Posts Tagged: fruit drop

Fruit Load Effect on Tree Water Use

As fruit gets heavier and heavier on a tree, does the tree use more water?  Or is it sensitive to more water stress?  It's not clear if there is a difference.  We've all seen a lemon tree sigh in relief after the crops been pulled off.  And avocado trees last summer that got hit by the Santa Ana seemed to be more affected the more fruit they had.  In fact, about two weeks after the wind blew, many trees with a heavy fruit load showed clear signs of salt burn or leaf blight.

Now according to work done on olive, the trees actually need more water to avoid stress.  In an Israeli study they measured water use by olive trees with and without fruit and found that there was significant more water use when fruit was present.

http://treephys.oxfordjournals.org/content/early/2016/01/22/treephys.tpv138.full

This is a lot more than we usually allow for in our irrigation scheduling.  Often we don't adjust at all for fruit load.  As fruit sizes most growers do recognize this as a period when stress might cause fruit drop and pay more attention to the trees, but may not alter the irrigation cycle. 

Fruit have stomata just like leaves do, so they do lose water.  Losing too much water leads to flaccid fruit and in the case of avocado easy fruit dehydration and drop.  So we know it's a sensitive period, but maybe we need to be irrigating more frequently.  That's where soil moisture measuring devices help adjust the schedule.  If they are using more water, then irrigate more frequently.

We'll be doing some work over the next few years to verify this.  Stay tuned.

fruit drop drought

Wow,There are so Many Avocados on my Tree! Should I Apply NItrogen at this Time?

Nitrogen and its Effect on the Balance of Vegetative and Reproductive Growth

With the big avocado crop out there, the question came up about whether a grower should add extra nitrogen to encourage more canopy growth to protect the fruit from sunburn or whether that would cause the fruit to drop. There is in our understanding the idea that there is a competition for resources and that in that competition one must best another. A commonly held belief is that if you apply nitrogen at the wrong time it will push resources to vegetative growth at the expense of fruit. This is somewhat true for annual plants that get most of their nutrients from outside sources (soil, air, fertilizer, water), but trees have a huge buffer in their storage organs (roots, stems, leaves, etc.). Most growth in trees occurs from this storage source and most importantly from photosynthesis and the sun. The more sun captured the more energy for flowering and fruit production.

So it is this competition for photosynthates that becomes the most limiting factor. When there is not enough to go around, the tree sheds fruit. If you see fruit dropping off a tree after applying a slug of fertilizer, it's a salt effect. Too much salt and it causes a water competition and the tree is stressed.   It's not the nitrogen, but too much salt. With fertigation this is not so likely to happen as when dry fertilizers were applied and someone got too aggressive with the application

In fact a dose of nitrogen fertilizer is a good idea at this time when there are lots of clusters of fruit. This can encourage a flush of leaves that will protect the fruit from sunburn and damage that would cause fruit to drop. Also at this time, persea mite has been building its population and is starting to cause leaf drop. A bit of nitrogen to encourage leaf replacement is a good approach to dealing with this mite damage.

For further reading about the competition between vegetative and reproductive growth as affected by nitrogen (or little affected in fruit trees by nitrogen), D.O. Huett wrote a wonderful review of past research on this topic:

http://www.publish.csiro.au/?act=view_file&file_id=AR9960047.pdf

avocado fruit

Wow, so many avocados! Should I add more nitrogen?

Nitrogen and its Effect on the Balance of Vegetative and Reproductive Growth

With the big avocado crop out there, the question came up about whether a grower should add extra nitrogen to encourage more canopy growth to protect the fruit from sunburn or whether that would cause the fruit to drop. There is in our understanding the idea that there is a competition for resources and that in that competition one must best another. A commonly held belief is that if you apply nitrogen at the wrong time it will push resources to vegetative growth at the expense of fruit. This is somewhat true for annual plants that get most of their nutrients from outside sources (soil, air, fertilizer, water), but trees have a huge buffer in their storage organs (roots, stems, leaves, etc.). Most growth in trees occurs from this storage source and most importantly from photosynthesis and the sun. The more sun captured the more energy for flowering and fruit production.

So it is this competition for photosynthates that becomes the most limiting factor. When there is not enough to go around, the tree sheds fruit. If you see fruit dropping off a tree after applying a slug of fertilizer, it's a salt effect. Too much salt and it causes a water competition and the tree is stressed.   It's not the nitrogen, but too much salt. With fertigation this is not so likely to happen as when dry fertilizers were applied and someone got too aggressive with the application

In fact a dose of nitrogen fertilizer is a good idea at this time when there are lots of clusters of fruit. This can encourage a flush of leaves that will protect the fruit from sunburn and damage that would cause fruit to drop. Also at this time, persea mite has been building its population and is starting to cause leaf drop. A bit of nitrogen to encourage leaf replacement is a good approach to dealing with this mite damage.

For further reading about the competition between vegetative and reproductive growth as affected by nitrogen (or little affected in fruit trees by nitrogen), D.O. Huett wrote a wonderful review of past research on this topic:

http://www.publish.csiro.au/?act=view_file&file_id=AR9960047.pdf

avocado fruit

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