Study On Biological Control Of Wheat Flag Smut Using Wheat-edophytic Bacteria

Wheat flag smut (WFS) is an important systemic disease caused by Urocystis tritici Korn. The disease incidence of WFS has a tendency to increase in some wheat-producing areas in China year by year. In recent decades, the incidence of WFS was approximately 10% in general, and reached up to 90% in severe cases. WFS became a potential threat to wheat production. Although WFS can be effectively controlled by some chemical fungicides and resistance breeding, both chemotherapy and resistance breeding had their respective shortcomings for sustainable wheat production, especially the overuse of fungicides might result in chemical residues in the environments and agro-products, and has become a public concern. For sustainable development of wheat production, it is necessary to develop ecofriendly means for reducing fungicide use in WFS management. Biocontrol is considered an ecofriendly technique to reduce pesticide use in pest management.In this study, a series of experiments regarding biocontrol of WFS were carried out, including a survey on the occurrence of WFS in Nanyang city, optimization of the conditions for germination of U. tritici teliospores, identification of wheat resistance to WFS, characterization of endophytic bacteria isolated from wheat plants, and field control of WFS using endophytic bacteria. The main results obtained are summerized as follows:1. The occurrence of WFS was confirmed in all of the counties/districts except for Xixia and Sheqi counties where the disease could not be found during the survey in 2011. The disease incidences varied from 0% to 10.7% in 65 wheat fields investigated, with 40% of them infected. The diseased area of wheat field was estimated to be approximately 700000 hm2 in Nanyang city. The teliospores of U. tritici were treated by sodium hypochlorite, with an enhanced germination rate of 26%.2. Six endophytic bacterial isolates with high wheat growth-promoting activities were screened out from 1931 isolates obtained. Three of them were Bacillus thuringiensis (58-2-1, 37-1 and 39-1). The remaining three were Pseudomonas koreensis (58-1-1,8-2 and 138-1). The B. thuringiensis and P. koreensis isolates are the first cases as wheat endophytic bacteria with wheat growth-promoting activities in the world.3. The resistance of wheat plants against WFS markedly varied from one variety to another. Out of the 15 wheat varieties investigated, seven were identified as highly resistant (HR), three as moderately resistant (MR), three as susceptible (S), and two as highly susceptible (HS) varieties. The results showed that the HR varieties were greater than 40%, and the HS and S ones were greater than 30%4. The WFS-controlling and yield-enhancing efficacies of four endophytic isolates (8-2, 58-2-1,37-1 and 58-1-1) significantly varied with wheat varieties. The influence of the endophytes on susceptibility to WFS on the HR varieties was minor, compared to their significant influence of susceptibility to WFS on the MR/S ones. The utilization of the endophytic bacteria for the control of WFS might be pratically aimed at S/HS the varieties. The four endophytic bacteria had different efficacies in controlling WFS on different varieties. The control efficaciy of isolate 58-2-1 was 63.2% on the S variety (Yubao-1), and 53.9% on the HS variety (Yumai-416). The isolate 37-1 could enhance WFS resistance on part of MR/S/HS varieties (25%), while had litlle effect on the remaining varieties (75%). The isolate 8-2 could enhance WFS resistance on Zhengmai-9023 and Yunong202, and enhance the resistance on the HS variety (Yunong012).5. The yield recovery rates were more than 90% and 300% on the HR varieties treated by the endophytic bacterial isolates 58-2-1 and 37-1, respectively. But the two endophytes were not completely consistent with each other in WFS-controlling and yield-enhancing efficacies on the same varieties. There were 11 varieties inoculated by isolate 58-2-1 on which their yields were enhanced, and 3 varieties with a reduced yield compared to their CKA plots. There were 10 varieties inoculated by isolate 37-1 on which their yields were enhanced, and 5 varieties with a reduced yield compared to their CKA plots.6. The endophyte-treated wheat plants showed an enhanced activity in at least one of the three enzymes (POD, SOD and CAT) tested in the CKA plots compared to the CK-B plots at the tillering stage on the R/HS varieties. The SOD activity of the CKA wheat plants was higher than the CKB ones except Yunong-012. The author speculated that the pathogen-inoculated wheat plants might produce a large amount of free radicals during host-pathogen interactions based on the enhanced activities of related enzyme(s).