Which characteristic is true of smut fungi?

Smut fungi, belonging to the group of plant pathogens known as fungal pathogens, are indeed characterized by their relationship with plants. They primarily infect and cause diseases in various plant species, particularly grasses and cereals. This group of fungi is known for producing dark, soot-like spores, which is where the term "smut" originates. The relationship is often detrimental to the host plants, leading to reduced crop yields and economic impacts in agriculture.

While smut fungi do have ecological roles in their specific niches, their primary identification and impact stem from their effects on plants, making the assertion that they primarily affect plants accurate. The other statements regarding their use in food production, ecosystems, and habitats do not characterize smut fungi in the same direct manner or significance as their role as plant pathogens.