At first glance, there may not be much in common between the Midwest and Great Plains of the U.S. and Brazil. In Brazil's tropical and subtropical climate, high temperatures are consistent, and depending on the location, they can reach 80 to 90 degrees even in winter.
Meanwhile, annual rainfall ranges from 50 to 65 inches in southern Brazil, where much of the country's crops are grown.
This climate lends itself to fungal crop diseases and development of fungicide-resistant pathogens. As Flavio Medeiros, professor at the Federal University of Lavras in Brazil, noted at the recent Alltech ONE Conference in Lexington, Ky., these circumstances have pushed Brazil's crop growers to look beyond fungicides for disease control — and there may be opportunities for U.S. growers to follow their lead.
"We have a huge problem [in Brazil] because the fungicides that are sprayed are very few; we have very few options, so we exert selection pressure," Medeiros says. "There are some alternatives in terms of combinations of fungicides to tackle this resistance pressure, but as a community, the university research centers and the private sector, we are considering alternatives in terms of biological control to cope with resistance management."
There are several kinds of biological control — including mites, nematodes and even viruses. However, mass produced, commercially available products are limited to bacteria (mostly Bacillus) and fungi (mostly Trichoderma). Still, Medeiros notes there are a growing number of players on the market.
"Last year to this year, just in Brazil we had a 70% increase in the sales of biological control agents," he says. This includes soil-applied biocontrol agents and seed treatments. In most cases, however, Medeiros says seed treatments will best fit the grower's management system.
How do they work? It starts with the spermosphere — the region around the seed. When the seed absorbs water, it releases exudates into the soil, which then can be used by soilborne pathogens such as fusuarium and rhizoctonia to colonize and infect the plant.
When biocontrol agents such as bacillus are applied, they colonize around the spermosphere, which releases antibiotics, forming a biofilm around the seed and roots to provide long-term protection against pathogens.
Medeiros outlined several strategies for using biological control agents.
Application timing. Applying a biocontrol agent as a seed treatment or during the pathogen's survival stage — when it's overwintering — not only helps target the pathogen when it's at its weakest point, but also prevents compatibility issues when fungicides are applied, Medeiros says.
For example, when using a fungus-based biocontrol product, it's not a good idea to apply a fungicide at the same time — but that doesn't mean they can't be used in the same season.
"A fungicide seed treatment lasts for 20 to 30 days maximum," Medeiros says. "When we have a fungicide treatment, you get rid of some of the native microbiome, some of the native bacteria and fungi. Then you have a favorable environment for pathogens to colonize the root and the stems. If we find the right partners, the biocontrol and the fungicide that can be combined, we have a synergistic effect and an extended duration of root and stem protection."
Resistance induction. Ordinarily, when a harmful pathogen is present, it binds to the plant's cell wall. However, with resistance induction, a beneficial strain such as Bacillus or Trichoderma binds to the plant cell wall and triggers the plant's defense mechanism.
"[It happens] regardless of the plant cultivar — it doesn't need a receptor to recognize the signal. All plants have a cell membrane that can recognize this elicitor," Medeiros says.
Preventive measures. In places such as Brazil, high soil temperatures can render the environment unsuitable for beneficial bacteria and fungi. On the other hand, these conditions are favorable for the onset of soilborne diseases.
Extended crop rotations that include cover crops — particularly grasses that leave more crop stubble — keep the soil cooler, more humid and less exposed to sunlight, which creates a more conducive environment to beneficial bacteria and fungi.
With these strategies, Medeiros notes, the modes of action of biocontrol are completely different, which prevents the buildup of fungicide-insensitive populations of plant pathogens. However, biological control agents won't be a silver bullet — it takes a combination of steps to keep resistance from spreading.
"The Fungicide Resistance Action Community, FRAC, provides a list of pathogens that have developed resistance, and they also keep a list of the products that can be used to cope with them, among which are biological control agents," Medeiros says, adding this most often involves Bacillus and Trichoderma — and modes of action different from fungicides. "It reduces the selection pressure exerted by fungicide."