Asher (1956) observed and described unexplained foliar
symptoms on ponderosa pine. He named the condition ‘xdisease’.
A field experiment using branch chambers was
designed to examined the relationship between urban smog
containing high levels of ozone and visible foliar symptoms
on ponderosa pine needles (Miller et al., 1963). This field
experiment and later ozone fumigations of ponderosa pine
seedlings (Richards et al., 1968) confirmed that ozone was
the cause of the chlorotic mottle and early abscission of
affected needles.