Development of a Multicomponent Microbiological Soil Inoculant and Its Performance in Sweet Potato Cultivation

Abstract

The cultivation and consumption of sweet potato (Ipomoea batatas) are increasing globally. As the usage of chemical fertilizers and pest control agents during its cultivation may lead to soil, water and air pollution, there is an emerging need for environment-friendly, biological solutions enabling increased amounts of healthy crop and efficient disease management. Microbiological agents for agricultural purposes gained increasing importance in the past few decades. Our goal was to develop an agricultural soil inoculant from multiple microorganisms and test its application potential in sweet potato cultivation. Two Trichoderma strains were selected: Trichoderma ghanense strain SZMC 25217 based on its extracellular enzyme activities for the biodegradation of plant residues, and Trichoderma afroharzianum strain SZMC 25231 for biocontrol purposes against fungal plant pathogens. The Bacillus velezensis strain SZMC 24986 proved to be the best growth inhibitor of most of the nine tested strains of fungal species known as plant pathogens, therefore it was also selected for biocontrol purposes against fungal plant pathogens. Arthrobacter globiformis strain SZMC 25081, showing the fastest growth on nitrogen-free medium, was selected as a component with possible nitrogen-fixing potential. A Pseudomonas resinovorans strain, SZMC 25872, was selected for its ability to produce indole-3-acetic acid, which is among the important traits of potential plant growth-promoting rhizobacteria (PGPR). A series of experiments were performed to test the selected strains for their tolerance to abiotic stress factors such as pH, temperature, water activity and fungicides, influencing the survivability in agricultural environments. The selected strains were used to treat sweet potato in two separate field experiments. Yield increase was observed for the plants treated with the selected microbial consortium (synthetic community) in comparison with the control group in both cases. Our results suggest that the developed microbial inoculant has the potential to be used in sweet potato plantations. To the best of our knowledge, this is the first report about the successful application of a fungal-bacterial consortium in sweet potato cultivation.

Keywords: Arthrobacter; Bacillus; Pseudomonas; Trichoderma; biocontrol; microbiological soil inoculant; plant growth promotion; sweet potato.

Figure 1

Extracellular enzyme activity values of 7 tested Trichoderma strains (mean ± SD, n = 3).

Figure 2

Colony diameter extension rates of the tested Trichoderma strains at different water activity values (mean ± SD, n = 3). Asterisks (*) indicate statistically significant (p ≤ 0.05) differences in relation to the colony diameter extension rate of the respective strain at the water activity optimum (a_w 0.997).

Figure 3

Colony diameter extension rates of the tested Trichoderma strains at different temperatures (mean ± SD, n = 3). Asterisks (*) indicate statistically significant (p ≤ 0.05) differences in relation to the colony diameter extension rate of the respective strain at the optimum temperature.

Figure 4

Colony diameter extension rates of the tested Trichoderma strains at different pH values (mean ± SD, n = 3). Asterisks (*) indicate statistically significant (p ≤ 0.05) differences in relation to the colony diameter extension rate of the respective strain at pH 5.

Figure 5

In vitro biocontrol index (BCI) values of the tested Trichoderma strains towards strains of fungal species known as plant pathogens (mean ± SD, n = 3). Higher values refer to more intensive overgrowth of the plant pathogenic fungus by the Trichoderma strain. A BCI value of 100 means complete overgrowth.

Figure 6

In vitro antagonism of Bacillus strains towards strains of fungal species known as plant pathogens. (A). Colony radius of the tested fungi in the presence of biocontrol candidate Bacillus strains. (B). Distance between fungal and Bacillus colonies (mean ± SD).

Figure 7

Indole-3-acetic acid (IAA) production of potential plant growth promoting glyphosate-tolerant bacterial strains measurep by HPLC (mean ± SD).

Figure 8

Effect of microbial treatment on sweet potato storage root yield and average tuber size at Szentes in 2019. The same letters above the data in the columns mean no significant difference (p ≥ 0.05) (mean ± SD) by the LSD test * or Tukey test **. T1: soaking secondary cuttings in the mixed microbial suspension; T2: same as T1 with an additional inoculation of the soil with the mixed microbial suspension; T3: inoculation of the soil with the mixed microbial suspension near the rows before planting; T4: same as T3 with an additional inoculation of the soil with the mixed microbial suspension; C: control (no microbial treatments applied).

Figure 9

Effect of microbial treatment on the performance of sweet potato storage root yield per plant at Ásotthalom in 2020. T1: soaking sweet potato cuttings in the mixed microbial suspension before planting in the fertilized unit; T2: same preparation of cuttings as in T1 with planting in the non-fertilized unit; T3: same as T1 with an additional inoculation of the soil; T4: same as T2 with an additional inoculation of the soil; Control 1: no microbial treatments applied, planting in the fertilized unit; Control 2: no microbial treatments applied, planting in the non-fertilized unit.