- Author: Alison Collin
If you have fruit trees you are probably now getting a little tired of raking up fallen fruits which are the result of a natural occurrence known as the “June Drop”. This year the phenomenon is extending well into July, to the point that one wonders if there will be a crop left to harvest. So what is normal, and when do our trees need help?
Apples and pears produce several flowers on each cluster in spring, and depending on pollinator activity, and the local availability of another apple to cross pollinate, many, many flowers may set fruit. However, there will also be numerous little fruitlets that did not get pollinated properly, are damaged by insects, or require more carbohydrates than the tree can produce, and these fruits wither and fall off, frequently in large numbers.
As the remaining fruits begin to swell many more get attacked by codling moth, are affected by lack of sufficient water, or otherwise get damaged, and since the tree can only support a certain sized crop it goes through another round of shedding, usually beginning in June and continuing until early July, often resulting in carpets of larger fruit on the ground - frustratingly too immature to use for any practical purpose.
It is thought that warm nights may cause the fruit drop to be excessive since the respiration rate of the tree causes more carbohydrates to be consumed during the hours of darkness, leaving less available for fruit production.
Another factor could be lack of nitrogen, so a careful fertilization regime should be followed, being aware that too much nitrogen can also cause fruit drop and excessive growth. For information on fertilization see: http://homeorchard.ucanr.edu/The_Big_Picture/Fertilization/
In areas such as ours lack of water will also be a factor, so make sure that you check your irrigation system and remember to seasonally adjust the amount. A mature semi-dwarf tree can produce over 400 pounds of apples which, after all, are made up mainly of water!
On a large, heavy cropping tree, thinning young fruits early in the season reduces the natural shedding, and picking off any obviously damaged fruit as the season progresses will also help, but not eliminate the problem entirely. If a fruit is scarred or misshapen this will only grow with the fruit, so one might as well remove it and save the tree from wasting resources trying to get it to maturity. Infected fruits should be properly disposed of so that they do not create a source of infection. For information on thinning: http://homeorchard.ucanr.edu/The_Big_Picture/Fruit_Thinning/
Generally speaking, if the tree looks healthy and has a heavy crop of fruit falling in mid-summer should not be cause for concern.
- Author: Ben Faber
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.
- Author: Ben Faber
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
- Author: Ben Faber
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
- Author: Janet Snyder
Do you have an olive tree, Olea europaea, in your landscape? We do - a 'Kalamata' olive. It's a beautiful evergreen tree that, once established, requires very little water. The pollen can be highly allergenic. Ten years ago, we brought it home from the Vacaville Home and Garden Show. It was a spindly little stick, not more than ten inches tall from root ball to tip. We were excited at the prospect of harvesting our own olives and curing them with a method my husband read about that used only water and salt instead of lye. Let me say, LOTS of water and salt. We planted it in the back portion of our vegetable garden, about two feet from the fence line. No, I was not a master gardener ten years ago, or I would have known to plant it farther from the fence.
Over the years, it's grown to about 20+ feet tall. Since then, that vegetable garden has been relocated to accommodate a swimming pool, the size of which was dictated by the location of the olive tree. My husband and I are out there pruning the tree a few times a year to keep it from hanging over the neighbor's yard. In the spring, the yellow pollen from the tree coats the patio and chairs in a thick yellow dust. From now until January, I am constantly picking up dropped olives to alleviate staining of the concrete. And the olives we were going to cure? Haven't eaten a single one. On the upside, the tree provides a terrific area of shade to sit under, and lends a distinctively Mediterranean atmosphere to the backyard. It is a beautiful tree.
So what have I learned from this tree? Two feet between a tree and a fence is nowhere near enough. Curing olives takes time and patience. Even though olive trees grow well in Solano County, don't plant them near a pool!
Don't put a pool next to an olive tree.