This early after planting, it's important to keep the rootball evenly moist. Doing so often requires that the water be applied directly to that area. Every other day for 15 minutes may be accomplishing that, or it may be keeping the soil too wet. The only way to know for certain is to stick a finger, or a trowel, in the soil, then look/feel. Avocados require excellent drainage. Because of that, the frequent recommendation is to mound up the soil in the planting area to at least 8 inches, then plant the tree in the center of the mound. In that way, one can avoid excess water at the crown of the tree. Soggy soil is the all too frequent reason for root rot in avocados. Unfortunately, no treatment is available to stop root rot once it begins. I suggest you contact your county's Extension Service office. More than likely they can offer specific suggestions for an avocado in a home garden. Locate contact info for the Ventura County's office using this map: http://www.csrees.usda.gov/Extension/ Jean, who gardened for 30-some years in Long Beach, CA Here is a link that might be useful: locate your county's Extension Service office

I suspect everyone hardens their plants a little differently, and if you search this forum you'll undoubtedly find many methods. But I start by setting the plant in the shade for an hour, the next day 2 hours, the next day 4... maybe starting to give it a little sun, gradually building up to them being outside all day (and all night - here, nighttime temp is a concern.) Sometimes I kinda have to do it based on my work schedule and what the weather is like. I'd say take at least a week to 10 days to harden. It looked like your seedling was healthy when it went out, so I think you'll be successful this time around!

Where did you get your tree from? Home Depot? I wonder is the issue is the roots i.e. Rootbound.

I am located in northern California, just south of the Bay in San Francisco. Our weather is usually mild & fairly consistent. Temps during the day have been 75-85 give or take. The temps do have a tendency to drop quite a bit come nightfall as the coastal fog starts coming inland bringing the temps down to the low 60s, at times in the upper 50s. My soIL has a PH of about 6-6.5. I used chicken manure, some sulfur to bring down the ph, it was at 9, so had to make adjustments. I also added peat moss, as my soIL is a bit on the clay side, it held water & wasnt the best at draining. Its much much better now. I initially had the soil tested & was given recommendations as to how to improve it, I made the corrections prior to planting. The strange color that is seen is not the soil but is mulch, gorilla hair, it's shredded redwood bark. It was what was recomended to me by my local nursery. A side note; All my other tomato plants that I planted at the same time are doing great, it's just the one. I water every 2-3 days dependent on the temperatures & according to the moisture levels that I test frequently. My soIL holds onto water a bit so I'm careful at being consistent. Initially I did have aphids but have eradicated them with use of compost tea which was really effective, there has been no sign of them in almost 2 weeks. There's been no sign of any other pest at all.