Physiological leaf spot and chloride

Physiologic leaf spot (PLS) is a malady of winter wheat that causes flag leaf spotting and destruction of lower plant leaves in severe cases. Yield losses from zero (when incidence is low) to as much as 10 percent (in severe cases; Smiley et al, 1993a) have been reported. PLS leaf lesions (oval to irregular shaped, chlorotic to necrotic spots) are similar to those associated with the fungal diseases tan spot and septoria, but no casual pathogen has ever been successfully isolated from PLS lesions. While foliar fungicides can be effective in treating tan spot and septoria, PLS seems unaffected by such fungicides (Smiley et al., 1993b; Engel et al., 1996). There is a substantial body of evidence that suggests PLS is not a disease, but if not, then what is the cause?