Brown rot, a prevalent fungal disease in the southeastern United States, significantly impacts stone fruits like peaches, cherries, and plums.
Monilinia fructicola
is a fuzzy yellow-brown fungus that thrives on the flowers, twigs, and fruits of these trees, being the primary culprit behind brown rot. Once a tree is infected with brown rot, the damage is irreversible, leading to considerable crop losses in commercial orchards. Farmers are actively working to mitigate the spread of this infectious disease
M. fructicola
by applying a chemical spray known as
fungicide
during the spring months.
In
M. fructicola
, a biosynthetic gene called
MfCYP51
encodes a vital protein essential for fungal survival. Certain fungicides categorized as
Demethylation Inhibitors
(DMI) aim to disrupt this protein’s function, but over the past two decades, DMI fungicides have shown decreasing efficacy against brown rot, with
M. fructicola
developing increased resistance.
Live microorganisms and their metabolites present a promising alternative to traditional chemical fungicides.
Biological disinfectants
can contain bacteria that produce harmful compounds like hydrogen cyanide and pyrrolnitrin, which are detrimental to fungal cells. However, a commercially viable biofungicide has yet to be identified. A research team from Clemson University in the US and Huazhong Agricultural University in China has initiated tests to evaluate the efficacy of soil bacteria
Pseudomonas chlororaphis
and
Bacillus subtilis
in combatting brown rot.
The researchers conducted culture experiments to analyze how fungal cells express the key protein
MfCYP51
under various fungicide treatments. They investigated three strains of
M. fructicola
susceptible to conventional DMI fungicides, alongside three resistant strains, utilizing five different treatment variations, including DMI fungicide,
P. chlororaphis
metabolites, a combination of
Bacillus subtilis
cells, and their respective combinations with DMI fungicides. A control group using sterilized water instead of a disinfectant was also established.
After six hours, the researchers extracted RNA molecules from the fungi to measure gene expression. Treatments with
P. chlororaphis
and DMI fungicide +
P. chlororaphis
led to a significant decrease in
MfCYP51
expression in both susceptible and resistant isolates when compared to controls. In contrast, DMI fungicides combined with
Bacillus subtilis
increased
MfCYP51
expression in resistant isolates. The researchers concluded that
P. chlororaphis
possesses a unique capability to reduce gene expression compared to other biofungicides.
To further explore how biofungicide treatments operate, the team investigated how
P. chlororaphis
and
Bacillus subtilis
produce the antifungal metabolite pyrrolnitrin. They analyzed treatment solutions using a high-performance liquid chromatograph to separate and identify the liquid compounds. Their findings suggested that pyrrolnitrin was potentially involved in reducing gene expression.
Additionally, the researchers performed five treatments on Gala apples to assess brown rot. Each apple underwent surface cleaning and disinfection, followed by fungicide application. After 24 hours, they punctured the apples and added 20 microliters of
M. fructicola
cells, then placed them in a humid environment for five days to evaluate the brown rot spots.
Despite the reduced
MfCYP51
expression observed, treatment with
P. chlororaphis
alone did not lower brown rot incidents compared to the control. However, treatments with DMI fungicide and
Bacillus subtilis
effectively reduced brown rot spots. Specifically, the DMI fungicide +
P. chlororaphis
treatment significantly prevented brown rot compared to the control.
The researchers concluded that while biofungicides may lack standalone efficacy, their application could help farmers minimize reliance on DMI fungicides. By reducing DMI fungicide usage, infection rates of
M. fructicola
resistance could be slowed. They propose future studies to test biofungicide mixtures containing pyrrolnitrin in real-world settings on stone fruit trees.
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Source: sciworthy.com
