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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