Some people think a simple stretched white line as seedy toe, but it is actually a hollow pocket created by a fungus that's entered a weak white line. Horses with shoes may be more prone to it, due to several factors from the general nature of wearing shoes. Nails introduce an easy path, the toe will usually have more stress on it than a beveled trim, and the shoe holds more dirt/mud/manure/moisture that breeds fungus and bacteria, plus the shoe itself is tight up against the foot, and doesn't allow much air in that area to dry it. Also, shoes slow blood flow, no matter how good the farrier, and the germs take advantage more easily.
Once it starts, it can become bad enough to nescessitate a "re-section" where a large portion is removed to allow meds complete access. Most of the time, you can avoid that by removing the shoes, soaking with the appropriate meds and a good trim. ACV is good, but may not always take care of it, and your farrier or vet can recommend what is right in your situation. If it persists, or comes back, something in your horse's daily routine needs a change, either change in trims or a cleaner environment. Sometimes, it can be a side-effect of chronic founder, the laminae seem to be forever weaker and diet plays a major factor. Cutting back on sugars/starches helps with those horses, as the fungus seems to be opportunistic, and horses that have diabetic-like issues will be more prone to foot problems (like founder, seedy toe, thrush, and canker). Takes experience to know what exactly you are dealing with. Even with a great barefoot trim, if it isn't frequent enough, the white line will beging to stretch, allowing for the infection, so be sure to stay on a regular trim cycle, even if the hooves "look okay" as barefoot hooves tend to look good longer, but they still need to be done in the usual 6-8 weeks cycle.