'Foolish seedling' disease of rice led to the discovery of

The 'bakanae' or 'foolish seedling' disease of rice had been known in Japan since the early 1800s.
The diseased rice plants were thin, pale green and spindle-shaped, longer by about 50 percent than the healthy plants, and were sterile.
The disease is caused by the fungal pathogen Gibberella fujikuroi.
E. Kurosawa (1926) showed that the symptoms appeared in healthy rice seedlings when they were treated with sterile filtrates of the fungus.
The active substance was later identified as gibberellic acid (GA).
ABA is a growth inhibitor, IAA is a natural auxin (discovered through experiments on coleoptile curvature) and 2,4-D is a synthetic auxin used as a herbicide.
So the foolish seedling disease of rice led to the discovery of GA (gibberellins).