May 31, 2011: This article has been updated with the current links.
Brown leaves are a common problem towards the end of our long, dry, windy winters due to dry soil and lack of watering. But there are other possible causes, but fortunately it is more likely to be an environmental stress rather than a plant disease.
Marginal necrosis (browning along the edges) may result from dehydration, high soil salinity, herbicide exposure, or an iron deficiency. Plants can become dehydrated from lack of water, windy conditions, and root problems. Brown leaf tips are also a symptom of too much salt. Brown blotches on the leaves may result from dehydration, too much sun, or high temperatures. Herbicide exposure can produce spots. Interveinal necrosis may be caused by herbicides or a severe manganese deficiency. When entire leaves have turned brown, the culprit could be dehydration, herbicides or cold winter winds.
Symptoms develop gradually when due to high salinity or nutrient deficiencies. Damage will appear suddenly if caused by high temperatures, too much sun, or herbicide exposure. Salt will cause more extensive damage to the older leaves, while iron deficiencies are more severe on new growth. Sun and high temperatures will damage the foliage with the most exposure. Root damage will affect the leaves at the edges, farthest from the roots.
The information provided for this blog article came from the University of California publication entitled Abiotic Disorders of Landscapes Plants: A Diagnostic Guide by Laurence R. Costello. An excellent source for finding out what is ailing your plant. http://anrcatalog.ucdavis.edu/LawnGarden/3420.aspx