Date of Award
8-2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Plant and Environmental Science
Committee Chair/Advisor
Guido Schnabel
Committee Member
James E. Faust
Committee Member
Elizabeth Cieniewicz
Committee Member
Hehe Wang
Abstract
Biological fungicides are increasingly investigated as sustainable alternatives to synthetic fungicides in fruit crop protection. This dissertation examines the interactions between two commercial biological fungicides, Bacillus subtilis AFS032321 (Theia) and metabolites of Pseudomonas chlororaphis AFS009 (Howler EVO); and demethylation inhibitor (DMI) fungicides, with a focus on compatibility, synergistic potential, and resistance implications. In vitro assays revealed that commonly used DMI fungicides difenoconazole, metconazole, tebuconazole, and propiconazole inhibited vegetative growth of B. subtilis at concentrations above 50 µg/ml, whereas mefentrifluconazole promoted colony expansion. Under controlled conditions, lesion development on detached fruit was inhibited synergistically when Theia was combined with the growth-promoting DMI mefentrifluconazole, while antagonistic effects occurred with inhibitory DMIs like difenoconazole. Fungicide formulations also influenced compatibility: non-emulsifiable concentrate (non-EC) formulations of propiconazole and metconazole were less inhibitory to B. subtilis and resulted in smaller lesions on detached apples and cherries when used in combination with Theia. The product reduced blossom blight in nectarines and peaches, suggesting its potential as an alternative to chemicals for early-season sprays.
Synergistic interactions between DMI fungicide propiconazole and Howler EVO were observed and reduced rates of the DMI mixed with Howler EVO were as effective as the full rate of the synthetic fungicides. Investigations into modes of action revealed that Howler EVO suppressed constitutive and DMI-induced expression of the MfCYP51 gene in DMI-sensitive and -resistant Monilinia fructicola isolates. This was later confirmed to be caused by the presence of pyrrolnitrin (PRN), a secondary metabolite produced by P. chlororaphis and present in Howler EVO. Field trials confirmed that Howler EVO combined with a reduced rate of propiconazole reduced blossom blight and brown rot in nectarines. Our findings also established that at least some biologicals should be used in resistance management programs. Under controlled conditions, Botrytis cinerea isolates resistant to the synthetic fungicide fludioxonil were also resistant to biological fungicide (Howler EVO). This cross resistance was likely due to the chemical similarity between active ingredient PRN in Howler EVO and fludioxonil. Overall, this work highlights both the promise and the complexity of integrating biological fungicides into modern disease management programs and provides critical insights into optimizing their use for sustainable fruit crop production.
Recommended Citation
Wesche, Johanna, "Interactions of Biorational and Synthetic DMI Fungicides for Control of Fruit Diseases" (2025). All Dissertations. 4081.
https://open.clemson.edu/all_dissertations/4081
Author ORCID Identifier
orcid logo https://orcid.org/0000-0001-6902-4690