Fungal disease of some kind. How often do you water and for how long? How much rain have you gotten? Are you down in a low area or has that area been covered with brush or something? It looks like it was smothered or otherwise protected from sun and wind for a time. I take an organic approach to lawn fungal diseases. I use ordinary corn meal at a rate of 20 pounds per 1,000 square feet. Corn meal attracts another fungus which is predatory on the fungus causing your disease. That fungus is called trichoderma (try ko DER mah). It takes 3 full weeks before you see any improvement. That can be the longest 3 weeks of your life, but after that you should see less yellow and much more deep green. Corn meal is also an organic fertilizer so the dark green color will be very noticeable at the 3 week point. If you have used a chemical fungicide recently, then the corn meal will not work against the disease but it will definitely green up the grass. And yes this is the same corn meal you cook with. You can get it in 50-pound bags at the feed store. And no, it will not attract varmints. Seems like it would but it does not. It does seem to attract birds but they don't eat it. They just come and visit. You can apply corn meal any day of the year, rain or shine, or every day of the year, without fear of hurting anything. If you wan to take a chemical approach, I cannot help you. Our temperatures, humidity, and rainfall are usually outside the recommendations on most chemical fungicides. I gave up on them years ago when I found out how well corn meal works. On another issue you did not ask about: You are mowing your St Augustine much too low. Raise your mower all the way to the highest setting. It will take a month or so before all the grass gets tall enough to be mowed but it eventually will. Mowing it low favors the bermuda which is mixed in with it. Mowing it high can have the effect of choking out the bermuda. Tall St Augustine looks very lush.

I think I saw through yardtractor's comments, and found them humorous. He hit all the antiquated high spots from back when the only version of organic gardening was Rodale's approach using 100% compost. I would add stinky since a lot of people use manure based compost which was often not completely composted. You would have to go back to pre-2005 to really get into the flame wars between organics and synthetics. Lots of people on lawn forums around the Internet were banned for life. And I agree that a lot of nonsense is repeated as fact on the forums. I prefer to use photos showing results and let the reader decide what to do. For example this photo taken by former Lawn Care forum participant, mrmumbles, shows his zoysia lawn about 30 days after spot treating with alfalfa pellets. You don't have to look too closely to see the improved color, density, and growth provided by a true organic fertilizer (not compost). But back to the OP... Are you in love with that large tree in the middle? If so then I would suggest creating a planting bed around it and fill the area with something that will hide the roots. Go to my member page and look at the trees in my yard. The three trees on the right are surrounded by "cast iron plant" for about 4-6 feet out from the trunk. The one in the back did not have roots at the surface so it was left alone. Whatever you do, if you bring in soil to cover your roots, do not bury any of the trunk of the tree. Leave some of the root flare showing and stop with the soil. Yes you did not use enough seed, or you did not keep it moist everywhere. You did seed at a poor time of year, so if you want this fixed, get ready for late August. That's the time. It will give the new grass plenty of time to develop roots which can resist the summer heat. Also the summer annual weeds are not germinating in August, so your weed pressure diminishes a lot. Sometimes it is expedient to spot spray with chemical herbicides and get on with your lawn life. You will still benefit from having been on an organic fertilizer program and continuing with that afterward. I call it tough love. They used to make a foaming Weed-B-Gone that was amazing. You could leave a spot that looked like shaving cream on individual weeds. There was absolutely no overspray and nothing touched the soil. Don't know why they don't sell it anymore. It would wipe out dandelions in 2 days. In the mean time practice deep and infrequent watering, mulch mowing at a high (highest??) setting, and fertilizing.

I am really tired today running between all the big box stores. At the end, I didn't option the lawn boy because I really can't justify the extra cost vs sears promo today. Plus, a technician at Rona's service center gave me a friendly warning about the Tecumseh engine, the supply of the parts might be good for next 2-3 years but can't promise anything at long run. I still have a thought that a good mower should last me 10 years ironed in my head... However, I really like the design of the lawn boy 10684, everything seems very well design and build quality is better than others. If it's around $350CND range and none self-propelled model, I would pull the trigger, no question ask. I went back to Sears and ordered the Craftsman 36933 model (3-in-1 with bag, side discharge, mulching), which has B&S 850 platinum engine (8.5 ft/lb torque), no choke and no primer needed. The price is just what I am aiming for, 289.99+taxes. The engine seems pretty well build but the rest of the parts are very standard, a regular every week lawn mower...

Well....tractor starts fine now, so today I decided to cut the grass. Not! When I put the "blade-lever" into the "engage" position...the blades did not engage. Total silence. There is still good tension on lever when moving up to engage the blades, so spring is still connected, There was a lot of built up dirt/grass on deck. Thought maybe it was keeping the idler-pulley from moving to tighten belt, but that made no difference. The belt does not seem to tighten when lever is engaged with motor off. Does motor need to be running to tighten belt? Couple more things worth noting: 1-when it was working, it would squeal when first engaging the blades, but I just thought that it was just some initial slipping of the belt 'til fully engaged. Maybe not. 2-Blades turn, somewhat harder when lever is not in the "engaged" position, very freely when in "engaged" position. 3-Belt is not tight in either position. Anybody help with this issue? I will scan other entries to see if someone else has had same issue while waiting. Thanks for your help and time.