We’re addressing two different problems here. First, Quackgrass and how to control it. Second, a problem with fungus on petunias and how to control it.
Here is what Quackgrass looks like.
To rid yourself of this pesky weed, Tom Samples of the University of Tennessee recommends the following products:
If your petunias are dying, but your other flowers, such as marigolds are flourishing (as in this picture), you probably have Phytphthora crown and root rot, according to Dr. Mark Windham of the University of Tennessee. This type of scenario is not uncommon, especially when there has been a lot of rain and/or plants kept wet with irrigation. High levels of nitrogen fertilizer may also enhance problems from this disease. In petunias, the disease usually kills from the top down.
The most effective way of preventing rot diseases is to provide good drainage and to practice good water management (avoid soil saturation). Along with appropriate cultural controls, the fungicide Fosetyl-AI (Aliette) may be used. When applied as a foliar spray it is absorbed by foliage and moves into the roots. However, do not rely on fungicide applications alone to control root and crown rot diseases.