- Author: Ben Faber
This is being posted again because there have been so many calls recently with the same problem - lack of good water management accentuated by lack of rain and salt damage.
When you see dieback in an avocado it could be due to several reasons, and here are three very common causes of dieback, one of which has been especially common when there is little rain. The first major cause of dieback is an overgrown tree where there is no light that penetrates into the canopy. Branches with leaves in a darkened interior will naturally shut down and dieback, leaving these twiggy dried out branches. This is a natural process whereby the tree just gets rid of leaves that are not performing.
Another cause of dieback is our old friend Avocado Root Rot, Phytophthora cinnamomi. This causes dieback, also called “stag horning” because of the dead branches standing out from the surrounding canopy. This is normally accompanied by a thinning canopy with smaller, yellow leaves and a lack of leaf litter because of lack of energy. It's also hard to find roots and if they are found, they are black at the tips and brittle.
http://www.ipm.ucdavis.edu/PMG/r8101311.html
The third major cause of dieback is a result of water stress. This shows up with low water pressure, at the top of the hill where the most wind occurs, where a sprinkler gets clogged, when the irrigation schedule is not meeting tree's needs or when there is not adequate rainfall to get sufficient leaching. And after four years of drought, this is very common. This appears as dead spots in the canopy, a branch here and there where the leaves have died and are still hanging. It's been called “salt and pepper” syndrome, because it can have a few branches here and there that have died back while the rest of the canopy is normal, the leaves are normally sized and green. In young trees, in severe cases, the fungus that causes this blight can work its way down to the graft union and kill the tree. In mature trees, it just causes an unthrifty look to the tree. Although we have always seen this problem in avocado orchards, this has become a very common affliction in orchards these last few years
http://www.ipm.ucdavis.edu/PMG/r8101311.html
1) Lack of light dieback
2) Phytophthora cinnamomi dieback
3) Stem and Leaf Blight
- Author: Ben Faber
Avocado is a fruit tree that is notable for its sensitivity to cold. There are tropical varieities that are very sensitive to cold most notably many of the varieties that are of a West Indian origin. The subtropical varieites that are grown in California are of Mexican and Guatemalan origin or hybrids of these two subraces. ‘Hass' is a variety that has genetic origins of both of these subraces. Mexican origin typically have more cold tolerance than Guatemalan sources. Mexicans can often sustain cold down to the mid-20 deg F for a few hours when trees are mature. Young trees can sustain short periods under 30 deg but can be severely damaged or killed for prolonged times (more than 2 hours). Temperature and duration are not the only factors for determining damage potential. Humidity, wind and surrounding environments (proximity to open water, enclosed areas with reflective heat, etc.) are also important factors.
We know from experience that some varieites of Mexican origin are more cold tolerant than other varieites, some being able to the very low 20's for prolonged periods are survive. They may not produce fruit the following year because all the fruiting wood may be killed, but they will still come back. Varieties like ‘Bacon, ‘Zutano', ‘Stewart', ‘Susan' and even ‘Fuerte' are notable for their greater cold tolerance than ‘Hass'. They may not have the same eating and shipping qualities as ‘Hass' so they will not be commercialized on the scale that ‘Hass' has. But some people like these lower oil content fruit.
It recently came to my attention that there are some low chill avocado varieties that were selected in Texas where winter temperatures are usually killing for most of the avocado varieites we have have. Texas has a much smaller acreage of avocados than California and even Hawaii. So small (under 500 acres) that yields are not recorded for this crop. Most of the trees are backyard trees that have much more protected environments than a normal orchard setting. No really systematic data has been collected on their cold tolerance, but word-of-mouth has identified several varieties that might be cold tolerant. These are ‘Holland', 'Wilma', ‘Joey, ‘Fantastic', ‘Lila' and ‘Poncho'. They may not be the best tasting (that is in the opinion of the taster, as always), the best shipping (thin skinned and damage easily), or hang on the tree for a long period of time, but may make for a good backyard tree in colder environments. There has been no systematic study to determine if and which of these varieties might be more cold tolerant that those that we already are growing in California. Desperate gardeners might find them worth growing in marginal environments.
The avocado as an evergreen subtropical will never likely be a commercial tree in Canada (greenhouses?) but there may be more cold tolerance in the species than we normally associate with the crop. For a description of some of the characteristics of the Texas industry see the bulletin: http://aggie-horticulture.tamu.edu/fruit-nut/files/2015/04/avocados_2015.pdf .
- Author: Ben Faber
Avocados and Water
Avocados are the most salt and drought sensitive of our fruit tree crops. They are shallow rooted and are not able to exploit large volumes of soil and therefore are not capable of fully using stored rainfall. On the other hand, the avocado is highly dependent on rainfall for leaching accumulated salts resulting from irrigation water. In years with low rainfall, even well irrigated orchards will show salt damage. During flowering there can be extensive leaf drop due to the competition between flowers and leaves when there is salt/drought stress. In order to reduce leaf damage and retain leaves, an excess amount of water is required to leach salts out of the roots zone. The more salts in the water and the less rainfall, the greater leaching fraction.
Drought stress often leads to diseases, such as black streak, bacterial canker, and blight (stem, leaf, and fruit). Leaf blight (Figure 1) is often confused with salt or tip burn (Fig. 2), but is actually a fungal disease that forms an irregular dead pattern on leaves and leads to defoliation. Blight is associated with lack of water, while salt burn is due to poor quality water and poor irrigation habits. Leaf blight often shows up after Santa Ana conditions, when growers get behind on their irrigations and the root zone dries out suddenly. There has been a high incidence of this disease the last two years. In both cases, defoliation leads to sunburned trees and fruit which can be severe economic losses. The only way to prevent these conditions is to keep up with your irrigation schedule.
Irrigation Management
To get your water to go further, it is important that the system is tuned in order to get the best distribution uniformity (DU). Many of our systems were installed 40 years ago and old age can lead problems, such as clogging, broken emitters, mixed emitters that put out different amounts and leaks. With poor DU, some trees get too much water and others do not get enough. Even fairly new irrigation systems can have poor DU, especially after a harvest. Poor water pressure on our step slopes is probably our main problem. A DU of 80% means 10% of the emitters are putting out more than the average and 10% are putting out less. The irrigator to compensate for the under irrigated 10% will run the system 10% longer to make sure the under irrigated trees get enough and over irrigating 10% of the trees with 20% more water than they need. A call to the local Resource Conservation District office can get a free DU evaluation and recommendations that are usually pretty reasonable to follow.
Aside from improving DU, it is important to know when and how much water to apply. When to apply can be evaluated by the hand or feel method (https://nutrientmanagement.tamu.edu/content/tools/estimatingsoilmoisture.pdf) which is fast and cheap. Or it can be done by tensiometer, Watermark or some of the more expensive electronic sensors. But these tools only tell you when to irrigate, not how much. This can be done by turning the system on (once you have made sure you have a good DU) and over the period of the irrigation insert a piece of rebar into the soil to determine the depth of infiltration. The rod will go down as far as the soil is moist and stop when it hits dry dirt. When you have about two feet of infiltration you will know how long to run the system to get an appropriate amount of water. A typical loam usually takes about 150 gallons per tree to two feet. Another way to get an approximation of the amount to apply is to use the Irrigation Calculator at http://www.avocadosource.com/tools/IrrigationCalculator.asp.
Managing the Tree Canopy
Significantly pruning trees can reduce the amount of water transpired by the tree. Trees that are about 15 feet in height, can be pruned by half and they will use half the water. Massive 30 foot trees would need to have a major pruning to significantly reduce water use. In extreme drought conditions and for the long term welfare of the grove, large trees should be stumped (Figure 3) or scaffolded (Figure 4) and paint white to prevent sunburn. Scaffolding usually produces fruit much sooner than stumping, because retaining a significant part of the trunk and branches the tree does not exert as much energy to regrow and retains buds that have been under apical dominance for less time. When new shoots appear they should be headed back to force lateral branches which is where the flowers will form.
All the prunings should be chipped and left in the field. This will help conserve water and help control Phytophthora root rot. Root rot or crown rot trees should not be pruned until they have been brought to health with one of the phosphorous acid formulations. They all are effective. Pruning a sick redirects the trees energy to fighting off the disease when it starts pushing new growth and then does not have the energy to fight off the disease. Or if you do have areas that are diseased (sunblotch, root rot, crown rot, etc.), windblown, in shallow soils or areas of recurrent frost, you might just remove the trees completely to save water.
White kaolin (Surround) applied to leaves has been shown to reduce leaf temperatures and water loss. This can be used, but under the direction of the packing house, since it if it is applied to fruit, it is very difficult to remove.
These are some steps that a grower can take to improve water management and create a more efficient use of water to help survive this period of not knowing how long this drought will last.
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Figure 1. Leaf blight is a disease that occurs with lack of water of any quality.
Figure 2. Salt damage from poor quality water and poor irrigation habits.
Figure 3. Stumped avocados for lack of water.
Figure 4. Scaffolded avocado that should produce fruit sooner than a stumped avocado.
- Author: Ben Faber
This is a great website to view plant nutrient symptoms by plant or by nutrient. It is of ornamental plants, but hey, once you can see it on one plant you will something very similar on avocado, lemon, apple, almond, lychee or whatever alphabetic fruit you work with. Kudos to University of Florida. Of course once you see what iron deficiency looks like, you'll be able to identify it on most other plants:
http://hort.ufl.edu/database/nutdef/index.shtml
Below is Boron toxicity from a grower who got too excited about correcting B deficiency,
- Author: Sonia Rios
The California Avocado Commission is joining forces with Kevin Turner, UC Cooperative Extension goldspotted oak borer (GSOB) program coordinator (http://ucanr.edu/sites/gsobinfo/) in producing and distributing a number of roadside signs to help control the movement of pest infested firewood.
They are asking for avocado grower's participation in areas that are susceptible to the GSOB and/or the Polyphagous Shot Hole Borer (PSHB). GSOB susceptible areas include oak woodlands and forests while PSHB environments include avocado groves. Some of these susceptible locations are primarily focused in the Fallbrook, Escondido, Valley Center, and Alpine areas. Eventually signs will be developed that will feature both the GSOB and the PSHB and discourage movement of oak and avocado wood.
The signs can go in a variety of frame designs. Most signs are 4' x 8'and are normally inserted into a redwood frame, but can be as simple as posts and a piece of exterior plywood onto which the thin aluminum sheet sign can be attached. The signs should be placed visibly along major roadways or busy streets.
If you would like to participate by permitting signs to be placed on your property along major streets, please contact:
Kevin Turner, GSOB Program Coordinator, UCCE/UCR
kevint@ucr.edu
951-827-5115
951-452-6427 cell