Microalgae Triple Team Beats Bean Damping-Off and Boosts Yields in Egyptian Field Trials

A combination of three microalgal strains applied to soil cut damping-off disease in common bean so sharply that seedling survival rose 14-fold compared with untreated plots, while yield per feddan climbed 33-fold, according to a two-season field study conducted at the Sids Horticulture Research Station in Beni-Suef, Egypt. The work, published in the Egyptian Journal of Biological Pest Control, tested the green microalga Desmodesmus abundans NA1 and two cyanobacterial isolates, Nostoc paludosum NA2 and Tildeniella torsiva NA3, both alone and in mixtures, against one of the most destructive soil-borne diseases of bean crops.

Damping-off attacks seeds and emerging seedlings, particularly in wet, poorly drained soils, and is attributed to pathogens including Rhizoctonia, Fusarium, Pythium, Alternaria, and Macrophomina. Infested plants show stem constriction, collar discoloration, and dehydration, and in severe cases entire seedlings die, causing stand losses and reduced yields. Losses from the disease range from 20% to complete harvest failure under certain environmental conditions. Conventional control has relied on chemical fungicides, but intensive use of such chemicals poses risks for human health and environmental stability, which has driven interest in biological control agents as eco-friendly alternatives.

The researchers began by isolating the causal pathogen. Bean plants with damping-off symptoms were collected during the 2020-2021 growing season from Giza, El-Fayoum, Beni-Suef, El-Sharkia, and El-Menoufia, and four isolates of Rhizoctonia solani were purified and identified from four governorates. In greenhouse pathogenicity tests on the susceptible cultivar Nebraska, all four isolates caused damping-off with varying severity. The Beni-Suef isolate proved the most aggressive, producing the highest pre-emergence damping-off at 66.6% and the lowest plant survival at 26.6%, and it was selected for all subsequent experiments.

Laboratory assays revealed marked differences in the antifungal capacity of the three strains. Cell-free filtrates of Tildeniella torsiva NA3 showed the strongest inhibition of R. solani mycelial growth, rising with concentration from 34.0% suppression at 0.1% to a maximum of 77% at 1.0%. Desmodesmus abundans NA1 was weaker, inhibiting growth by 11.4% to 21.8%, while Nostoc paludosum NA2 had the least impact, with negligible inhibition at lower concentrations and only mild suppression of 15.9% at the highest dose. Biochemical profiling showed that D. abundans NA1 produced the highest levels of total phenols at 1127.3 ppm gallic acid equivalents, flavonoids at 4739.9 ppm quercetin equivalents, and antioxidant capacity at 15442.1 ppm ascorbic acid equivalents, whereas no detectable phenols were found in the other two strains.

Greenhouse trials compared two application methods, soil drenching and seed soaking, at concentrations of 5, 10, and 20%. All algal isolates significantly reduced pre- and post-emergence damping-off and increased surviving plants relative to untreated controls. Soil drenching proved more effective than seed soaking, a result the authors attribute to improved root colonization, sustained activity of bioactive metabolites, and direct exposure of the inoculant to soil-borne pathogens. The field experiment was then built on the most successful application technique from the greenhouse stage.

Under field conditions, the study ran across two successive growing seasons, 2022 and 2023, using a randomized complete block design with three replicates per season, giving six replicates in total. Twenty-two treatments comprising individual and combined inoculants were randomly assigned to plots of 10.5 square meters each. Among single strains, D. abundans NA1 at 20% delivered the lowest pre-emergence disease at 30.8% and the highest plant survival at 68.3%, while T. torsiva NA3 at the same concentration raised survival to 52.9%. Low concentrations of N. paludosum NA2 were less effective, with survival rates falling below 20%. Increasing inoculum concentration consistently enhanced disease suppression and crop establishment.

The combined treatments produced the study’s most striking results. The triple mixture of D. abundans NA1, N. paludosum NA2, and T. torsiva NA3 at all tested concentrations provided the most effective control, reducing pre-emergence damping-off to as low as 30% and completely preventing post-emergence losses at the highest concentration, with the highest survival rate of 70%. Among two-species combinations, D. abundans NA1 plus N. paludosum NA2 at 20% performed significantly better than the other pairings, holding pre-emergence damping-off at 35.8% and survival at 59.1%. The untreated control, by contrast, suffered severe pre-emergence losses of 93.3% and survival of just 5%.

Growth and yield parameters responded in parallel. The triple mixture at 20% produced the tallest plants at 70.0 centimeters, the highest plant biomass at 118.0 grams, the heaviest seed weight per plant at 97.6 grams, and the greatest yield per feddan at 2836.7 kilograms. Among individual inoculants, D. abundans NA1 at 20% reached 3703.8 kilograms per feddan with plants averaging 64.2 centimeters and 99.0 grams of biomass. The untreated control yielded only 85.2 kilograms per feddan. The abstract additionally notes gains of 1.7-fold in pods per plant, 3.1-fold in seeds per plant, 3.6-fold in seed weight per plant, and 1.6-fold in 100-seed weight under the best treatment. Photosynthetic pigments rose significantly, with chlorophyll a peaking at 1.3 milligrams per gram fresh weight under D. abundans NA1 at 20%, and the triple combination at 20% lifting chlorophyll a to 1.2 milligrams per gram from a control value of 0.88.

Seed quality improved as well. The triple mixture at 20% achieved the highest protein content at 16.0%, along with maximum phosphorus at 0.3%, potassium at 1.7%, and nitrogen at 2.5%. Among dual combinations, D. abundans NA1 plus N. paludosum NA2 at 20% and 10% raised nitrogen to 2.5% and 2.4% and protein to 15.7% and 15.2%, while T. torsiva NA3 alone at 20% produced the highest individual-strain values for nitrogen at 2.4%, potassium at 1.4%, and protein at 15.5%. The authors link these improvements to nitrogen fixation, growth hormone production, and enhanced nutrient bioavailability, and they note that Nostoc paludosum NA2 was the most promising strain for improving pod formation and increasing seeds per plant, possibly due to its nitrogen-fixing capacity.

The study’s authors state that this is the first investigation of the combined application of these three strains, and they highlight the potential of algal bioformulations as sustainable biocontrol and biofertilization agents in legume cultivation. They also point to practical feasibility, noting that large-scale cultivation in open raceway ponds and photobioreactors has made algal biomass cost-effective for agriculture, and that inoculants can be delivered as dried powders, encapsulated beads, or liquid suspensions through seed coating, soil drenching, or fertigation. The findings rest on two seasons of field data at a single Egyptian research station in a semi-arid region with clay soil of slightly alkaline pH, so broader validation across other soils, climates, and farming systems would be needed before generalizing the yield and disease-control benefits.

Subject of Research: Use of microalgal inoculants as biocontrol agents and biofertilizers against damping-off in common bean

Article Title: Harnessing microalgal inoculants for integrated damping-off management and growth promotion in Phaseolus vulgaris

Article References: Abdelwahab, N., Goma Mohamed, A., & Ahmed Abd El Moneam Ahmed, S. (2026). Harnessing microalgal inoculants for integrated damping-off management and growth promotion in Phaseolus vulgaris. Egyptian Journal of Biological Pest Control, 36(1), Article 14. https://doi.org/10.1186/s41938-025-00889-6

Image Credits: AI Generated

DOI: 10.1186/s41938-025-00889-6

Keywords: microalgae, damping-off, Rhizoctonia solani, common bean, biocontrol, biofertilizer, Desmodesmus abundans, Nostoc paludosum, Tildeniella torsiva, field trial, plant survival, seed yield