During a wet spring in a zoysiagrass lawn, which disease is likely causing small, straw-colored patches that are enlarging?

The presence of small, straw-colored patches that are enlarging in a zoysiagrass lawn, particularly during a wet spring, is indicative of Dollar Spot disease. Dollar Spot is a common turfgrass disease that thrives in warm and moist conditions, which aligns with the environmental factors described in the question.

In the case of Dollar Spot, the lesions start small and can quickly expand, creating visible patches that may take on a bleached or straw-colored appearance. This symptom matches what is observed in the scenario provided.

While the other diseases listed—Brown Patch, Rust, and Powdery Mildew—can also occur in lawns, they have distinct characteristics and conditions under which they flourish. For example, Brown Patch typically shows as dark, water-soaked spots, while Rust presents as a more yellowish color with noticeable rust-colored spores on blades. Powdery Mildew is characterized by a white powdery substance on the blades of grass and does not generally manifest as straw-colored patches. Hence, in the context of the symptoms and environmental conditions presented, Dollar Spot is the most likely cause of the observed damage in the lawn.