Washingtonia Robusta leaf yellowing

[RobustaEnvirons]

Hello everyone, recently I've noticed that my one particular leave on my Washingtonia Robusta has been steadily turning yellow. Its been gradually turning yellow, and hasn't come on all of a sudden. I recently last week (with the arrival of summer temps; 80s & lower 90s) began putting all my Washies outside in the front yard. There they receive sunshine beginning around 12:38pm until about 8:30pm-9pm. I leave them out there until after the sun sets and then I bring them inside my sun-room for the night. Then, the next morning I check the hourly forecast and if there is going to be sunshine (which since its summer weather it has been sunny) I put them out for the day.  

All my other Washies are not showing any signs of yellowing, its only this one leaf on my more mature Washy. The leaf that is yellowing is the oldest leaf on that Washy, so I kinda suspect that its reaching the end of its life-cycle. As I understand it, the leafs fall off as new ones push through. Well, it does have a fresh new leaf growing vibrantly up day-by-day, and it also has a bigger/larger leaf next to the yellowing leaf too. Its still green and growing nicely, its just that leaf that's turning yellow/light green.  

I had been keeping my Washies in my sun-room, there they received a lot of sunlight (albeit indirect sunlight), but still got it. I try to water them just enough to keep the soil moist and I try to let them dry out a little between watering. Since they're a dessert palm I have never tried to water them a lot since I could accidentally over-water. So I don't think its due to lack of water. I just watered it the other day as well anyhow. I first noticed the yellowing about 3-4 weeks ago (could have been happening a long time before that though).   

That leaf has been on that Washy for over a year now and was at one time the only leaf that Washy had until the 2 other began grow soon after. I recieved this palm last August in the mail and it suffered Transplant Shock. This leaf was its primary surviving leaf that pulled this Washy through until the other leaves could join it. This leaf burned a little on the tip about 6 months ago, but stopped soon after. 

I'm just wondering if the yellowing is from the leaf beginning to dye off, too much sunlight, not enough water, or something else?   

 

Edited June 20, 2016 by RobustaEnvirons

[RobustaEnvirons]