Disease suppressive effect in soil against Rhizoctonia root rot induced by mimicking wheat monoculture
Authors:   Agricultural Academy, Sofia, 1Institute of Soil Science, Agro-Technology and Plant Protection “Nikola Pushkarov”, Plant Protection Department - Kostinbrod, Yanashkov Ivo , Vatchev Tzenko ,   Agricultural Academy, Sofia, 1Institute of Soil Science, Agro-Technology and Plant Protection “Nikola Pushkarov”, Plant Protection Department - Kostinbrod, Petkova Galina ,   Agricultural Academy, Sofia, Institute of Soil Science, Agro-Technology and Plant Protection “Nikola Pushkarov”, Department of Physics, Erosion, Soil Biota
Published: March 2024

Disease decline and induction of soil suppressiveness by continuous (5-6 years) monoculture with susceptible plant-host have been reported for various crop-pathogen systems. The present study achieves the goal of inducing suppressive effect in soil towards Rhizoctonia root rot (Rhizoctonia solani AG 4) through monoculture of winter wheat (Triticum aestivum L.). Twenty four high-quality wheat (c. Exotic) seeds were planted in plastic containers, each containing 1.1 L Alluvial-Meadow (sandy-clay) soil (Eutric Fluvisols, according to FAO classification). According to the treatment applied, the resulting plants were grown for 0-4 successive short cropping cycles, each lasting three months. Disease suppressiveness was measured in a bioassay in which the experimental units were rectangular, plastic containers (26 x 6 x 8 cm), with drainage holes in the bottom and detachable trays allowing for manual irrigation from the bottom of the container, 7 cm height of the column of tested soil in each container and a two-row crop of wheat (c. Exotic, 2 rows x 12 plants, 2 x 3 cm plant spacing). The spread of the disease was measured from a focus (point) source of inoculum of the pathogen – 20 inoculum grains from a pure culture developed on autoclaved (30 min at 121 °C) barley, introduced into the soil at the beginning of both rows after emergence of the plants. Immediately after termination of the soil suppressiveness test, the abundance of the major groups of soil microorganisms were determined in rhizosphere soil in each treatment by using the method of decimal dilutions and subsequent culturing on agar media. Separately, bacteria of the genus Pseudomonas were isolated from the rhizosphere soil on King’s medium. The antagonistic activity of 10 Pseudomonas spp. isolates from each treatment was tested against R. solani AG 4 in vitro by the double culture method. The monoculture of wheat imitated under controlled conditions resulted in a significant (P<0.05) reduction of the spread of Rhizoctonia root rot from the source of infection. Furthermore, statistically significant differences (P<0.05) were also found between the individual treatments with pre-cultivation of wheat: with the increase in the number of cultivation cycles from one to four, there was a significant reduction in disease spread – by 19.5%, 31.0%, 40.5% and 54.8%, respectively, as compared to the untreated control. Simultaneously, no substantial alterations were observed in the population densities of the monitored groups of microorganisms in the rhizosphere of the experimental plants, including ammonifying and spore-forming bacteria, microscopic fungi, Actinomycetes and bacteria, utilizing mineral N. With the increase in soil suppressiveness from the first to the fifth growing cycle of the crop, a significant increase in the average antagonistic activity of the isolated Pseudomonas spp. against R. solani AG 4 was observed. The thesis is argued that shattering rachis of the ripe ear in wild forms conditioned a continuous “natural” monoculture in which bread wheat and its predecessors grew and evolve during the prehistoric era. We speculate that this condition, along with the lack of genes for resistance to necrotrophic pathogens, have given rise to evolutionary stable defence mechanism and the latter based on mutualistic relationships between the host-plant and bacteria that naturally inhabit rhizosphere and exhibiting high antagonistic (antibiotic) activity against soil-borne plant pathogens. A manifestation of this mechanism is the increasing suppressive response of the soil and the disease decline during or after continuous wheat monoculture. The results obtained in this study are discussed in the light of the microbial eco-evolutionary dynamics in the plant rhizosphere. The progressively increasing pathogen-antagonistic activity of Pseudomonas spp. isolates is discussed as an example of “rapid microbial evolution”, accomplished by possible horizontal gene transfer within one or more successive cultivation cycles of the wheat crop.