Navel Oranges Splitting Open

Question from Jon:
I trust you are enjoying your Sunday and rooting for the Chargers.

I noticed today that some (or most) of my naval oranges have split on the bottom. Can you tell me what you think might be causing this? There is some white “fuzz” which might be aphids. Can aphids cause this to happen. I’m attaching a couple of photos that show you what they look like.

I hope you are doing well and that you have a very enjoyable Thanksgiving.

Answer from Pat:
What a shame that many if not most of your immature navel oranges are splitting open on the bottom. It is really heart-rending when this happens and there is nothing much you can do about it now except clip off the fruit that is splitting open and pick up those that fall off the tree. Splitting of citrus fruit is not caused by any diseases or pest, so there is no spray or chemical or beneficial insect that can cure it. The white fuzz you mention has nothing to do with the splitting fruit, though open fruit can attract ants and ants bring aphids and other pests.

Splitting fruit is caused by unusual temperature swings or by wind and dry air alternating with rain or heavy irrigation. Such conditions can create uneven soil moisture, which can cause fruit to split. The problem may also be exacerbated by uneven fertilizer levels. This problem usually happens in fall as you have experienced and most often afflicts navel oranges, again which is exactly what happened to you. It is especially bad with citrus that has thin skins, and it appears from the photograph you sent me that the fruit you are holding has thin skin. Also, the fruit usually splits on the bottom where the skin is weakest. Once the oranges have split open there is nothing one can do to save them and one cannot eat them, either, since the fruit with split skins is usually under-ripe and too sour to eat. (I suppose you might try making marmalade out of it, but it might be too bitter even for that purpose.) Most of the damaged fruit will eventually drop to the ground. Be sure to keep it picked up regularly or it will rot on the ground thus leading to future fungus problems with rotting fruit on the tree itself.

Here is what the mechanism seems to be in regard to split fruit: Imagine a tree covered with immature citrus fruit in fall. Perhaps the gardener or farmer has a regular routine of watering, but the weather may not act according to any routine. Let’s say that there is a hot dry spell, perhaps with wind also and the tree gets dried out. The leaves may wilt or curl from dryness and the fruit especially may become somewhat dry, including the skin, which makes it a bit brittle. Then comes heavy rain or heavy irrigation. The thirsty tree pumps in all that water and the fruit gets full of juice plumping up suddenly. But unfortunately the skin of the fruit can’t grow fast enough to keep up so the pressure of the juice and plump segments inside pushes outwards and bursts the skin, tearing the fruit open on the bottom, the blossom or navel end, which is weaker than the top. Another possible scenario is when fruit trees are fed too much all at once, resulting in a growth spurt on the inside of the fruit but not the outside.

The way to protect your tree from having this happen again will be from now on to be highly observant of the weather in fall and also be aware of soil moisture levels, especially in fall. (It might be worthwhile to purchase a professional tensiometer that you could shove down into the ground a foot or two and be sure that soil moisture levels stay even.) If there is dry weather, such as an expected Santa Ana, be sure to water your trees well in advance so they do not dry out. If you travel in fall adjust your irrigation with these possible problems in mind. Also try to keep fertilizer levels in the soil even instead of creating spikes in the nutrient level. In a home garden organic fertilizers, which are slower acting combined with organic mulch under the tree can greatly help to avoid problems with splitting fruit. This is based on my experience and observations. Most commercial growers do not mulch their citrus trees and they may have reasons for not doing so. In a home garden, however, mulch combined with wise irrigation and fertilizing practices can help maintain even fertility and moisture in the soil.