Expanding soybean diseases

Diseases are among the main factors that limit the achievement of high yields in soybean crops (Glycinemax (L.) Merrill). In Brazil, approximately 40 diseases caused by fungi, bacteria, nematodes and viruses have already been identified.

The economic importance of each disease varies from year to year and from region to region, depending on the climatic conditions of each harvest. Some diseases can cause losses of up to 100%. Around 15 to 20% of annual reductions in crop production are caused by diseases.           

Despite the great evolution of soybean breeding in Brazil, monoculture and the adoption of inadequate management practices have favored the emergence of new diseases, called soybean end-of-cycle diseases or DFCs. 

DFCs have been causing considerable reductions in crop yields. The disease complex is made up of: brown spot (Septoria glycones), cercospora blight (Cercospora kikuchii Mat. and Tomoy. M.W. Gardner), anthracnose (Colletotrichum dematium var. truncata), stem and pod drying (Phomopsis spp.) to mildew (Peronospora manshurica (Naoum.) Syd).                              

 The action of these fungi causes plant defoliation and causes the crop cycle to be brought forward by up to 25 days, thus leading to less grain filling, reducing crop productivity due to deficiency in graining, which can reach more than 30% in relative to a healthy plant.            

The beginning of the development of these diseases can happen already during the implementation of the crop, through contaminated seeds. In areas with a history of these diseases, the pathogen may be surviving in crop remains. In this scenario, under favorable conditions, infections can begin in the vegetative state of soybeans and, from there, evolve so that higher levels of severity are often observed from the beginning of pod formation.                

Sometimes, producers believe that the damage is small because the increase in severity is more concentrated at the end of the cycle. Much of the inefficiency in controlling these diseases is the concern and late placement of fungicides. Considering that these diseases can start even in the vegetative stage, concern should begin from the first applications, which should focus on patches to reduce inoculum.

There are several factors that contribute to reduced effectiveness with delayed positioning. One of them is the limitation imposed on the application technology in depositing drops on the lower leaves, which is where the spots are evolving. It is worth mentioning that in current cultivars, the productive load is most often located in the lower third of the plant and, therefore, it is very important to protect the bass leaves so that they remain active for as long as possible on the plant.

The damage caused by S. glycines are normally underestimated due to the erroneous attribution of the brown color of the leaves to their normal senescence, but under conditions favorable to the development of this fungus, losses can reach 25% to 34%. The damage caused by C. kikuchii range from 15 to 30%, but can reach 30% due to purple spot and 7% due to cercospora leaf blight. A anthracnose caused by the fungus Colletotrichum truncatum it is closely related to pod infection or under conditions of severe infection in leaves and petioles, which can cause losses ranging between 10% and 20% of productivity.