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.
You come on a leaf with the margins munched on. It's got to be a beetle or a looper or some insect doing the damage, right? Not necessarily. It's not time to drag out the Raid. Look at the damage closely. In the photos below you can see the dead leaf margins caused by either salt damage or more likely leaf blight. Leaf blight is a disease that shows up with water stress and is caused by a fungus, one of the Botryoshpaerias. It causes an uneven marginal necrosis that goes along the margin in a somewhat irregular pattern and often not at the leaf tip. In this case it does affect the leaf tip, and since salt burn and leaf blight are caused by the same conditions of water stress, it's probably a bit of both.
Lepidopteran larvae will more commonly feed in a smooth pattern, not the rough pattern seen here. Now with this dead tissue, the wind blows it out, and what's left is the uneven margin. No it's not time to spray an insecticide. It's time to reflect on irrigation. There's a lot of this damage out there now. On avocados, citrus, landscape plants. It's going away until the leaves drop and are replaced with new ones, that will hopefully be well hydrated by rain and proper irrigation.
Top photo is salt/leaf blight damage
Bottom is necrotic tissue that the wind has blown out
I get a call.
He: My trees not doing well.
Me: What's the problem?
He: It's yellow?
Me: Have you looked at the roots? Are there roots?
Me: I'll be out next week, but in the meantime, look at the root system.
This is a pretty common exchange and when I got out, you find out that the emitter is clogged, the ground is soggy, there's weed whack damage, there's gopher damage, there's…………………. All kinds of things that pop up and until you see the context that the poor tree is in, it's hard to diagnose the problem. Too much fertilizer, black drip tubing that had water heated in the sun and burned the roots, the trunk buried in mulch, it's hard to imagine all the possibilities. But start with the roots and then go where that leads you.
So, here's the scenario.
You get a call/email.
He: What's wrong with my young tree? It snapped off in the wind. Here's a picture of what's left. Corroded, bulbous graft union. Incompatibility? Extensive decay that indicates a problem of long standing. The leaves are green though, so it means it was hanging in there until the wind blew. Was the trunk buried at planting leading to asphyxiation and crown rot? Is it some sort of wound that started it off? Oh, and about 1% of the planting is like that.
Me: OK, I better take a look at it. I'll get some samples and send them into Akif Eskalen at UC Riverside and bring our plant pathologist Jim Downer out to look at it with me.
So tune in to find out what we diagnose as the problem. In the meantime, if you have an unknown avocado or citrus problem that looks of root origin. You can contact Akif and send him a sample after following the “Sample Submission Form” at: http://eskalenlab.ucr.edu/
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