Promoting Agricultural Sustainability through the Use of Rhizosphere-Competent Fungi as an Alternative to Soil Fungicide

Promoting Agricultural Sustainability through the Use of Rhizosphere-Competent Fungi as an Alternative to Soil Fungicide

Summary

Summary
This research determined that growers can effectively use a recently developed biocontrol fungus, Trichoderma harzianum 1295-22. This strain is derived from a common rhizosphere inhabitant, and was bred at Cornell University for its ability to better colonize roots and kill root pathogens. It is commercially available under the name T-22. Using sweet corn as a model, researchers examined five parameters — financial return, application method, interaction with cover crop, soil type, and soil temperature — that growers need to be concerned about.

The potential result of these findings will be higher production (estimated at 5%) with all other inputs remaining the same. This increase is about a fiftyfold return on investment for the farmer. The research is developing delivery systems that are usable by family farmers and small producers. T-22 application qualifies as organic management, permitting added value for those specialized markets.

Objectives
* Evaluate delivery methods for Trichoderma harzianum strain 1295-22 to find which is most effective in commercial farming operations. The methods are in-furrow application, seed treatment, and cover-crop inoculum.

* Evaluate the economic impact of different delivery systems.

* Test additional cover crops for effectiveness in increasing the population of the biocontrol organism so that a broader choice of delivery systems might be identified.

* Identify the properties of Northeastern agricultural soils that affect the ability of Trichoderma to colonize crop roots, thereby identifying the most promising places to begin implementation.

* Identify growth-reducing stresses that are mitigated by Trichoderma.

Key Findings
Using Trichoderma is profitable. The cost is so low; the risk is small. The return averages 15 to 30 times the cost.

Trichoderma can be considered biological insurance against unpredictable crop loss. T-22 preserves the crop’s yield potential under certain adverse conditions.

Mixing Trichoderma in the planter box works best because it provides a highly vigorous inoculum in close contact with the seed. Pretreated seed should be effective when it becomes available.

Colonized cover crops are not an effective way to inoculate sweet corn with Trichoderma.

Colonization was good on all soils suitable for sweet corn, but liming acid soils could improve colonization.

Trichoderma is effective when the daytime soil temperature is above 55°F. Colder soil restricts Trichoderma growth and favors pathogens.

If seed-applied fungicides are necessary for stand establishment, they need to be used regardless of whether Trichoderma is also used.

Methods and Findings
This research has, as a broad objective, to determine what growers need to know to effectively use Trichoderma harzianum 1295-22. The results are also available on a web site at . Follow the link “Implementation of Trichoderma.”

Better root growth of Trichoderma-colonized roots should allow the plant to capture soluble nitrogen and phosphorous that would otherwise leach to the ground water, thus preventing a process that is both wasteful of resources and detrimental to water quality. In addition, use of a biocontrol fungus is one more element that will increase farmer awareness of the active management of soil microflora, an awareness that will lead to better management of soil health.

One goal of this project is to reduce the need for fungicides used as treatments on seeds. Our experiments showed that routine fungicide treatments were not necessary in warm soils.

The preferred application method for sweet corn is a powder that is mixed with the seeds in the planter box at sowing. Seed coating produces excellent colonization as well, and commercial seed treatment may be commercially available in the near future. The response to seeds purchased with the seed treatment already applied should be similar to that described in this research.

The product is inexpensive to use on corn because of the small amount of inoculum necessary. The return on investment was ten- to fortyfold on sweet corn, depending on the market.

The soil requirements for colonization were studied in experiments done at twelve different field sites in 1996 and seven sites in 1997, with twelve additional soils in a common garden each year. These showed that all tested soil types supported high populations of Trichoderma on roots inoculated with T-22. Wild Trichoderma have much lower populations and are more sensitive to soil type. An analysis of soil characteristics that are associated with good colonization confirmed previous findings that roots growing in soils high in calcium are better colonized.

The minimum useful temperature for Trichoderma was lower than expected. There was no problem at 55°F or above. In very early plantings, one each in 1995 and 1997, colonization was somewhat reduced as a result of cold. Even though the growth of T-22 was slower, its value in ameliorating stress is higher in cold soil. Although overwintered or early-spring crops may give unsatisfactory colonization, T-22 is useful at any temperature appropriate for sowing sweet corn.

Research on this and other crops has shown a clear pattern of what to expect from Trichoderma. The yield enhancement is primarily seen when plants in a well-managed soil are weakened by stress. Trichoderma restores their original vigor.

The yield-reducing conditions encountered in this study were water stress, early planting or harvest, moderate nitrogen fertilizer, and low plant population. In all these situations, greater root growth has obvious value. However, Trichoderma did not work if the field was flooded or crusted because both roots and fungi need air to grow. The restorative effect occurs when control yields are reduced below a threshold amount. It occurred when untreated sweet corn yielded less than five tons an acre. Near the threshold yield, the increase was about 10%; with greater stress Trichoderma increased yields by 50% to 100%.

Delivery Methods
Treating seed with a slurry of spores places the spores in direct contact with the seed, but this placement also means that the spores are in contact with fungicide and stickers on treated seed. Extended exposure to these materials may kill or inhibit the beneficial fungus. T-22 treatments were tested with several combinations of the fungicides Apron, Captan, Demosan and Imazilil. T-22 was compatible with all except Imazilil, which is used to protect against Penicillium. Since Penicillium is such an important seedling disease of sweet corn, we also investigated compatibility with one substitute for Imazilil called Maxim. This fungicide was completely compatible with T-22.

In three field trials, we tested the ability of an uninoculated sweet corn crop to be colonized by Trichoderma when it followed inoculated winter rye that had been plowed down before sowing the sweet corn. At all three farms, no Trichoderma colonized the subsequent corn crop. Do not expect Trichoderma to colonize crops that follow the crop that was inoculated with T-22.

Economic Impact
T-22 had a positive economic impact by increasing yields. The effect of Trichoderma was pronounced where untreated plots had low yields. The yields were low at these sites because of intentional management practices to get a particular market, such as early planting, premature harvest, or organic nitrogen management. None were low due to poor management. In 1995 and 1997, both dry growing seasons, the response to Trichoderma was seen where control yields were below five to six tons, whereas in 1996 the threshold was around four tons. In dry years, root development, which is often increased by Trichoderma, is more limiting to growth. The wet season in 1996 allowed even crops with minimal root systems to grow well.

The economic return was substantial. The average return paid for the treatment many times over, on sweet corn and on other crops. Therefore the economic risk of using T-22 is minimal. Also, the maximum benefit comes when most needed by the farmer. When conditions arise during the season that compromise the yield and result in a low-income year, the response to T-22 is greatest. This effect could help manage economic risk in addition to having an overall positive return.

The estimated cost to use the different formulations of T-22 on sweet corn are: granules — $10 to $30 an acre; planter box–$1.17 to $1.50 an acre; seed treatment — $5 to 10 an acre. The return was established only for planter box. The return for seed treatment should be similar, while granules would probably be somewhat less. The average return for fresh market sweet corn was $50 an acre, and the average return for processing sweet corn was $17 an acre.

Additional Cover Crops
Cover crops may differ in their ability to support a population of Trichoderma that is sufficient for colonizing a subsequent crop. The colonization of Trichoderma on several species used as cover crops was examined in greenhouse trials. Six cover crops of regional significance were selected: annual ryegrass, canola, red clover, grain rye, hairy vetch, and winter wheat.

These results suggest wheat as the most likely cover crop to use for carrying over and multiplying Trichoderma inoculum, with canola the likely second. We are not especially optimistic about any of them in light of the field results.

Soil Characteristics
In order to identify specific traits of soils that are important to colonization, a variety of soils were tested in a common-garden experiment. This experiment was conducted in all three seasons.

We concluded T-22 will colonize sweet corn on a great diversity of soils. The only northeastern soils where there would be a concern are those very low in calcium and acidic. Liming in accord with normal recommendations for sweet corn should eliminate even the small chance of inadequate colonization.

During the common-garden experiment, we also examined whether different types of soil management alter the ability of Trichoderma to colonize. Of specific interest is whether organic matter management that promotes growth of a diverse soil microflora will make it more difficult for Trichoderma to invade.

Stable microbial communities will not prevent colonization by Trichoderma. This organism may be effectively used by organic growers.

The performance of Trichoderma on different soils was tested with grower trials. Sweet corn is colonized on a wide variety of soil types. Soil type does not need to be a consideration in deciding whether to use Trichoderma.

While we cannot distinguish wild strains from strain 22 in the assay, other tests have shown that strain 22 displaces the wild ones on inoculated roots. Even when the wild strains reach high populations, they are less effective at protecting from disease or increasing growth. They also do not colonize the roots as quickly when the seed germinates. These are the traits that were bred into strain 22 that sets it apart from the wild strains.

Stresses
Cold. Trichoderma grows sparingly below 60°F and not at all below 50°F. This may limit colonization immediately upon germination. Super-sweet corn also does not germinate at low temperatures, so the germination of both may only be delayed by cold. Colonization of roots was strong at the four-leaf stage regardless of the soil temperature. Early colonization was affected by low temperature, confirming the slow growth of Trichoderma in cool soil.

These data are the first strong field evidence of the remarkable rhizosphere competence of T-22. No other biological on the market has a comparable ability to colonize roots under field conditions.

Oxidation. Treatment of the seeds with dilute hypochlorite to cause oxidation injury resulted in considerably reduced vigor. Subsequent colonization with Trichoderma completely restored the vigor of these seedlings. The effect of both hypochlorite and Trichoderma treatment on seedling vigor occurred mainly in the moderately vigorous seedlings. In contrast, the strongest seedlings in each group performed similarly.

Allelopathy. Soil treated with the allelopathic compound benzoxazolinone (BOA) resulted in truncated roots with the same biomass as roots grown in the absence of BOA. Trichoderma increased the biomass growth (dry-weight gain) by 52%, with no BOA effect.

Fungicides. Seed-applied fungicides, particularly Imazilil, represent a stress that can potentially diminish the effectiveness of the beneficial fungus. Seed-applied fungicides are often used in conjunction with T-22 to control seed rots, which are not affected by T-22. The effect of fungicide seed treatment on colonization and establishment was tested with the granule formulation in 1995. We found Trichoderma colonization was unaffected by the fungicides and fungicides were sometimes necessary for establishment, and Trichoderma did not substitute.

Reported March, 1998. 1999 Northeast Region SARE/ACE Report.

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