Grass, just like any other living organism, can catch diseases which affect its health. Because your lawn is an interconnected ecosystem of related plants living together, any disease causing damage in one part of the lawn can quickly spread. Thankfully, there are things you can do, not only to prevent this damage from spreading, but also from beginning in the first place.
Just like with diseases that affect people, grass can catch a number of various strains. Instead of viruses or bacteria though, it is generally fungi that will cause issues for a lawn. Some symptoms of common bad fungi to watch for are dead spots in the lawn, white, spiderweb-like threads going from blade to blade, blades going from green to red to brown and even slime covering sections of grass.
The difficult issue with fungus is depending on the strain and the condition of your lawn, it can show up for directly opposing reasons. One main cause to look-out for is general ill health of a lawn. When the lawn is in a diminished state, it cannot fight off attacks from diseases as efficiently. This can even just apply to a warm season grass during the winter or cool season grass during the summer. A humid spell can lead to issues as can punishingly-high heat. Since the fungi already live in the lawn, but are just balanced by friendlier organisms, it only takes the right conditions for them to start to win the battle.
Watering too much can lead to some fungi that thrive in damp environments, while watering too little can lead to others if your lawn is impaired by drought. Similarly, too little fertilizer will harm and weaken the lawn but too much fertilizer can attract fungi that thrive in a nitrogen rich environment. So, finding the right balance of watering and fertilizing is important because erring too much in either direction can leave your lawn open to disease.
Also, it is generally wise to mow at a higher setting and to mow more regularly so as not to stress the lawn with drastic cuts. In addition, water your lawn in the morning rather than in the afternoon. This is because the moisture will protect the grass blades from the day’s hottest hours but will then evaporate by later day so as not to leave a moist state for fungi to thrive.
Ok, but what if you’ve done all that and still get a lawn disease? At this point it will be important to stop the spread of the fungus across your lawn. All of the factors above that help prevent it also help your lawn fight back. So if you can increase the health of the lawn, the disease will likely subside. If all else fails, you will need to use a fungicide, though. There are both contact and systemic fungicides, which can either work by directly contacting the fungus or working through the roots of the grass. These can damage the beneficial organisms which protect from diseases, but if a specific disease is getting control of the lawn, a fungicide should be used regardless.