Question
I planted petunias from more than one grower this year.  Several days or up to 2 weeks after planting they started wilting.  Upon checking them the stems had turned brown and were hollow when they were mashed between my fingers.  This browning started at the crown of the plant.

Answer
I am assuming you watered the plants when needed. Petunias require 1.5" of water per week; more in hot conditions. They should also be mulched with straw or dried grass clippings to cool the soil and retain moisture.

If watering is not the issue, another possibility is a soil-borne fungus disease. Soil should be amended every spring with organic matter such as compost, aged manure, or peat. This composition helps ward off disease and pests. It also provides a constant source of nutrients to the plants throughout the year.

Another possibility is excess watering. Too much water will drown the roots and prevent them from absorbing nutrients. Once the roots begin to rot, the stem will wither and die. This is especially prevalent in heavy clay soils due to poor drainage.

The only other possibility is slugs eating away at the stem. You can usually spot them in the early morning hours or early evening. They can be controlled with a product called Escar-go or another one called Sluggo. Most garden centers carry slug pellets to control this insect.

I hope this answered your question. Please write again if I can ever provide assistance.

Regards,

Mike