This season, Iowa soybean producers have been challenged by several disease problems and some problems may revisit in the next soybean crop if proper management is not applied. Although tillage has been considered a measure to reduce disease risk, not all diseases (for example, viral diseases with insect vectors) are lessened by tillage practices. Effectiveness of tillage in disease control varies depending on the biology of a disease. Knowing how tillage affects the occurrence of a disease can help minimize tillage efforts and achieve maximum control. This article discusses effects and timing of tillage measures for major soybean diseases this past growing season.

Tillage effects

Tillage reduces disease risk mainly by

1. reducing disease inoculum by destroying infested residues,
2. physically separating a pathogen from soybean plants by burying the pathogen deep in soil, and
3. changing the soil conditions (for example, increasing temperature and reducing moisture).

Efficacy of each of effect depends on the biology of a disease and the timing of tillage application. Table 1 lists how tillage affects the major soybean diseases that occurred in the 2002 growing season.

Tillage is very effective in reducing risks of the foliar diseases frogeye leaf spot, Phomopsis, and Cercospora leaf spot (also called purple seed stain). Pathogens of these diseases have to survive on infested residues in the absence of soybean crops. Tillage helps decompose infested residues. The earlier a tillage operation is made, the more the pathogen population decreases. Operations after harvest of infested soybean crops provide better control than those made after rotating with corn if a 1-year corn-soybean rotation scheme is used.

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Frogeye leaf spot caused by Cercospora sojina..

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Tip blight caused by Phomposis.

For sudden death syndrome (SDS), tillage or rotation with corn has no effect on pathogen survival. SDS pathogen infects seedlings when soil temperature is low and moisture is high. Tillage reduces disease risk by increasing the soil temperature and reducing moisture early in the season when infections take place. Therefore, tillage after corn rotation provides better control than operation right after harvest of an infested soybean crop for soybean-corn rotation.

Phytophthora does not have to survive on infested residues in the absence of soybean, although tillage can reduce pathogen density. However, the fungus survives in soil as oospores, one of which can produce numerous zoospores to infect soybean plants. Tillage reduces the risk of this disease mainly by separating the pathogen from soybean plants and by reducing soil moisture. Tillage right after harvest of an infested soybean crop may not be better than the one made after rotation with corn in terms of separating the fungus from soybean plants and reducing soil moisture.

Finally, there is no need to work on the entire field if a disease occurred only in portions of a field. Limited tillage on problematic areas instead of entire fields should be considered to reduce soil erosion and save energy.

Table 1. Effect of tillage on soybean disease risk.

NA, not applicable.