It's that time of year when grower's think about pruning their avocado trees. What scars many growers is the regrowth that occurs. If the cut is to the middle of a branch, there is all kinds of wild growth that results. The best cuts are back to a crotch or a subtending branch. Then there is little regrowth. Sometimes those cuts cannot be made and a mid-branch cut needs to be made.
Tre-Hold, naphthalene acetic acid was registered on avocado several years ago to retard that regrowth. The growth hormone is mixed with white latex paint so it is possible to see where it has been applied. We recommended that the paint be hand applied to a distance two times the width of the cut. So if the cut is 2 inches in diameter, paint back 4 inches around the base of the cut. It works.
Many growers don't use the material, but recently I saw a neatly pruned grove that has been using NAA for a number of years and in order to avoid any confusion about how much paint to put on, they just apply a 2 inch width of paint to all the cuts. This is easier for the pruners and speeds up the process. Try it, you may like it. It certainly saves time in maintenance over the long haul after pruning.
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
It ain't over yet, and this last week we had a wonderful 2 day meeting with folks who have dealt with drought in many different ways. Here are presentations made by people from Israel, Australia and California. Soon the actual videos will be available, but now see the powerpoints.
The grower panels are wonderful, but are not uploaded at this point
DAY I: UNDERSTANDING IRRIGATION WATER MANAGEMENT
Session I: California Response to Drought
9:15am // Overview of California Drought Response
Session II: Technology of Water Management
9:45am // Soil Water Sensing
11:00am // Salinity Measurement
12:00pm // Precision Water and Fertility Management During Deficit Irrigation
Session III: Alternative Water Supplies
2:00pm // Effects of Irrigation With Poor Quality Water on the Soil - California Experience
3:00pm // The Challenge of Sustainable Irrigation with Water High in Salts: Lessons from Dates, Olives and Grapevines
Session IV: Water Management Strategies During Drought
Day II: WATER MANAGEMENT FOR INDIVIDUAL CROPS & GROWER EXPERIENCES
Session V: Technology Demonstrations
8:30am // Using the Pressure Chamber for Drought Mangagement Decisions
9:00am // Irrigation System Evaluation
9:30am // Salinity Mapping for Water Management
Session VI: Conncurrent Breakout Groups
10:30am // Citrus - Israeli Experience: Long Term Effects of Deficit Irrigation, Salinity, and Rootstocks on Orchard Productivity
10:30// Almond Irrigation, the Israeli Experience
11:15am // Grapes - Wine Grapes Irrigation - Coastal Vineyards
11:15am // Deciduous Nut Crops - Almond Irrigation - Israeli Experience
1:00pm // Avocado - Israeli Experience
1:00pm // Grapes - Wine Grapes Irrigation: San Joaqin Valley
1:45 // Subtropical Crops - California Experience
2:00pm // Deciduous Nut Crops - Walnut Irrigation
2:30pm // Grower Panels
3:15pm // Concluding Remarks
So I got an email with an attached set of photos showing a number of 18 month avocado trees that had broken at the ground line in a wind storm. It looked something like an incompatibility between scion and rootstock, since below the graft union the rootstock girth was substantially smaller than the scion. The problem is, I'd never heard of this between ‘Hass' and ‘Dusa' and the combination has been around for at least 15 years in trials.
So it was time to go out to the orchard and see the setting in which the trees had failed. And there was the answer. All of the trees had been planted too deeply. The trunk sleeves had been buried and the graft union was below grade and had been infected by disease organisms. The crown roots are the most active physiologically and that union area is a weak spot. Burying it encourages disease and a weak union.
The trees had been planted with an auger and over time the trees had settled into the soft earth and had been buried. The first time I had ever seen something like this was in Guatemala and Costa Rica. There growers had created these massive 3 x 3 X 3 foot holes and amended them 50% with compost. And over time, the trees had settled as the compost decomposed and the unions were covered with dirt. Occasionally you see people moving too fast when they are planting and this shows up in a few trees, not usually a whole orchard.
The lesson here, is that if you are going to err on plant depth, plant high. A few exposed roots won't hurt, and the settling problem doesn't become a problem. Of course it's best to plant just right.
The Buried Avocado Graft Union
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/