Fungi of the genus Trichoderma from the soils of Brazil's semi-arid Caatinga have traits that matter for biological disease control — and an unusual tolerance of heat — according to a study by the Brazilian Agricultural Research Corporation, Embrapa. The researchers collected fungi from native vegetation and from irrigated melon fields in the states of Bahia and Piauí and identified 14 isolates of five species: Trichoderma asperellum, T. asperelloides, T. koningiopsis, T. virens and T. spirale, with community composition varying by land use and soil chemistry.

The Caatinga, found only in Brazil, has scarce and irregular rain, high temperatures and drought-adapted vegetation, yet irrigated farming of high-value crops such as melons has expanded across it. Intensive cropping brings soil-borne diseases that are hard to control — Fusarium, Macrophomina, Rhizoctonia and the oomycete Pythium, which cause root rot and vascular wilt. Understanding how beneficial microbes interact with pathogens is essential to farming less dependent on chemical fungicides and more resilient to climate change, said Embrapa Environment researcher Gabriel Mascarin.

Trichoderma species are known worldwide as biocontrol agents: they compete for nutrients, parasitise other fungi, secrete compounds toxic to pathogens and prime the plant's own defences, explained fellow researcher Wagner Bettiol, and several also promote growth and help plants withstand drought and salinity. The team tested two mechanisms — direct confrontation through competition and mycoparasitism, and volatile compounds that inhibit pathogens without contact — against Fusarium sulawesiense, Fusarium solani, Macrophomina phaseolina, Rhizoctonia solani and Pythium myriotylum.

Grown at up to 40 °C, all isolates performed alike at 25 °C and 30 °C but separated at 35 °C: T. asperellum, T. asperelloides and T. virens from the melon fields showed high heat tolerance, which the authors read as adaptation to the irrigated semi-arid environment. That is a strategic trait for future bio-inputs, since many commercial microbes lose effectiveness in climates unlike the ones they came from.

The control mechanism depended on the pathogen: against Rhizoctonia solani and Macrophomina phaseolina direct confrontation worked best, with the Trichoderma isolates overgrowing and parasitising the colonies, while against others the volatile compounds dominated, in some cases cutting pathogen growth by more than 70%. Different species used different strategies, the authors note, so strains must be chosen for each cropping system and disease; one species performed consistently against all five pathogens by both routes.

Spore output is the industrial test. Most Brazilian Trichoderma biofungicides are still made by solid-state fermentation on cereal grains, Mascarin said, so the team grew the isolates on parboiled rice: the best produced more than 1.4 billion conidia per gram of substrate after ten days — the kind of yield a commercial product needs, and a promising start for biofungicides suited to hot, water-scarce farming.

Source: AgroPages