What are Two Things that Cause Major Problems in a Landscape?
By far, overwatering causes the most problems in the landscape. Many plants will take on extra water to a point. Then they weaken and become susceptible to bugs and disease.
2. Wrong plant in the wrong place
Every plant has a specific environment in which it thrives. Some plant species can tolerate a wider variety of environments than others. When a particular plant is planted in an environment that it doesn't like, you will have a constant uphill battle trying to keep the plant bug and disease free.
Here's an example: Have you ever seen gardenias at the nursery and think, "Oh, that smells so good. I am going to buy one and plant it in my yard!" Well, if you don't plant it in some shade and completely amend the soil to create an acidic pH, you will always have a sick gardenia.
What's bugging your plant?
Critters, weeds, diseases, and other assorted disorders can damage or even kill your plants. Go to ipm.ucanr.edu for an incredible (Really!) wealth of information from the most definitive source: the University of California.