From a Correspondent:
I have a sole Navel orange tree. The fruit suddenly began to turn color from green toward orange in the past two weeks. Any fruit that achieves all-over light orange drops from the tree. They taste bitter as they have not sweetened yet. In addition, those that drop have thin skins.Dear Hopeful, The best thing you can do right now is follow your friend's advice: water the tree deeply, down three feet into the soil, and at least as wide as the drip-line. If you did this last week, wait another week before doing it again; every two weeks is as frequent as you want to water right now. If last week's watering wasn't a full three feet down, go ahead and do that now.
The rest of the fruit seems healthy but is galloping toward ripeness. Since the tree only has about 35-40 oranges on it, better than last year's urban crop, we hope to preserve the rest. A friend told me to be sure to deep water the tree as it may have been caused by our warm fall and lack of deep watering. I did so last week but it might be too little, too late.
Is my entire crop in trouble? Is there anything I can do now?
Hopeful in Chandler
I addressed the problem with Navel Orange fruit drop in a previous post earlier this year. What you're seeing now is much the same problem, exacerbated by an extremely stressful Summer. As you know, your July was the hottest on record, the monsoons rains were AWOL, and you have yet to have any Winter rains. All of that has been hard on your tree, and it's responding by shedding fruit.
The early coloring of the fruit, usually a hopeful sign of impending harvest, is in this instance a signal that the tree is giving up on the given fruit, and letting go. As much as I hate to say it, the deep watering is the only thing you can try, and it may not be enough to save this year's crop; Navel Oranges are temperamental in the Phoenix area in the best of circumstances, and these are not the best of circumstances.
For future years, take a look at the approach outlined in the earlier post, and be certain to supply the tree with deep but infrequent irrigation during the worst Summer months. If it's any comfort at all, know that you are not alone in getting whacked this year. With any luck, next year will be back to normal, or we will at least be prepared if it's not.
I hope this helps,