Incorporating animal manure into soybean fields is one way to manage manure, but it also poses a new challenge for Iowa soybean growers as well as plant pathologists.
Manure applications affect fungi, bacteria, and other organisms in the soil and, therefore, influence soilborne diseases. Many people have asked how manure applications affect soybean diseases, and ISU plant pathologists are searching for answers. Plant pathologists are not ready to make specific recommendations because many studies were conducted in laboratories and greenhouses, and more field tests are needed, but we are taking a proactive approach to keep you informed about the latest research.
Studies have shown that animal manure affects soybean diseases in several ways. Manure can increase or decrease the potential for soilborne disease, depending on three factors: 1) the type of disease; 2) the type of manure, and 3) the time of application. One study shows that manure can increase soybean Pythium and Phytophthora damping-off, and Phytophthora root rot. On the other hand, some studies, including several from Iowa State University, have shown that soil incorporated with cattle manure can suppress Rhizoctonia root rot, and cause less damage. Such an effect has been used as a biological control to manage an avocado disease in Australia.
Manure application may affect soybean disease for several reasons. First, animal manure can increase biological activity in the soil; beneficial bacteria and fungi may increase and compete with pathogenic fungi.
Secondly, manure can influence the soil environment, which is critical to fungal activity. For instance, a layer of manure on the ground may increase soil moisture, needed for fungal growth. One study found an increase in soil pH after manure was applied, resulting in more root rot.
Manure also introduces nutrients, such as carbohydrate and nitrogenous substrates, which can stimulate fungal spores to germinate. If spores germinate without the presence of soybean roots, the fungi die; if abundant spores germinate in the presence of soybean roots, infections occur.
The time when manure is incorporated into soil may be a critical factor in managing disease. Results of one ISU study, supported by the Leopold Center for Sustainable Agriculture, suggest that planting crops right after manure application in the spring could increase seedling and soilborne disease, while having several weeks between manure application and planting could reduce the potential for disease. We recommend you plant varieties resistant to Phytophthora when you apply manure to fields where Phytophthora could be a problem. A combination seed treatment containing Apron also may be beneficial when you plant right after spring application of manure.
Always remember to watch your crops after manure application because observations are invaluable in making management decisions.