Ornamental grass is a luxury for the homeowner who wants her yard to look its best. There are many different varieties, known for their different blade shapes, color variations, and ability to withstand different climates. As a result, the species suffer from different problems, but they do have a few problems in common thanks to rampant propagation practices designed to enhance their growth rates.
Oedema is the condition in ornamental grass where the grass roots absorb water more quickly than the water can actually be used. The internal cells which forms the veins in the grass through which water passes are called mesophylls. Because the plant cannot use the water passing through these mesophylls as quickly as it's traveling, the internal pressure of the mesophylls rises. This creates blisters along the lengths of the veins which will eventually harden. Water will continue to pool in these hardened blisters even after the period of oedema has passed. This is normally called by overwatering or a lack of sufficient light.
Nematodes are a large group of roundworms comprising roughly 80,000 species, of which almost 15,000 are parasitic. Only a small portion of them affect plants, let alone ornamental grasses. Depending on the species, nematodes attack ornamental grass in two ways. They can attack the roots, generating galls, or abnormal growths through which fluids and chlorophyll leak, slowly killing the grass. They can take up residence in the veins and vascular tissue of the grass, clogging the flow of nutrients and other essentials with their own waste. Outwardly this appears as a mottling spreading away from the veins.
Pesticides are not normally effective at killing infestations, so removing infested grass from the root up is generally the only accepted method of treatment.
Powdery mildew constitutes numerous species of fungi. They affect ornamental grass when they're exposed to an already-infected plant. Also, when ornamental grass is soaked by heavy rains or over-watered so consistently, this creates the moist environment in which fungal spores can germinate. They appear as a light dusting of white or grey spots, typically on the undersides of the leaves.
While powdery mildew does not kill or severely damage ornamental grass, it does leach nutrients from the leaves and can stunt further growth. General fungicides are effective in remedying the problem.