Question Statement:
A farm research station observed that fields treated with compost brew showed a $25\%$ reduction in fungal leaf spot incidence compared to untreated fields. The lead investigator concluded that compost brew directly synthesizes chemical fungicides that destroy fungal spore walls.
Which of the following, if true, most seriously calls into question the investigator's explanation?
Options:
- (A) Compost brew introduces competitive beneficial bacteria that physically occupy foliar infection sites and outcompete fungal pathogens for nutrients without synthesizing fungicides.
- (B) Fungal leaf spots can cause yield reductions in wheat and tomato crops.
- (C) Compost brew application slightly increases soil potassium availability.
- (D) Chemical fungicides are widely used in commercial horticulture.