Trichoderma for Control of Soil Pathogens

Soil contains many microbes, including beneficial ones that are essential to good crop growth. Recent research has begun to show how to manage the soil microflora to favor the good microbes. One approach has been to add some of the best ones to fields in order to create a more favorable soil environment. Most of these introductions fail because the native microflora has the competitive advantage.

One biocontrol fungus has been bred to have the rare but essential property of being able to colonize roots under field conditions. This special strain is called Trichoderma harzianum 1295-22 and is sold under the trade name “T-22.” This strain was developed through a 15-year breeding effort led by Prof. Gary Harman at Cornell. Its essential characteristics are a greater ability to colonize roots and a better ability to kill root pathogens.

Vegetable growers can use this biological control agent to reduce the risk of crop loss to stress and root rot. Research in my lab has provided answers about how and when to use T-22. We determined the role of several parameters that growers need to be concerned about, using supersweet corn as a model. We examined application methods, soil types, soil temperature, some other crops, and financial return.

How to use T-22. The preferred application method is a planter box seed treatment. It puts all inoculum where it needs to germinate on the emerging root, and it allows the fungus to largely escape long-term exposure to seed-applied fungicides. Having the spores on the seed also means that very little is used, only about 1/2 oz per acre.

What does T-22 do? Trichoderma grows on the surface of roots, where it provides disease control and enhances root growth. Its spores survive in the soil, but the food it lives on is mostly secreted from the root surface. Since the fungus multiplies on its own, it is different from seed-applied fungicides. First, only a little needs to be applied because it will grow to continually cover the roots. Second, because it grows, it protects all the roots for the whole growing season. Chemical controls protect only the seed where they are applied, and their protection lasts, at most, a few weeks.

Once Trichoderma has colonized roots, it can improve growth in two main ways. First, it kills the fungi that cause root rot. Second, it protects roots from certain physical stresses, allowing the roots to grow faster.

Trichoderma kills several major root rot fungi: Pythium, Rhizoctonia, and Fusarium. The process is called mycoparasitism. Trichoderma secretes an enzyme that dissolves the cell wall of the other fungi. It can then get inside the bad fungi and consume them. The strain used in T-22 secretes much more of the key enzyme, endochitinase, than wild strains, thus T-22 grows better and secretes more enzyme than wild strains. This combination allows it to protect crop roots against root rot fungi in the field.

Research in the greenhouse has shown a clear pattern of what to expect from T-22. The yield enhancement is primarily seen when plants are weakened. Trichoderma restores their original vigor, rather than increasing the vigor of plants already growing at their potential.

Where to use T-22. Experiments done at 12 diverse field sites and with 12 additional soils in a common garden showed high populations of Trichoderma were found on roots inoculated with T-22 on all soil types. The most difficult soils for wild Trichoderma are those that are extremely well drained. They are presumably more difficult for strain T-22 as well, but even on these, the populations were high enough to be effective. An analysis of soil characteristics that are associated with good colonization showed that roots growing in soils high in calcium are better colonized.

Does T-22 work at any planting date? In culture, Trichoderma grows slowly if the temperature goes below 60o F. That raises the question of whether it can colonize well enough when soils are cool. The minimum useful temperature for T-22 was lower than expected. As expected, it colonizes well at 60o F or above. In very early plantings, colonization was somewhat reduced as a result of cold. Colonization was lower than desired when the soil was 55o F at sowing, but growth was improved. Even though the growth of the fungus was slower, its value in ameliorating stress was higher. At 50o F, neither supersweet corn nor T-22 establish well enough, so T-22 does not make it possible to plant earlier. It is safe to conclude that if the soil is warm enough for supersweet corn, it is warm enough to benefit from T-22.

What crops does T-22 work on? Trichoderma has shown little host specificity, colonizing most plants. It should be good on most vegetable crops. It has been tested on potatoes, radishes, and cucumbers. In addition, repeated drench applications work well on bedding plants, potted flowers, and greenhouse tomatoes. In some cases, the production schedule or fertility management needed to be adjusted to accommodate the increased growth. Related forms of this fungus are effective on root rot in raspberries and on gray mold in strawberries.

Yield responses. In the field, we have seen responses when yield-reducing conditions were acting by not allowing the roots to use the full soil resource. Examples are early planting or harvest, moderate nitrogen fertilizer, and low plant population. In all these situations, greater root growth has obvious value. For sweet corn, the restorative effect occurred when control yields are reduced below a threshold amount of 4 tons/ac. Then the yield increase was about 20%. It has been seen when yield of the untreated crops was less than 40 bu/ac (soybeans) or 120 bu/ac (field corn). At those yields the increase was about 10%; in a few cases the yields were increased by 50 to 100%.

However, there are exceptions. T-22 does not work if the field is flooded or crusted because both roots and fungi need air to grow. Also, it was not possible to obtain consistent colonization by growing the inoculum on the roots of an overwintering rye crop. Apparently, the rye roots grow during the late fall and early spring, when it is so cold that Trichoderma is dormant. The growing roots therefore fail to spread the inoculum. Lower soil temperatures, encountered with early-seeded crops like peas or early field corn, and overwintering crops, such as spinach or wheat, will probably give unsatisfactory colonization.

Economic return. The product is inexpensive to use on corn because of the small amount of inoculum necessary. The return on investment was 14- to 40-fold on sweet corn, depending on the market. The cost to use it is about $1.50 per acre, the increased yield was worth an average of $20 for processing corn (@ $75/ton) or $60 for fresh market corn (@ $2/doz.). Some selective fresh-market outlets use only large ears; in this situation, Trichoderma can push a larger proportion of the ears into the harvestable size class and have a bigger economic effect.

In cooperative industry trials, the results for other crops have been investigated as well. With snap and dry beans, the large number of seeds makes the product more expensive to use, so the economics need closer scrutiny for this crop. With large-seeded field crops, the return was 2.5- to 17-fold. For small seeded crops, the cost would be minimal and the benefit similarly large.

Conclusion. Trichoderma is likely to have a positive effect on summer crop performance if something causes the roots not to use the soil resource fully. Therefore, the effect is likely to be largest if the soil has been well-managed, so there is a large soil resource, quality criteria are high, and there is bad weather limiting plant growth.

T-22 is effective against root rots, but it does not control seed rot. For that purpose, seed-applied fungicides are still useful. The manufacturer lists the susceptibilities and resistance of T-22 (and other Trichoderma strains) to many pesticides.

This research was funded by the Sustainable Agriculture Research and Education program of the USDA.
Thomas Bjorkman, Assoc. Professor, Department of Horticultural Sciences, Cornell University, Geneva, NY 14456

Originally published: Proceedings. 1999. New England Vegetable and Berry Growers Conference and Trade Show, Sturbridge, MA. p.310-312.

Reviewed by: Mary Concklin, UConn IPM. 2012

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