Neal ....
What fungal diseases ? If the article was referring to black spot or mildew ... no, the sulfur in the root zone will not prevent foliage diseases.
Sulfur is often used to discourage fungal disease as a spray on foliage, but the leaves have to be completely covered for the spray to work and it needs to be re-applied after rains.
I use eggshells in with my mulch because as they decompose, the provide calcium to the root zone.
I stopped putting chopped up banana peels out in the garden because they drew skunks. YUK! I don't think it would be a problem putting them into a planting hole.
