Saprotrophic survival of Magnaporthe oryzae in infested wheat residues

Wheat blast caused by Magnaporthe oryzae is a relatively new disease that has caused considerable losses in wheat fields of several South American countries, including Brazil. The 2016 report of wheat blast occurrence in Bangladesh raised concern in South Asia where wheat represents a significant crop. The sources of primary inoculum and survival from season to season of the fungus remain largely unknown. The effect of wheat residues on the onset of blast epidemics and the potential for survival of M. oryzae in the residues were studied under subtropical climatic conditions, in the South of Brazil. The objective of this study was to monitor the saprotrophic development of M. oryzae on wheat debris and explore the relative importance of crop residues as a source of inoculum. The wheat cultivars BRS 229 and Anahuac 75, moderately and highly susceptible to the disease, respectively, were inoculated with a spore suspension of 10−5 conidia mL−1 using an aggressive (Py 12.1.209) and a less aggressive (Py 12.1.132) isolate. At maturity, a portion of leaves, stems and spikes were detached from plants, and a group of ten lesions were randomly selected and marked on each type of plant organ. The air-dried plant organs were placed separately inside bags and exposed outside. The experiment was conducted over three different time intervals. Each 14 days, samples were taken from the field and tested for sporulation. The survival of the blast fungus decreased rapidly on the rachis when compared to stems and leaves. Sporulation of the fungus was observed on the wheat residues for up to five months. Based on the results of this study, the possibility that the causal agent of wheat blast survives under Brazilian conditions from one crop to another in wheat residues is very low. The management of crop residues is not a key point to control the development of wheat blast. A strong emphasis should be placed on the presence of other hosts.