Lawn diseases are caused by pathogens, usually fungi. The fungi are almost always around, living off dead and decaying material in the soil. You just don't notice them because the grass usually fends them off. Check out pictures of the common lawn diseases in New York. Preventing disease problems is the best strategy, because once the symptoms are visible chemical rescue treatments aren't recommended for home lawns.
Prevention means maintaining a healthy turf through proper mowing, watering and fertilizing.
If you water , do it early in the morning so that the leaves dry quickly. Avoid watering at night. Too much fertilizer can stress plants and leave them vulnerable to diseases. Plant disease-resistant grasses. Choose the right grass for the site.