- Author: Ben Faber
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.
- Author: Ben Faber
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.
Citations
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.
- Author: Ben Faber
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).
Citations
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.
- Author: Ben Faber
Recently I was asked why an irrigation schedule could be projected for almond and citrus in the Central Valley (Almonds:http://cekern.ucanr.edu/Irrigation_Management/Almond_Drip_-_Microsprinkler_-
_Flood_Weekly_ET/Citrus: http://cekern.ucanr.edu/Irrigation_Management/Citrus_ET_by_age/ ) and why the same couldn't be done for the main avocado growing areas. Here was my response:
Generating a generic irrigation schedule for avocados along the coast is very difficult and if done would be terribly misleading. Scheduling gets really hairy along the coast where avocados are grown. As you get further from the coast the water demand (ETo) increases in many months, typically increasing in the summer. This can be most pronounced in the late winter/spring when the fog along the coast really causes a contrast between coastal and inland conditions. May in Ventura, the sun comes out for about 2 hours and in Fillmore 20 miles inland it may be 90 F at 4 PM. The fog is a major determinant for irrigation demand and it varies daily, monthly and year to year from Monterey to San Diego. So fog can throw off an irrigation schedule.
The next variable to area-wide scheduling is the topography where avocados are grown, usually slopes to improve air and water drainage. Depending on the aspect and slope position, the ETo can vary tremendously depending on the sky conditions and what those conditions are depending on the time of day (such as foggy in the morning and clear in the afternoon). So west and south facing will always be higher than north and east. The top of the slope that intercepts more wind than the bottom and will have higher ETo than the bottom of the slope. And if the trees intercept more evaporative conditions midday when the sun comes out, it will be much higher than the east side in the morning when fog is dripping off the trees (zero evaporative demand). Then as you go south from Monterey to San Diego the ETo goes up, just because of latitude and sun interception. These conditions are very different from Fresno where ETo in July is 0.6 inches per day and is the same until Sept, the sky is clear most days and trees are grown on fairly flat ground.
Now throw in rainfall. Almonds are deciduous and only count on the value of rainfall as that which is stored in the rooting zone going into spring when leaves are come out. Avocados rely on winter rain for transpiration and salt leaching. In a good year a significant portion of the total yearly ETcrop can be subtracted from the irrigation demand. In a low/no rainfall year that all needs to be made up by supplemental irrigation.
An almond grower in the Valley might be able to go onto a calendar, set the clock if they have water on demand and walk away. That's never going to happen in a coastal avocado orchard. Depending on where the avocado is grown and the ETo at that site, applied water might vary from 1.5 ac-ft per acre to 3.5. This will depend on rainfall (when and how much), water quality (which determines leaching requirement) and the system delivery (system efficiency). This system issue can be further complicated by whether the delivery is on-demand or whether a certain amount will be delivered at a certain date for a certain length of time - 24 hours or 48. This makes it difficult for the grower to put on exactly what ETo and other issues the trees would demand. In this case, the delivery system determines the schedule.
So this is why there's no chart showing ET demand for coastal avocados where the bulk are grown in California.
A CIMIS (CA Irrigation Management Information System) DWR weather station for calculating crop water requirement.
- Author: Ben Faber
There is some variability in citrus susceptibility to both the psyllid (Westbrook et al and Hall et al) and the bacterial infection (Shokrollah et al and Stover and Mc Collum). It is not fully clear at this point which of these species would be best to include in a breeding program, but both scions (http://www.concitver.com/3erEIIC2011/Viernes_23_sep/Ed_Stover/Ed_Stover.pdf) and rootstocks (http://www.flcitrusmutual.com/files/4cbb1e3c-1e1f-4b04-a.pdf) are being evaluated.
Some varieties like Australian Finger Lime (Microcitrus australasica) might show more resistance/tolerance to HLB than other species. At the UC Citrus Variety Collection website, it's possible to see the wide range of citrus species that are available for breeding and the commercial availability of those species. The collection is a view into the different materials that breeders can select from, in order to improve resistance to this citrus disease.
Citrus Variety Collection
http://www.citrusvariety.ucr.edu/
Australian Finger Lime Collection and Availability
http://www.citrusvariety.ucr.edu/citrus/microcitrus.html
Susceptibility to Infestation by Asian Citrus Psyllid
Catherine J. Westbrook1, David G. Hall2, Ed Stover, and Yong Ping Duan
http://www.ars.usda.gov/SP2UserFiles/Place/66180000/Westbrook%20et%20al%202011%20HortScience.pdf
D. Hall, C. Westbrook, Y.-P. Duan, E. Stover, R. Lee and M. Richardson http://citrusagents.ifas.ufl.edu/events/fl_citrus2011/Hall%20ACP%20IR%20Citrus%20Expo%202011.pdf
Susceptibility to HLB
Hajivand Shokrollah, Thohirah Lee Abdullah, Kamaruzaman Sijam,Siti Nor Akmar Abdullah and
Nur Ashikin Psyquay Abdullah
http://thescipub.com/PDF/ajabssp.2009.32.38.pdf
Ed Stoverand Greg McCollum
http://hortsci.ashspublications.org/content/46/10/1344.full.pdf+html
Photos
Australian Finger Lime Fruit
Australian Finger Lime Tree