The Soil Bacteria That Fight Rice Disease as Well as a Fungicide

Sheath blight is the kind of disease that resists simple solutions. The fungus behind it, Rhizoctonia solani, survives in soil and crop debris as hardened structures called sclerotia for years at a time, attacks an unusually wide range of host plants, and — critically — has no major resistance gene that breeders can work with to build resistant rice varieties the way they can for many other diseases. That leaves chemical fungicides as the default tool, with all the downsides that come with repeated use: rising cost, the risk of the fungus evolving resistance, and collateral damage to the beneficial microbes living in the same soil.

The alternative researchers have been exploring for years is biological control — recruiting bacteria that already live in the rice rhizosphere (the zone of soil immediately around plant roots) to fight the fungus directly. The problem, per this paper, is that most of that research has stopped at laboratory screening. Few strains have actually been tested under realistic growing conditions to see if lab success translates to a living plant.

Rather than working from a lab culture collection, the team collected soil directly from the root zone of healthy rice plants growing across four locations in Karnataka — Dharwad, two sites near Gangavati, and Shimoga — reasoning that bacteria already coexisting successfully with rice in real field conditions were more likely to be effective, well-adapted biocontrol candidates than bacteria sourced elsewhere. That yielded 32 distinct isolates: 30 actinobacteria (a group of bacteria well known for producing antibiotics and antifungal compounds) and 2 fluorescent Pseudomonas-type bacteria, identified initially just by colony color, texture, and growth pattern.

Each isolate went head-to-head with a confirmed virulent R. solani strain (RS4) in a dual culture test — both organisms growing on the same plate, with the width of the zone where the fungus stops advancing serving as a direct visual measure of how strongly the bacterium fights it off. Out of the full set, two isolates stood out clearly: one labeled GVTAM 8, which inhibited fungal growth by 90.61%, and another labeled DWRAM 10, at 88.38% — both edging out even AUDT 502, an already-known reference biocontrol strain from earlier research, which scored 87.77%.

Lab-dish success doesn't always survive contact with a living plant and real disease pressure, so the team moved the top candidates into a glasshouse trial — growing a sheath-blight-susceptible rice variety, deliberately infecting it with the pathogen, and testing 17 different treatment combinations. The clear winner was applying GVTAM 8 and AUDT 502 together, as both a seed treatment before planting and a foliar spray afterward: this combination brought the "relative lesion height" (how far up the plant the disease had spread, as a percentage of total plant height) down to 14.52%, essentially matching the chemical fungicide hexaconazole's 14.04% — a genuinely rare result for a biological alternative to match a chemical standard this closely. For comparison, untreated infected plants reached 65.40% relative lesion height, more than four times worse.

DNA sequencing settled the identity question precisely. Using the 16S rRNA gene — a standard genetic marker for bacterial identification — the team confirmed GVTAM 8 as Streptomyces cinnabarinus (98.06% sequence match to the reference) and DWRAM 10 as Streptomyces pseudogriseolus (98.21% match), with both sequences formally deposited in the NCBI public database under accession numbers OQ512000 and OQ512154. That matters beyond bookkeeping: it means other researchers can now order or compare against these exact strains rather than working from a vague "promising actinobacterium."

The paper goes a useful step further than just reporting that the bacteria work — it identifies two specific chemical weapons behind the effect. The first is hydrogen cyanide, which disrupts the fungus's electron transport chain, the cellular machinery it needs to produce usable energy, effectively suffocating it at the cellular level. The second is siderophores — small iron-binding molecules the bacteria release that grab up the available iron in the immediate environment before the fungus can use it, starving R. solani of a nutrient it needs to survive. Together, these give a specific rather than vague explanation for why the bacteria work, which matters for future formulation and quality-control work on any eventual commercial product.