Answer: Unfortunately based on your description I am not certain what happened. Leaf mold refers to a mulch or soil amendment made from chopped up leaves that have half way rotted so I do not think that would be the problem here.
Fungal or bacterial infections could cause foliage problems, however usually they are not along the edge of the leaf as you described, instead they are in spots or blotches. They also would not kill a plant virtually overnight.
Sometimes a mishandled mulch is hot (smells like ammonia or turpentine) when it is applied and it gives off toxic gas for a short time that can damage plants and cause the symptoms you described. Once the mulch no longer smells bad it is no longer dangerous to the plants, but the damage is done. If this is what happened, there is nothing to be done for it now. They should not show much growth this fall because it is the season when plants naturally begin to slow down in preparation for winter. Hopefully they will be able to recover eventually -- in the spring.
Another possibility is that some sort of weed control product was applied. I would strongly suggest you consult with the owner about what types of chemicals they may have used (or previous occupants may have used) to see if there is something with a residual effect that could be affecting new plants.
Another possible cause for browning edges could be excessive fertilizer or fertilizer burn. Again, this is a wait and see problem. If you suspect overfertilization occurred, do not fertilize your plants next spring. Instead use a top dressing of good quality compost. If a granular fertilizer was heavily applied to the soil surface, you might be able to scrape it away unless it has already fully dissolved.
Yet more possibilities might be due to transporting the plants in an open vehicle, poor planting procedure, insufficient soil moisture, or even transplant shock.
You might also want to consult with the nursery where you purchased your new plants and with your local county extenstion to try to obtain a specific diagnosis of what is happening. They may appreciate photos of the overall plant and closeups of the affected foliage, as well as freshly collected foliage samples kept cool and enclosed in a clear plastic bag so they stay fresh.
I hope the problem is nothing too serious.
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