Integrated control of Fusarium head blight of wheat.
Mngadi, Zandile Nothile Consolate.
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Fusarium head blight (FHB) is a destructive fungal disease of wheat (Triticum aestivum L) world-wide. There are no resistant cultivars currently available, nor are there any effective fungicides against FHB of wheat. The use of biological control agents (BCAs) has been reported to be effective, if combined with other control measures. Therefore, this study was aimed at isolating and screening for antagonistic BCAs against F. graminearum Strain F.32 (F.32) in in vitro and in vivo bioassays. Subsequently, single and combined applications of potassium silicate (KSi) and BCAs treatments were tested for their ability to reduce FHB of wheat under greenhouse and field conditions. In vitro bioassays showed that eight Bacillus isolates and two Trichoderma strains inhibited F.32 on potato dextrose agar (PDA) plates. Bacillus Isolates B13, B14 and B15 inhibited mycelial growth of F.32 by between 41 - 49%, whereas B1, B7, B9 and B16 inhibited mycelial growth of F.32 by between 20 - 28%. Two commercial Trichoderma harzianum strains, Strains T.kd and T.77 inhibited mycelial growth of F. graminearum F.32 by 30% and 24%, respectively. None of the 16 yeast isolates tested inhibited the germination of F.32 macroconidia within 7 h when screened on yeast dextrose chloramphenicol agar, ¼ strength PDA and water agar plates. Eight Bacillus isolates, two commercial T. harzianum strains and four yeast isolates were tested against FHB under greenhouse conditions. Bacillus Isolate B7 significantly (p = 0.05) reduced FHB severity by 16% compared to the pathogen-inoculated Control treatment. None of the four tested yeast isolates significantly reduced FHB compared to the pathogen-inoculated Control treatment (p = 0.90). Single applications of T. harzianum Strains T.kd and T.77 did not significantly reduce FHB compared to the pathogen-inoculated Control treatment (p = 0.22). Repeated applications of T. harzianum Strains T.kd and T.77 did not significantly improve the reduction of FHB compared to the pathogen-inoculated Control treatment (p = 0.76). Granulated KSi fertilization at 3.0 and 4.5 g reduced FHB of wheat under greenhouse conditions by 49.0 and 54.0%, respectively, on Day 12 post-pathogen inoculation (ppi) compared to the pathogen-inoculated fertilization at 600 mg L-1 was not effective in reducing FHB of wheat under greenhouse conditions in the first 12 days after pathogen inoculation. There was no significant difference in the severity of FHB of wheat heads after treatments with KSi alone, and KSi combined with the T. harzianum Strains T.kd or T.77. None of the single and combined treatments with Bacillus Isolate B7, T. harzianum Strain T.kd and KSi treatments significantly reduced FHB severity and incidence compared to the pathogen-inoculated Control treatment on Day 20, Day 30 and Day 40 ppi under field conditions. None of the treatments tested under field conditions reduced deoxynivalenol levels in the wheat grains below 2 mg kg-1, which is the legally recommended limit. Biological control agents that demonstrated antagonistic potential against F.32 on agar plates were ineffective in reducing the severity of FHB by F.32 on wheat under field conditions. Potassium silicate demonstrated a limited capacity to reduce the levels of FHB under greenhouse conditions. The search for effective BCAs against FHB of wheat grown under South African conditions will need to continue.