Use of green manures to reduce inoculum production of Fusarium graminearum on wheat residues

Use of green manures to reduce inoculum production of Fusarium graminearum on wheat residues

Summary

A series of greenhouse and field experiments were conducted. In each experiment, two green manures, sorghum-sudangrass hybrid and common buckwheat, were evaluated and compared to a fallow (no green manure) treatment. Changes in total bacterial density in the soil, the density of streptomycetes in the soil, the density of F. graminearum-antagonists in the soil, and the inhibitory activity of F. graminearum antagonists were evaluated before and following the incorporation of the green manures. Soil characteristics were examined along with the frequency of Fusarium spp. especially F. graminearum, in the wheat residue. The decomposition rate of the infested residue was also evaluated over time.

Objectives/Performance Targets

• Determine the effects of green manures on the frequency of soilborne antagonists inhibitory against pathogenic F. graminearum, and on the intensity of their F. graminearum inhibition. Quantify the impacts of green manures on the survival of F. graminearum on wheat residues. Evaluate the effects of green manures on the rate of decomposition of F. graminearum-infected residue.

Accomplishments/Milestones

We have successfully conducted a series of greenhouse and field experiments to evaluate the effects of green manures on the frequency and intensity of Fusarium-inhibitors, wheat residue decomposition and Fusarium survival in residue by soil microbes.

In each experiment, wheat was grown and wheat residues, naturally infested by Fusarium, were incorporated into the soil. Two green manures, sorghum-sudangrass [Sorghum bicolor (L.) Moench-S. bicolor (L.) Moench var. sudanense (Piper)] hybrid and common buckwheat (Fagopyrum esculentum Moench) were evaluated and compared to a fallow (no green manure) treatment. Changes in total bacterial density in the soil, the density of streptomycetes in the soil, the density of F. graminearum-antagonists in the soil, and the inhibitory activity of F. graminearum antagonists were evaluated before and following the incorporation of the green manures. Soil characteristics (pH, organic matter, K, P) were examined along with the frequency of Fusarium spp. especially F. graminearum, in the wheat residue. The decomposition rate of the infested residue was also evaluated over time.

Five greenhouse experiments were conducted in the greenhouse facilities at the University of Minnesota, Saint Paul, MN. Different combinations of wheat residues and soil type were used in each experiment.

Field soil was sieved using a 6 mm mesh sieve to remove large pieces of plant material. Stem pieces, each 2.5 cm long and containing a node, were cut from the collected residue. Residue pieces were mixed thoroughly with the collected soil and perlite (5:1, vol/vol) at a rate of 100 pieces per 7 kg of soil using a cement mixer. The soil-perlite-residue mix was then transferred into 11-L plastic pots (approx. 25 cm diameter).
The experimental design was a randomized complete block with 15 replicates and three treatments. Seeds of sorghum-sudangrass (cv. Excel, 30 seeds per pot) and buckwheat (cv. Mancan, 20 seeds per pot) were planted 1-cm deep and distributed uniformly across the pot.

Pots were watered and weeded by hand as necessary. The greenhouse was maintained at 20-22C throughout the experiment. Two weeks after planting, pots were thinned to 20 sorghum-sudangrass plants and 15 buckwheat plants per pot, respectively. Six weeks after planting, the aboveground biomass of the green manure treatment from each pot was harvested, cut into approximately 1-cm long pieces and incorporated by hand into the soil of the pot in which it had grown. Soil from fallow-treated pots was similarly disturbed although no residues were incorporated.
Field experiments were used as scaling factor. Two adjacent field experiments were simultaneously established at the Minnesota Agricultural Experiment Station in Rosemount, MN (UMore Park). Plots (3 x 5 m) were established in a randomized complete block design with six replicates. Plots were separated by 4 m within a block and 5 m between blocks. Experiments were separated by 20 m. Wheat (cv. Olsen) was planted, and inoculated.
The experiments were both plowed using a chisel plow. The green manures were planted on August 18 for Experiment A and September 1 for Experiment B. Sorghum-sudangrass (cv. Excel; planting rate, 93 kg of viable seed/ha) and common buckwheat (cv. Mancan; planting rate, 109 kg of viable seed/ha), were planted in 21-row plots, 5 m long and with 17 cm inter-row spacing, using a 7-row drill planter. No fertilizer was applied.

On October 6, the green manures were incorporated in both experiments using a rotovator that incorporated the green manure to a 10 cm depth. The rotovator was also used in fallow plots to produce similar soil disturbance as in the green manure treatment plots.

Data Collection

Greenhouse Experiments

The aboveground biomass (fresh weight per m2) of each green manure treatment was determined at the time of incorporation in all experiments. Biomass was collected the day prior to green manure incorporation from three arbitarily placed 0.5 x 0.5 m quadrats in each plot. Biomass samples were placed in plastic bags and stored at 4C until processed. Samples were subsequently dried with forced air at 35C for 48 h to determine dry matter weight.

Soil and wheat residue samples were collected from every pot when each treatment was established, at the time of green manure incorporation, four weeks after green manure incorporation, and 12 weeks after green manure incorporation. For each pot, three cores of 2.5-cm diameter were taken from the top 20 cm of soil, bulked, and stored in plastic bags at 8C until they were processed. Twenty nodes were arbitrarily collected at each sampling time and stored in paper bags at -20C until processed. The remaining soil was returned to the pot after each sampling.

Field Experiments

Soil samples were collected when the plots were established, at the time of incorporation of green manures, five weeks after incorporation, and six months after incorporation. Soil samples consisted of five cores of 2.5-cm diameter collected from the top 20 cm of soil per plot. The five cores, collected at five sites distributed evenly across each plot, were bulked and stored in plastic bags at 8C until processed.

Wheat residue samples were collected when the green manures were planted, five weeks after the incorporation of green manures and six months after incorporation. Fifty residue pieces, each with a node, were collected from each plot at each sampling time. Specifically, 10 arbitrarily selected nodes were taken at each of the five locations where soil sampling was conducted within a plot. Only residue pieces where the node was below the soil surface were collected. Node pieces were placed in paper bags and stored at -20C until processed.

The day prior to green manure incorporation, aboveground plant biomass was assessed. Due to the high frequency of volunteer wheat plants in all plots, green manure biomass (sorghum-sudangrass or buckwheat) and wheat biomass were separated and the fresh and dry matter weight of each component determined as described above.

Streptomycete densities for all soil samples were estimated using a modification of Herr’s method (Herr, 1959) as described by Wiggins and Kinkel (2005a). Two replicate plates per sample were prepared on each medium at each dilution. Streptomycetes and total bacteria were counted on plates of WA/SCA and OA, respectively, and expressed as the number of colony-forming units (CFU)/g of dry soil.

The assessment of the antagonistic activity of soil streptomycetes against pathogenic F. graminearum was performed using a modification of Herr’s method (Herr, 1959) as described by Wiggins and Kinkel (2005a). Each soil sample was plated onto three replicate plates.

Streptomycetes were considered antagonistic if they produced a clear inhibition zone (no F. graminearum growth) of at least one millimeter surrounding the colony edge. Antagonist densities were expressed as CFU/g dry soil. The proportion of streptomycetes antagonistic against F. graminearum in each soil was expressed as a percentage (number of colonies identified as antagonists divided by the total number of streptomycete colonies x 100). The diameter of each zone of inhibition against F. graminearum was calculated by subtracting the diameter of the colony from the inhibition zone diameter including the colony size. For each Petri plate the mean inhibition zone size per antagonist colony was determined.

Soil tests were performed at the Soil Testing Laboratory at the University of Minnesota. Soil pH, phosphorus (P2O5), potassium (K+) and organic matter content were determined for the soil samples of six arbitrarily selected replicates from selected experiments.

The decomposition of wheat residue was quantified by measuring losses in dry weight over time. The collected residue for each sample was rinsed in tap water to remove adhered soil, and dried with forced air at 35C for 24 h prior to weighing. Twenty residue pieces from each plot or pot in greenhouse and field experiments were analyzed at each sampling time.

Samples were processed to determine the presence of F. graminearum in residue according to the protocol developed by Pereyra (2000).

We have completed the data collecting and now all the data and is going to be subject to analysis of variance (ANOVA) using SAS.

Impacts and Contributions/Outcomes

Even when we have not completed the data analysis, we can assure that this work will determine the effects of green manures on the frequency and intensity of soilborne antagonists inhibitory against pathogenic F. graminearum. We will also determine if green manures have any significant impact on the rate of decomposition of F. graminearum-infected residue or on the population dynamics of F. graminearum in the residues we recovered. After analyzing the data, and determining the impact of green manures on the analyzed variables, we will be able to estimate the economical impact of the adoption of this technology.

A manuscript will be submitted to “Plant and Soil” for publication.

Collaborators: