Iron deficiency could be cause of pansy foliage yellowing

If you are seeing interveinal chlorosis on newer pansy leaves, Univ. of Md. Extension personnel advise checking the growing medium pH. If the pH measures higher than 5.8, the disorder is likely caused by iron deficiency. At higher pH levels, iron in the medium is unavailable to the plant. A high pH also promotes Thielaviopsis, which causes black root rot. They recommend maintaining a medium pH level of 5.4-5.8.
A foliar spray of iron chelate (4 oz. per 100 gallons) can be applied. The iron chelate should be rinsed off the leaves to avoid foliar burn.

Pictured: At soil pH levels above 5.8, iron in the medium is unavailable to the plant, which can result in leaf chlorosis.
Photo courtesy of Univ. of Md.