Calling in the biological team: Control of Fusarium

One of the most common pathogens of ornamentals, and many other crops, is Fusarium. Although it is common, it is hard to diagnose without a laboratory to assist. Fusarium can cause a multitude of symptoms. These include leaf spots, cutting rot, crown rot, wilt, bulb rot, and root rot. Fusarium spores move easily with water and can be splashed from one plant to another. They often appear in propagation as cutting rot and are not always the only pathogen involved. Wilt diseases are usually caused by Fusarium oxysporum with a distinct type for each plant species or family affected.

Despite decades of research on all aspects of Fusarium prevention and control, Fusarium remains one of the hardest pathogens to control, whether the crop is grown in soil or in soilless media in a container. This is even true when plants are treated preventatively – before infection. The most effective fungicides include fludioxonil, triflumizole, strobilurins, and thiophanate methyl. When disease occurs in the propagative unit (bulb, corm, or cutting) even these products provide little benefit.
 

Impact of biologicals

Biological control agents range from Streptomyces (Actinovate, MycoStop) to fungi like Trichoderma (RootShield and RootShield Plus) and bacteria (Cease, GallTrol, Companion). There are also many others that are not as well identified, and studies that are used by our industry have shown that biologicals may affect plant growth, tolerance of stress conditions, or even aide in disease prevention.

Biological control agents are often used commercially to replace chemical products and, in most cases, are not as effective on diseases like Fusarium wilt. This is mainly due to our failure to use most disease control products preventatively. The biggest difference between using biologicals and conventional disease control products is that they are often living organisms that are more sensitive to extreme conditions. Most of the trials that have been completed are conducted in a side-by-side with conventional fungicides/bactericides and may not be preventative.

When researchers focus on the effects of biologicals on the crop without a pathogen involved, they have found that sometimes a biological agent (or groups of them) actually increases plant tolerance to stresses such as drought. They also improve the soil quality and reduce populations of pathogens in the soil by many actions including direct competition.

The nature of Fusarium diseases is that they often become a problem after plant stress (including flowering and fruit production) occurs. Thus, it seems likely that some of the benefits of biologicals are indirect, achieved through keeping the plant from reaching that critical level of stress that triggers a Fusarium disease.
 

Fusarium diseases

As part of the IR-4 project (a major resource for supplying pest management tools for specialty crop growers by developing research data to support new EPA tolerances and labeled product uses) trials were completed over several years and included many synthetic fungicides known to be somewhat effective against Fusarium diseases.

The only biological control product included in many of the trials was RootShield Plus, which is a combination of Trichoderma harzianum (T22) and T. virens (G41). In greenhouse trials, the experimental formulation of RootShield Plus, applied as drench or soil incorporation, provided good efficacy against stem root rot (F. solani) on Dracaena deremensis, but was poor against Fusarium root rot (F. oxysporum) on larkspur. There was no effect on damping-off and root rot (Fusarium commune) on Douglas fir and bulbous iris, and Fusarium wilt (F. oxysporum) on lisianthus. Also it provided no efficacy against stem based necrosis (F. oxysporum) when applied as corm dip or foliar spray on gladiolus in two trials.

Anyone who has been faced with controlling a Fusarium disease, especially Fusarium wilt, which is systemic, has been frustrated with even the most effective synthetic fungicides.
 

Integrating biologicals & fungicides

Even the most effective chemicals are not all that effective on Fusarium. One way the research community has sought to improve the situation is to combine biologicals and chemical products.

Dr. Wade Elmer (Connecticut Agricultural Experiment Station, Dept. of Plant Pathology and Ecology) is a worldwide authority on Fusarium diseases. His group performed greenhouse studies designed to determine if disease suppression of Fusarium wilt of cyclamen could be enhanced by integrating commercial formulations of beneficial microbes (biologicals) with registered fungicides. When biologicals were used as the first preventative treatment and inoculated a week later, there was no acceptable disease control even when plants were returned to a full conventional chemical fungicide program.

However, when biologicals were applied following a fungicide treatment or tank-mixed with a fungicide, there were combinations that yielded significant reductions in disease development. These interactions were specific to certain biologicals and certain fungicides, and no pattern could be determined from the pairings except that fludioxonil (active ingredient in Medallion) combined with a biological generally resulted in the greatest reductions in disease. Researchers concluded the efficacy of integrating biologicals with fungicides has potential.

A review of the research on many other crops, from tomatoes to chickpeas (seed treatment), has shown similar results. Biologicals do not affect the disease when used alone but do improve control when used in combination with chemical fungicides or in a truly integrated program. Other studies show benefits from combining a Trichoderma spp. with a bacterial species or mycorrihizal formulation.

Alternatives to soil fumigation with methyl bromide are needed for controlling Fusarium diseases in tree nurseries. Studies are under way for developing microbial mixtures (bacteria and ectomycorrhizal fungi) that control Fusarium diseases of eastern white pine (Pinus strobus L.) seedlings. Greenhouse studies in containerized production have shown that the application of rhizosphere bacteria to conifer seed, coupled with ectomycorrhizal fungi application at sowing, can protect seedlings against Fusarium root rot. Not only do seedlings have a reduced incidence and severity of root rot, they also have greater levels of ectomycorrhizal roots. When applied to seeds for field (bareroot) production, the bacterial strains are associated with increased stand numbers.
 

Conclusion

There are greenhouse practices that can make Fusarium diseases especially bad. Therefore, here are the don’ts:

• Don’t re-use pots or flats without a thorough cleaning and disinfesting.
• Don’t re-use potting media.
• Don’t use a single product (biological or chemical in nature) without rotation.
• Don’t guess what is wrong – many crown rot diseases look alike and if you treat for the wrong one your efforts will fail.
• Don’t use fungicide dips during propagation. Even when an effective fungicide is used, more disease is spread this way than is controlled. Instead, do a sprench after sticking cuttings to avoid spreading disease and to get the best preventative control.
   

Most importantly, use an integrated approach to disease control and always treat preventatively. Biologicals will probably not replace all chemical fungicides in our lifetime, but research has shown that they actually can be a critical part of crop production.

A.R. Chase is Plant Pathologist of Chase Agricultural Consulting LLC and Professor Emeritus of Plant Pathology at the University of Florida. For more, www.chaseagriculturalconsulting.com