases is one of the greatest
challenges facing modern agriculture, as Lloyd Phillips discovered during
an information day of the KwaZulu-Natal No-Till Club in Bergville.

Soilborne diseases are among the most important yield-limiting factors in crop production worldwide. A number of maize diseases commonly found in conservation tillage systems do not manifest on the visible plant structures above ground, yet ultimately have a severely negative effect on an enterprise’s profitability.
In 2001, agricultural scientist Dr Jim Cook said, “If we can’t see these symptoms then we don’t know what damage they’re doing to the maize crop. Diseased root systems are unable to explore the soil for nutrients. Root diseases have become so widespread and occur so uniformly within fields that we have come to accept these crops as normal or healthy. However, if the above-ground parts of our cereal crops were as damaged and diseased as the roots, there would be no end to support and efforts until the problems were solved.”
Speaking recently at an information day hosted by the KZN No-Till Club in Bergville, Dr Sandra Lamprecht, a plant pathologist at the ARC Plant Protection Research Institute in Stellenbosch, said that shifts in farming practices such as conservation tillage profoundly impact on plant pathogens.
“Many pathogens increase to ­damaging levels under conservation tillage,” she explained. “While there is a considerable amount of information available on the effect of conservation tillage on ear and stalk rot of maize, information on this farming system’s effect on soilborne diseases of maize in South Africa is almost non-existent.”
Maize crop residues routinely retained under conservation tillage provide an environment for soilborne pathogens such as Fusarium graminearum to survive until the next crop season. Increased soil moisture levels, a result of this farming practice, favour the establishment and proliferation of pathogenic Pythium spp In the United States, it has been found that incidences of ­Rhizoctonia crown and brace root rot have increased because the soil under conservation tillage is only minimally disturbed.
In 1999, the American agricultural scientist Dr Donald White said, “Root rots are among the least studied and least understood diseases of corn [maize], even though they occur on every corn plant in every field, every year. Yield loss cannot be accurately estimated, because techniques are not available to compare yields of healthy and diseased corn plants.”
Dr Lamprecht said experiments are underway in South Africa, using soil fumigation as an experimental tool to determine yield losses caused by soilborne diseases. Tests have already shown that maize crop yields can be significantly increased after soil fumigation.
“Farmers in KZN expressed concern about soilborne diseases in maize planted after wheat. Trials were set up to identify the dominant soilborne pathogens in maize following winter wheat in no-till systems in the province, and to quantify the effects on growth and yield,” explains Dr Lamprecht. “We also wanted to test the effects of two biocides, and assess the effects of soilborne diseases on nutrient uptake and the effect of soil fumigation and biocides on soil microbial diversity.”
The trials took place at Bergville and Winterton. Four treatments – control, methyl bromide fumigation and two biocides – were compared. Root samples were collected from all plots during the season and grain yields were determined at harvest. Root samples were analysed to determine the dominant pathogens.
Until flowering, the mass of plants from the fumigated plots significantly exceeded those from the other treatments, but thereafter the differences decreased. However, root and crown rot severity on the maize plants was significantly lower on the methyl bromide-treated plots until the last sampling (soft dough stage). The other three treatments didn’t differ from one another.
Dr Lamprecht added, “The effect of the four treatments on the grain yield of the maize under trial showed that methyl bromide-treated plots in Winterton yielded 16t/ha, while the other treatments did not differ from one another and averaged 14t/h. The Bergville yields were noticeably lower than Winterton’s, because of environmental conditions and a high incidence of grey leaf spot. The results were inconclusive.”
The trials identified a variety of fungi species in the soils, with Fusarium graminearum prominent in Bergville and Pyrenochaeta terrestris and Pythium spp prominent in Winterton. During the trial maize’s growing season, Fusarium graminearum and Pythium spp. increased, while Pyrenochaeta terrestris decreased. A number of plant parasitic nematode species were also identified in the soils, including Helicotylenchus dihystra and Meloidogyne spp.

Trials’ results
At the end of the trials, Dr ­Lamprecht and her team were able to reach a number of conclusions:

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Soilborne diseases cause significant losses of maize that follow wheat in no-till systems in KwaZulu-Natal.

Treatments had no effect on plant nutrition and growth, and yield responses were possibly the result of enhanced moisture recovery associated with improved root health.

Crown and root rot of maize respectively were caused by a complex of fungi and possibly nematodes.

The two biocides tested did not differ significantly from the control with regard to growth, yield, root and crown rot severity, and the pathogenic fungi isolated.

The trial confirmed results of previous studies with regard to the incidence of Fusarium graminearum in wheat-maize rotations.

Trichoderma spp were the most predominant fungi isolated from the crowns and roots in trial. Trichoderma spp have been listed as both pathogens of maize as well as bio-control agents against maize diseases.

Plant parasitic nematodes were isolated from the maize roots and rhizosphere soil, but their relative importance still needs to be determined.

Carbon utilisation profiles and soil enzyme activity measurements can provide useful information on the effect of microbial diversity and activity in the soil.

There was more enzyme activity in the Winterton soil than in the Bergville soil. Reduced enzyme activity is an indication of soil degradation.

Fumigation of soil as an experimental tool to reduce soilborne diseases can be successfully used to study the effects of soilborne diseases on maize yields.

Highlighted was the need to establish rotational systems capable of simulating the effects of methyl bromide on plant health, and to determine the relative importance of fungi associated with diseased maize crowns and roots.

There is a need to determine the impact of crops other than wheat – and bio-control agents – on soilborne pathogens of maize and microbial activity and diversity in the soil.
For more information contact Dr ­Sandra Lamprecht on (021) 887 4690/1 or ­­
e-mail [email protected]. |fw

Future research is imperative
“Crop rotation is one of the most important tools to manage crop diseases under conservation tillage systems,” said plant pathologist Dr Sandra Lamprecht. “Future research must evaluate the impact of crop rotation and some biocides on yield, microbial activity and diversity in the soil, and on soilborne diseases. The research must also evaluate the relative importance of soilborne pathogens such as fungi and nematodes on the growth and yield of maize, and markers must be developed for sustainable maize production.”
Dr Lamprecht said the management of soilborne plant diseases is one of the greatest challenges facing modern agriculture. Crop rotation with non-hosts of the pathogens that require control is considered the most important cultural factor for reducing overall disease problems in crops. Crop rotation should be used with other cultural practices such as weed control; however, obstacles to successful disease management using crop rotations are the long lifespan of the diseases, and the wide host ranges of some pathogens such as Sclerotinia spp.
While monoculture maize is known to increase the level of root rot experienced in the crop, agriculturist Dr Mart Farina found that rotating maize with soya increases the maize yield significantly. On the other hand, the rotation of maize with wheat results in an increase in scab of wheat, and ear and stalk rot of maize caused by Fusarium graminearum. Unfortunately, information on the effect of crop rotation on soilborne diseases of maize in SA is very limited.
According to agriculture scientist Dr Jim Cook research programmes must take into account the ”likelihood that clean [conventional] tillage is obsolete and that direct seeding [minimum tillage] will become the conventional method for the production of cereals and probably all broad-acre dryland crops worldwide”.

Soilborne diseases cause significant losses of maize that follow wheat production in no-till systems in KZN.