Effective no-till control of maize root diseases

Root diseases overwinter in crop residues on the land pose a significant threat to no-till enterprises. No-till experts have joined forces to find the best methods of overcoming this challenge. Lloyd Phillips reports.
Issue Date 25 May 2007

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Root diseases overwinter in crop residues on the land pose a significant threat to no-till enterprises. No-till experts have joined forces to find the best methods of overcoming this challenge. Lloyd Phillips reports.

Root diseases of maize under no-till conditions have become such a significant crop health threat in KwaZulu-Natal that the Maize Trust, the Cedara ­Agricultural Research ­Facility of the KZN ­Department of Agriculture and ­Environmental Affairs (DAEA), Omnia ­Fertilizers, farmer Ant Muirhead and the No-Till Club of KZN banded together to sponsor a trial to identify the most ­effective yet ­economically attractive ways of beating these diseases.

ccording to Dr Mart Farina, Omnia Fertilizers’ east coast ­consultant and a specialist in soil fertility, this trial was ­exciting in that it brought together various agricultural disciplines as never before in the country. In addition to Mart’s input, the trial used the skills of Dr Sandra Lamprecht, a plant pathologist at the ­Agricultural Research Council Plant Protection Research ­Institute in Stellenbosch and leader of the project; Guy Thibaud, a ­specialist scientist in soil fertility and plant nutrition for the DAEA; microbiologists Jacomina Bloem and Johan Habig; and nematologists Mariette Marais and Antoinette Swart from the Plant Protection Research Institute in Pretoria.

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Mart feels that such collaboration should have started decades ago. “Nobody has really appreciated how economically important root diseases are, as unlike stalk and leaf diseases, their effects are not readily visible to the naked eye,” he explains. “During a ­preliminary trial last year, we used methyl bromide to kill every ­living organism in trial plots before planting a maize crop into them. This action resulted in a 14% yield increase – 16 tons/ha instead of the 14 tons/ha in untreated plots. This shows just how much of an impact these hidden diseases have without us knowing.”

While the use of soil fumigants like methyl bromide has a ­positive impact on crops planted shortly after application, it is ­generally impractical, uneconomical, and environmentally unfriendly to use soil fumigants on a regular basis. Methyl ­bromide kills off both “good” and “bad” soil ­organisms, although this effect remains only for the short-term and it is also classified as a Class 1 ozone-depleting substance by the US Environmental Protection Agency.
Mart and his colleagues only used methyl bromide to prove how much of a negative effect soil-borne root diseases can have on crop health and yields. “We wanted to find economical and sustainable methods of simulating the effects of soil fumigation, without actually having to actually resort to it as a full-time root-disease treatment,” Mart says. “The possible strategies that we looked at to achieve this goal were alternative crop rotations, biocontrol agents, anhydrous ammonia, and winter fallows.”

Before planting the various trial blocks at Winterton, no-till farmer Ant Muirhead’s farm, the plots all received 200kg of nitrogen per hectare. Some plots were inoculated with ­biocontrol agents at both planting and four weeks after ­planting, while others received the treatments shown in Table 1. ­Thereafter, the plots were sampled by having 10 maize plants randomly taken from each of them at days 21, 70 and 99 after planting.

Detailed analyses
According to Mart, the above-ground parts of these plant s­amples were weighed, and their roots were taken by Dr ­Sandra ­Lamprecht to Stellenbosch for disease ratings. Microbial ­diversity and ­nematode analyses of plot samples were carried out in ­Pretoria, and soil and plant analyses were done at Cedara. Stringent controls were maintained to avoid sample contamination. As the results of these trials were announced before the plots could be harvested, grain crop yields have not yet been determined. “It is important to note that this is only the first trial of its kind that we’ve conducted and therefore its results cannot be considered conclusive,” Mart says. “The results (Table 2) could still be affected by the yield results that still have to come, but even so, further similar trials will need to be conducted to ensure that we achieve scientifically accurate conclusions.” In the tests conducted at Stellenbosch on the samples ­submitted from Ant Muirhead’s farm, the relationship between root disease incidence and yield of maize was excellent.

Crop yields using methyl bromide were 99% higher at 21 days after ­planting than they were in the control plots. At 70 days after planting there was a 16,8% difference between the top anhydrous ammonia plot at this stage and the control plot, in terms of this relationship. The relationship between crown rot and yield on the plots 70 days after planting was similar for that of between root disease and yield for the same sampling date (16,8% difference). The ­relationship between root rot and crown rot 70 days after planting was lowest for the methyl bromide treatment plot and poorest for the control. The difference between root disease incidence and yield 99 days after planting between anhydrous ammonia and the control was 17,3%. The difference between crown rot incidence and yield for the same sampling date was also 17,3% higher for anhydrous ammonia than the control.

“In terms of controlling root and crown rot, anhydrous ammonia performed better overall than the methyl bromide,” Mart says. “This was totally unexpected, an anomaly, because there is no previous research, locally or internationally, to support anhydrous ammonia’s exceptional performance in our trials.” Ammonia lowered the cations and increased the anions in the anhydrous ammonia plot in the first sample (Table 3). In the second sample, some of these figures had actually increased dramatically.

Manganese levels were twice as high as in the first sample of the anhydrous ammonia plot and acted as a potent fungicide. These results were remarkably similar to those obtained in 1991 in a trial at Cedara where another soil fumigant, Busan, was used. Busan is known to increase ammonium (NH4) and manganese levels. Both of these minerals are known to have fungicidal effects. The resultant improved root systems and their penetration into the soil, as a result of the positive effects of increased NH4 in soil, also results in the increased uptake of nitrogen, phosphorus, potassium and zinc from the soil. Benefits of anhydrous ammonia “Anhydrous ammonia also markedly reduced the population of Helicotylenchus nematodes. All these aspects gave us a sound explanation as to the benefits of anhydrous ammonia,” Mart says.

“The second anomaly we found was the dramatic improvement in the Eco-T plot. Until more information becomes available to us, this remarkable improvement over time cannot be explained. It was visually evident after the second application of Eco-T, and so this may mean that the initial recommended application rate of this product is too low.”

A third trial anomaly was linked to how poorly the maize and soya fallows performed over time. Mart says that the results from these two plots were quite unexpected and contrary to ­conventional wisdom, which says that planting crops after a ­fallow period is good for crop health and yield. These two treatments were third and fourth respectively at the first sampling, but by the second sampling were performing worse than the control plot. Mart says that nematodes appeared to be the ­primary cause of this drop in performance of the maize and soya ­fallow plots over time, but added that the reason for this is ­uncertain. He guessed that these nematodes had ­somehow ­promoted disease infection of the plots’ maize crop.

“To confirm and validate the results of this first trial, we are planning on repeating it this coming summer ­season. We will also be taking a closer look at the effects of ­conventional tillage compared to no-till in terms of controlling root ­diseases. We will do the same with double-cropping under ­irrigation versus single-cropping on dryland,” he explains.

In the 2008/09 summer, the no-till experts intend to test new seed treatments, maize cultivars, the relationship between root and stalk rots, the ­pathogenicity of root diseases, and to examine the interactions between fungi and nematodes. In 2010/11, they hope to test the best possible combination of strategies against root diseases in no-till, as indicated by the results from the various trials. Mart continues, “Root rots, in both conventionally tilled and no-till systems, are far more important than is generally appreciated. Their effects can be likened to that of aluminium toxicity – something that most no-till farmers appreciate these days. Fewer roots on a crop result in lower yield.

At this stage, it looks as if anhydrous ammonia offers exciting possibilities as an ameliorant.” “The effects of fallow on soil-borne diseases are ­contrary to conventional wisdom and could well be impacting on dryland no-till production,” Mart concludes. “Eco-T at higher than recommended application rates looks set to reverse previous findings from its use on maize, and to date, the effects of rotations and other biocontrol agents are too close to call.” Contact Dr Mart Farina on 082 889 5233 or e-mail [email protected]. |fw