A pattern I have observed over the last few years is that as soon as the temperature drops (we are in Texas), something appears to cause the lawn to lose its immunity, such that small regions turn yellow and then die rather quickly. I have been able to maintain a thick dark-green turf all year (since spring) and fundamentally, there is no reason why anything needs to change as soon as we get the first cold snap -- but something does occur. A yellow rust-type fungus sets in very quickly and grass in these regions dies rapidly, and all you see is dead grass. The remaining areas seem just like the rich dark green during spring-summer type. I pretty much keep the insect infestation down by use of neem oil, and have noticed there are no chomped up blades of grass suggesting anything like sod-webworms or cutworms. I also haven't been hit with any brown spot or fungal infection all summer long. Through the latter part of summer we only used the irrigation system (1/2" water) twice a week and now I will turn it off if it rains. There must be some type of cycling the plant goes through after the first few periods of temp drop, causing poor immunity. Are there any organic applications (rules of thumb, independent from soil test pH, etc.) which can knock or alter this dormant-immunity switching mechanism?