Recent News 53 : Effective, yet vulnerable: Review outlines key hurdles to using bacteriophages for plant disease control

Effective, yet vulnerable: Review outlines key hurdles to using bacteriophages for plant disease control

Using bacteriophages for plant disease protection faces key challenges. (https://ars.els-cdn.com/content/image/1-s2.0-S0160412019305410-ga1_lrg.jpg)

Using bacteriophages for plant disease protection faces key challenges including poor stability, environmental sensitivity, and timing issues, a new review finds.

A team from the Indian Council of Agriculture Research and the Department of Plant Pathology of the University of Florida reviewed the current state of bacteriophage (phage) therapy for plant disease control.

Bacteriophages, or simply phages, are viruses that target bacterial hosts and exhibit typical viral characteristics. They cannot replicate independently and must rely on a bacterial host for reproduction.

The review highlighted its potential as an alternative to antibiotics, especially in the face of growing regulatory restrictions on agricultural antibiotics.

The interest in phage therapy is primarily driven by concerns over rising antibiotic resistance as traditional antibiotics become less and less effective.

However, the review highlighted that real-world field trials have been mixed due to environmental challenges such as weather variability, as well as the challenge of finding the best ways and times to apply phages effectively.

Environmental vulnerabilities

While they are a promising alternative to antibiotics, utilising phages faces significant challenges.

The review highlighted that phages are sensitive to the external environment. In particular, they are susceptible to ultraviolet (UV) radiation, especially UVB, which can impair their ability to replicate.

As such, phages are best applied in the evening hours, said the researchers.

“Studies have demonstrated that phage application after sunset significantly extends their persistence within the phyllosphere, providing an extended window for bacterial infection and control. This strategy underscores the importance of optimizing phage application timing to maximize their efficacy in agricultural systems.”

At the same time, phages have been suspended in various formulations to extend its longevity against UV.

The leaf surface itself have hostile conditions for phages as they face temperature fluctuations and can have copper pesticide residues that affect phages.

Lastly, phage movement can be hindered in the rhizosphere, where soils are mixed and varied, and can help or hinder phage movement.

For instance, phages will move through sandy soils better compared to more compact clay soils.

If they cannot spread well, they will not be able to reach the bacteria it was designed to kill, hampering its overall efficacy.

Furthermore, phage stability in the soil depends on factors such as temperature, moisture, and materials released by plant roots.

Human farming practices also have an impact. For instance, continuous cropping can help phages stay in the soil and adapt to attack bacteria in future crops. Conversely, rotating crops tends to reduce this effect.

Application challenges

Given their instability in harsh conditions, it is likely phages need to be reapplied frequently.

Researchers also noted that application needs to be precisely timed.

Phages need to be on the plant just as the bacteria sets in. However, in real practice, farmers may not be able to predict an attack.

Furthermore, previous studies have shown that phages work well only when their numbers are high and cannot be effective enough when their concentration drops.

To counter this, phage application may need to be applied daily to keep their levels high enough.

The review also noted that applying phages over vast agricultural areas “would require substantial phage volumes”.

“For example, one quart of AgriPhage—Tomato Spot/Speck- is recommended for one acre and costs approximately 50 USD. Keeping in mind, phages should be applied repeatedly due to their instability in harsh environmental conditions.

“The application to large farms over a long time may not be viable. Although there has been modelling and research on lowering the cost of production of phages, commercial applications are still expensive when dealing with large acreages of farms.”

Source: Viruses

Bacteriophages as Agents for Plant Disease Control: Where Are We After a Century?

Authors: Choudhary et al.

https://doi.org/10.3390/v17081033

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