Orange and Mango Tree Problems and the Amazing Trifoliate Orange

I have a Valencia seedless orange tree that I set out about 40 years ago. The fruit has always been sweet and juicy. But the last couple of years the oranges are larger with thicker skin and not much meat and less juice. Can you tell me what I am doing wrong?
Charles Johnson, North Hills

Sour Oranges

In all likelihood, you are seeing sour orange fruit, growing from a branch or branches that have grown off the bottom of the trunk.  Nearly all fruit trees, at least those available in the nursery trade, consist of a scion (desired fruit bearing variety) grafted onto a rootstock species.  The fruit you describe is from a rootstock species known as sour or bitter orange (Citrus aurantium), which was commonly used forty years ago as a rootstock for sweet oranges (Citrus sinensis) such as Valencia and navel.

Scion varieties benefit from being grafted onto rootstock species by demonstrating qualities they would otherwise lack if clonally propagated from their own stem or shoot tip cuttings.  These qualities include one or more of the following:  greater vigor and productivity, disease resistance, drought tolerance, cold tolerance, and salt tolerance.  Sometimes a shoot below the graft union, emanating from the rootstock, develops into a branch or branches and begins to fruit, which seems to have happened in your case.  Follow this fruiting wood back to the trunk and remove it with a saw.
Your lack of sweet oranges could have to do either with rootstock growth overtaking and sapping vigor from the scion or it might simply be a consequence of getting old.  Seedless Valencias may decline significantly in productivity once they reach 40 years of age if not sooner. (Note: the average lifespan of all citrus trees is around 50 years.)  Rootstock species are chosen for the vigor they impart to the scion.  However, once the productive lifespan of the scion is exhausted, the rootstock species may still be going strong and it is that much likelier to take over the growth of the tree.
My mango tree has lot of flowers and tiny fruits every year;  however, the flowers and most of the  fruits don’t last because the flowers and stems are covered by a layer of white powder.  In addition, some branches turn brown for no apparent reason.  I ended up having only two mangoes last year, but they were big and sweet.  Please advise.
Nathalie Lee, Rowland Heights

powdery mildew fungus on mango flowers, photo by Nathalie Lee

Your tree is stricken with mango powdery mildew, a common malady of mango trees.  The powdery mildew involved is specific to mango trees alone.  Suggested remedies include spraying sulphur, ultrafine horticultural oil, ordinary milk, or this mixture: 4 level teaspoons of baking soda, 1 teaspoon of mild dish soap (Dawn or Ivory), 1 gallon of water.

Hygiene is also crucial in control of this disease.  You need to remove infected flower panicles at the first sign of mildew and immediately dispose of fallen panicles, leaves, and fruit since they carry spores of the fungus that may splash (during irrigation or rain) or float (from wind) back up into the tree.  Similarly, if you have a camellia whose buds drop before opening, it is due to a fungus that remains on those fallen buds and so they should be immediately picked up and disposed of as well.

trifoliate orange tree foliage changes color in the fall

Tip of the Week:  One of the most versatile, yet largely unknown, citrus species is trifoliate orange (Poncirus trifoliata).  It differs from your average citrus in being deciduous and extremely cold hardy.  Due to its wicked thorns, it was planted years ago as a living fence at the Brooklyn Botanical Gardens.  Beyond its thorniness, spring flowers are highly fragrant, its leaves and golf ball size green fruit turn yellow in the fall, and it is pest and disease free.  Trifoliate orange, sold for $2.59 per seedling on eBay, may also be utilized as an exotic indoor plant for sunny exposures.

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