Field topworking avocados and citrus can be a prolonged process that can take two years of maintenance before the trees are productive. Often, it's easier and cheaper to just start off with a new tree. Thegrafted tree needs to be pruned back and buds or bud sticks need to be inserted into the old trunk. Birds love to sit on these new perching spots, and can often damage the bud stick, so they need to be protected with some type of covering like a paper bag. The latent buds in the trunk need to be constantly removed to prevent the old scion from outgrowing the new one.
A nurse branch also needs to be maintained so that there is a steady flow of transpired water to cool the trunk. Bud sticks are leafless and when the tree is headed back to graft, the tree no longer has the leaf area to pump water through the trunk. The trunk can heat up and result in sun burn damage. This damage may not appear for a year or so after the incident has occurred. It normally will show up on the south or southwest side of the tree, which catches the afternoon sun. The damage may be so severe that the tree dies, or the tree may begin a healing process as with other wounds and start growing callus tissue along the margins of the sun burn damage. If the damage is nor too extensive, the callus growth can eventually cover over the damage with new bark. If the grafted tree has left some structural branches, damage can show up on upper surfaces of branches, as well as those sides facing the afternoon sun.
To avoid sunburn damage, retain the nurse limb for about a year after grafting and whitewash or cover the trunk with a protective paper. Managing irrigation is important, as well. If grafted trees are interplanted with trees with more mature canopies, the grafted trees should be on a separate irrigation system and schedule. This is to avoid water stress which can enhance sun burn.
Sun burn can happen to any grafted tree, especially those with thin bark, like citrus and avocado. Any time a portion of the bark is exposed to sun that has previously been in the shade, can also set up the tree for sun burn. So excessive pruning going into summer months can also cause significant sun burn.
Sunken area of trunk, showing sun burn damage and callus growth starting to grow along margins of damage. Grafted citrus with nurse limbs and whitewash.
Transpiration is essentially a function of the amount of leaves present. With no leaves, there is no transpiration and no water use. The extreme case is tree removal. If canopies are pruned there is reduced water use. The more canopy reduction, the more transpiration reduction. Most citrus produces terminal flowers, so there is also a reduction in yield, but there is also typically an increase in fruit size as competitive fruit growing points are removed. There is a balance between yield reduction and tree water use, but typically a 25% canopy reduction results in a 25% decrease in tree water use (Romero, 2006).
The severity of the drought will determine how drastic the canopy should be trimmed. The trees can be skeletonized so that only the main structural branches are left. The tree is whitewashed to prevent sunburn and the water is turned off. As the tree gradually leafs out, the water is gradually reapplied in small amounts. It's important to check soil moisture to make sure the tree do not get too much or too little water. The trees if pruned in the winter will often flower a year later in the spring, but normal production will often take three years for the trees to recover their previous yields.
Skeletonizing should first be practiced on orchards that are the poorest producing. In those areas that get too much wind and have lots of wind scarring or elevated water use, those areas that are most prone to frost damage, those areas that have been always problematic, such as fruit theft. In areas that are healthy and a new variety has been contemplated, this is the time to topwork and replace that old variety. In areas that have been poor producing from disease, this is the time to get rid of those trees.
Canopy sprays of kaolinite clay have shown some promise in reducing transpiration with negligible yield reduction (Skewes, 2013; Wright, 2000). If these are used, they should be done under the advisement of the packing house to make sure the clay can be removed in the packing house.
With a reduced canopy, there are often other benefits besides water reduction. There is better spray coverage for pest control. There is also reduced fertilizer use. New growth is normally coming from nutrients that are now being mined by a large root system and fertilizer applications can be significantly reduced or eliminated altogether for a year until fruit set recommences.
Kerns, D. and G. Wright. 2000. Protective and Yield Enhancement qualities of yield of kaolin on lemon. In: Eds. G. Wright and D. Kilby, AZ1178: "2000 Citrus and Deciduous Fruit and Nut Research Report," College of Agriculture and Life Sciences, University of Arizona. http://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1178_3.pdf
Skewes, M. 2013 Citrus Drought Survival and Recovery Trial. HAL Project Number CT08014 (16/12/2013). SARDI. http://pir.sa.gov.au/__data/assets/pdf_file/0004/238414/SARDI-Citrus-Drought-Survival-Recovery-Trial.pdf
Navel trees skelotinized and topworked, ready for rain and more profits in the future.
Water moves in a wetting front. When irrigation water hits the soil it moves down with the pull of gravity and to the side according to the pull of soil particles (more lateral with more clay). Soil is a jumble of different sized soil particles, from clay to silt to sand sizes and then often intermixed with stones of different sizes from gravels to boulder. The different textures determine how water moves. It moves fastest through coarse textures and slowest through finer ones – the clays, the ones with the smallest pores. But soils are a jumble of particle sizes and pores.
Water first moves down the larger pores and then it slowly moves through the larger ones. As water moves through the soil it carries salts that have accumulated in the soil. At the wetting front is where the salt accumulate. As the water moves through the larger pores, salts migrate/diffuse from the small pores to the larger ones. This diffusion takes a bit of time, so typically the small pores have a larger salt concentration than the larger ones.
So an initial application of water will carry the salts from these large pores and if the irrigator were to stop in mid-application, it allows time for the salts to move out of the small pores into the larger ones. Then when the irrigation recommences, it will carry more of the salts out of the wetted area – the root zone. This technique is called “bumping” where an irrigation is stopped and then restarted in order to improve not only leaching, but also reduce runoff.
This principle also is at play when there are two or more sources of water quality. Soil salinity can be no lower than the irrigation water that is applied. Then as the soil water is removed through plant absorption or evaporation, the salinity increases. The soil salinity can easily be two to three times higher than the irrigation water.
If there are two sources of water, the initial application can be with the poorer quality water, and once that has reduced the soil salinity, then the better water quality can be applied which will then bring the soil salinity closer to that of the better quality water. By doing this two part leaching, the amount applied of the better quality water can be significantly reduced. This is a type of “bumping” to improve leaching.
Watch this U-Tube video on how water moves through soil, thanks to the work at Walla Walla Community College.
Thank you Walla Walla Community College for the video
Growers are still calling in about avocados with thinning canopies, fruit drop and sunburn and leaf death. Coastal avocados are always difficult to irrigate. Mild weather followed by dry windy conditions means growers have to scramble to get water on. If the trees are on some sort of calendar schedule, it usually means the trees get stressed. If trees are on a slope where pressure is not properly regulated, some of the trees are going to get stressed. If emitter clogging is not addressed, then more trees get stressed. And lack of rains to leach salts from the root zone, and more trees are stressed.
This stress sets up the trees for disease and a very common one in an avocado orchard that is filled with lots of leaves where decay fungi are working is stem and leaf blight. The disease causes defoliation and exposed fruit sun burn and drop. In an orchard, it's possible to see healthy trees and sick ones at the same time. This may be due to the differences in soil type from tree to tree or the fruit load on trees - more fruit, more stress. Looking out over the orchard there may be a polka dot of sick trees. And it might all happen at once in a week or gradually.
So, this is a problem that is out there, if the irrigation issue is corrected, the trees usually recover. It might require white washing and pruning out dead tissue. If it is a young tree under two years, it might actually kill the little tree, but the disease is not usually fatal, just loosing the fruit.
This is a disease that goes to many different tree species - redwoods, eucalyptus, pine, Brazilian pepper, CITRUS. The cause is the same, water or salinity stress. To read more about this disease, go to:
A common sight in an orchard with stem and leaf blight. Healthy tree in foreground and thinning canopy in back. Another photo showing the blighted stems on a canopy with surrounding healthier trees. The die back characteristic on the leaves and random dieback in the leaf.
A common sight in an orchard with stem and leaf blight. Healthy tree in foreground and thinning canopy in back. Another photo showing the blighted stems on a canopy with surrounding healthier trees. The die back characteristic on the leaves.
Citrus response to irrigation water deficits have demonstrated that sensitivity of yield to water stress is dependent on the phenological phase in which water stress was applied. Adequate water supply is of major importance during citrus flowering and fruit set. A second critical period coincides with the period when fruit growth is rapid (fruit set to harvest). Depending on the level of water stress developed, the abscission of flowers and young fruits will be affected in the first case, as will fruit size in the second case.
For navels and mandarins it is possible to identify these critical periods in the crop and possibly allow stress when the trees are not in those critical periods. Some varieties though are complicated by having overlap of critical periods when another crop is present at the same time. Valencias can have two crops on the tree at the same time in spring and into summer harvest and coastal lemons can have fruit in all stages from fruit set to mature fruit at all times of the year. In the case of navels, reductions of applied water by 25% or more have resulted in no fruit yield reductions, if those water reductions do not occur during critical periods (Goldhamer, 2006; Domingo, 1996; Hutton et al, 2007). Water reductions during the rapid expansion period can result in significant fruit size reduction, though, and this period should be avoided if fruit size is critical to marketing (Goldhamer, 2006; Hutton et al, 2007).
In the case of coastal lemons, the stress should be avoided when the period of the most profitable crop is in rapid expansion, this is normally the summer crop. Each grower would need to identify, when the most profitable fruit size is important. Growers in areas that have more summer heat than the coast might practice a ‘Verdelli' irrigation practice, where water is withheld for a period of time, in order to force flowering that can often result in more summer fruit being harvested the following year (Maranto and Hake, 1985).
Domingo, R., Ruiz-Sanchez, M.C., Sanchez-Blanco, M. J. and Torrecillas. A.1996. Water Relations, growth and yield of ‘Fino' lemon trees under regulated deficit irrigation. Irrig. Sci.16: 115-123 http://link.springer.com/article/10.1007%2FBF02215619#page-1
Goldhamer, D. and N. O'Connell. 2006. Using Regulated Deficit Irrigation to Optimize Fruit Size in Late Harvest Navels. Citrus Research Board. http://citrusresearch.org/wp-content/uploads/2006-GOLDHAMER1.pdf
Hutton RJ, Landsberg JJ, Sutton BG. 2007. Timing irrigation to suit citrus phenology: a means of reducing water use without compromising fruit yield and quality. Australian Journal of Experimental Agriculture (47): 71–80. http://dx.doi.org/10.1071/EA05233
Maranto, J. and K. Hake. 1985. Verdelli summer lemons: a new option for California growers. California Agriculture 39(5): 4. https://ucanr.edu/repositoryfiles/ca3905p4-62870.pdf
Phenological stages of navel orange.