The calls are coming in and have been for the last several months. The trees are tired, worn out and look horrible. What's the problem? Well four years of drought, accumulated salts in the root zone and irrigation practices that aren't removing the salts from the root zone. It sets up a situation of tip burn, but much more extensive than tip burn is the water stress that results from salt accumulation. Salts compete with roots for water and they act to pull water away from the roots. It is as if less water is being applied. The water stress sets up the trees for a fungal infection called variously leaf blight, stem blight and in young trees, death. We used to call this Dothiorella blight, but since the work of Akif Eskalen at UC Riverside, it turns out it is one of many fungi that cause this problem, most of them Botryosphaerias.
The leaves show what would appear to be salt burn damage which increasingly causes leaf drop. In fact, there's often a pile of leaves under the canopy unless the wind has blown them away. The difference between this and salt burn is that there is not a regular pattern to it. It can start on the margins, or in the middle of the leaf, or wherever it darn well pleases. Whereas salt/tip burn always starts at the leaf tip and progressively moves back onto the main part of the leaf. Leaf blight (I don't like to use bigger words than that – Botryosphaeria. Try spelling it on the phone), on the other hand doesn't follow this regular pattern. It's a random pattern.
This a decomposing fungus. Wherever there is organic matter – leaves, twigs, branches, fruit, whatever is dead on the ground – there is a decomposing fungus. When the fungus finds a stressed plant, it invades the most susceptible part of the plant, usually the leaf. It starts growing through the tissue and down the leaf petiole. It then starts growing down the dead part of the plant. Most of a tree is dead. All that stuff under the bark and cambium is dead tissue, although it still carries water. In mature trees, there is a capacity to close off the decay and limit it. In young trees (younger than two or so), the capacity is lacking and the fungus can keep on growing down to the union and kill the tree.
As can be imagined, this fungus does not discriminate amongst the type of plants it feeds on. It will go to water-stressed, citrus, roses, apples, etc. It goes to every woody perennial that I am aware of. I've seen it on redwoods and eucalyptus. It especially goes after shallow-rooted species like avocado which are the most prone to water-stress. Like when a Santa Ana blows in and the irrigation schedule is slow to respond. Like when there is a heavy load of fruit. Fruit have stomata and the more environmental stress the more water they lose and pull on water from the tree.
Now imagine a tree loaded with fruit, in the later summer, with a Santa Ana and salt stress. Boom! Fruit drops and leaf blight shows up. And the damage doesn't go away, until it so severe that the leaf drops and new leaves come on in the spring.
Hopefully these rains will wash the salts from the root systems and refill the profile with high quality water. We are extremely reliant on winter rain to cover up the effects of the damage that irrigation water does to our soil and plants. And rain is the answer, as long as it's not too much.
Notice the even pattern of necrosis with tip burn
And the random pattern with leaf blight.
The disease pyramid describes how disease can eventually destroy a plant.
It is comprised of the presents of the pathogen that causes the disease, the plant or host, the environmental conditions that sets up the pathogen to go after the plant and time. It requires all four at the same time to have a disease. So in the case of avocado root rot, it's necessary to have Phytophthora cinnamomi there with a susceptible avocado root, in warm, moist conditions for a period of time for the disease to express itself. For a young tree, the disease may show up within a year but for older trees it may take several years to see the disease symptoms. For avocado crown rot, it may take several years for Phytophthora citricola to appear. In the case of Huanglongbing, not only is the pathogen required, but also the vector for spreading it, the Asian Citrus Psyllid. At this point in California, we have a rapidly spreading vector, but as yet, it appears the psyllid is not infected with the bacteria causing the citrus disease.
Not only time is important, but timing of sampling and point of sampling are important. In the case of avocado root rot, the best sample if from a recently infected root in the spring or summer. A thoroughly destroyed root taken in the winter will often give a negative response. The pathogen has done its ravages and is no longer active. If the wrong leaf or root is sampled for the Huanglongbing bacteria, the assay will show a negative response, even though it is in the plant. (This may have something to do with Einstein's Theory of Relativity in this 100th year of pronouncement, but I doubt it).
And then too, even if a disease is finally diagnosed, it doesn't mean the tree is going to die. If Phytophthora is caught soon enough, it can be treated with phosphites, but the environment needs to be changed at the same time. Meaning, the irrigation needs to be adjusted so that the treatment will work. If the irrigation is changed, the trunk canker will disappear.
It also means that all those terrible disease out there that threatened our trees in other countries won't hurt ours. We have a different environment from Thailand, India, Brazil, etc. and so even though there may be devastating disease there, since we don't have the environment, we wont get those diseases. Yeah, we are going to get huanglongbing and polyphagous shot hole borer fusarium, but we should be happy we aren't getting all the other problems other countries have. This is a way of saying, happy New Year. It could be a whole lot worse and we are doing pretty well.
There have been so many calls recently with the same problem, thinning canopy with dieback. Thiscan be caused by several problems, but the most common this time of year is lack of good water management over the year, accentuated by lack of rain and salt damage. The rain will often leach salts accumulated from the root zone over these many years of water from only irrigation. I thought as everyone else did that we would be having some gushers in S. California and some of these problems would be relieved. With rain, the cankers from black streak, bacterial canker and Botryosphaeria canker (Dothiorella) staining are washed away, and often times, so are the tree's problems. Well that hasn't happened. Or rather the rain hasn't happened to resolve this problem - Botryosphaeria or Bot Rot.
This problem has been ongoing and 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:
Thinning canopy, dieback and shriveled fruit in avocado and dieback and gumming in citrus
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.
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
1) Lack of light dieback
2) Phytophthora cinnamomi dieback
3) Stem and Leaf Blight
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.
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.
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.