- Author: Ben Faber
‘Hass' avocado fruit set, January 12, 2024.
I was looking for ‘Fuerte' fruit for harvest this winter and came across a grower who said that there was no fruit from last spring, but the tree is loaded with newly set fruit right now. This is in Santa Paula, 15 miles for the ocean. In the same grove there is also ‘Hass' fruit set going on. What is going on? It's too cold – day and night. Almost too cold for honeybees to fly and maybe too cold for pollen tube growth, even if there were pollination. Pollination is the movement of pollen from male to female stage flowers by a bug agent. Too cold for fertilization, pollen tube growth from the stigma down through the pistil to the ovule to start fruit formation.
Cold weather fruit set is often the condition for formation of ‘cukes', especially along the coast. These little bullet shaped fruit that are seedless can often form doubled fruit. We are sure of the actual cause of this occurrence but some varieties like ‘Fuerte' and in in some places like New Zealand have a greater occurrence of the fruit.
Stenospermocarpy is the formation of fruit after pollination and fertilization, but in absence of the seed, which has aborted for some reason. Somehow, the endocarp (the fleshy part of the fruit) continues to supply the necessary carbohydrates and hormones necessary for fruit development. So, there might be a lot of these cukes forming from this early set, or it may just not stick. The avocado is pretty clever, it keeps flowering through winter and spring until it has had a really good chance to set fruit that will hold to maturation.
Even the ‘Gordon' apple tree in my Ventura backyard is confused; it hasn't dropped its leaves this winter. This is not the winter that i was expecting.
- Author: Ben Faber
It wasn't supposed to rain this winter, but it did. The forecast was for La Nina which typically brings less rain and would continue the drought pattern we have had for the last three years. So with no rain, there is no need to spray a protective copper for brown rot or septoria, right? And with low prices for lemons, it pays to save money, right?
Well it did rain and we have had some problems. There have been problems, as well from the violent winds and the cold weather. According to Daniel Swain at Weather West, this was the coldest winter on record, even though the amount of time below freezing was low. The wet weather brought down trees and flooded fields, burying trees in some cases.
So we had unexpected rain, and violent winds and cold weather and that has led to concerns about the lack of copper applications. And there have been calls concerning symptoms on fruit that kind of look like disease, however in many cases it is not disease that is the ultimate cause of the damage. So let's look at some of this damage.
If copper had been applied, brown rot (Phytophthora sp.) probably would not have resulted from rain splash from the ground onto the fruit. There was a heck of a lot of mandarin and oranges that were lost in one case because of this.
In a few other cases, we have seen septoria fungal spot due to the fruit staying wet too long. Finding the black structures can identify this problem
But a lot of fruit spotting that might be associated with disease causing organisms, is actually abiotic, caused by non-living sources, like
Hail
Puncture
Oil spray damage
Cold, wet conditions that may have been on the edge of freezing.
So, it was a wet winter. Blustery and cold. It would have helped in some cases to have had copper on, but there were other causes of fruit damage than just all that wet. Some growers regret not having just put it on as part of the regular regime. This coming year, it's time to get regular again and make sure the trees have a protective copper cover. But if the rain and cold and wind and hail are back, there are some things that are a lot harder to provide protection for.
- Author: Ben Faber
It's winter time and avocados and other subtropicals are prone to frost damage. Little trees especially that haven't developed a canopy that can trap heat are the most prone. So it gets cold and all the orchard looks fine, but there's one tree that doesn't look right and in a couple of days it really stands out.
Here's an example of a year old tree that turned brown and it actually looks like it was doing better than the trees surrounding. It's bigger and has a fuller canopy..... or at least it did.
But there's all the symptoms of frost damage - bronzed leaves and dead tips.
A week after the cold weather, there is already sunburn damage on the exposed stems. See the brown spots on the upper fork? That will soon turn all brown and dry up.
This is still a healthy tree with green stems, in spite of the burned leaves. Now is the time to protect the tree from sunburn damage. This is what can kill the little tree. Time to white wash it.
Why did it happen to this one tree? Maybe it was a little bigger and needed more water than the surrounding trees. Maybe sitting on a rock and didn't have enough rooting volume for water. Maybe a touch of root rot (although the roots looked pretty good even for winter time). And there were ground squirrels in the area. Easy to bklamne them.
Listen to the sound of winter frost control
https://www.youtube.com/watch?v=rwTJveN8cIE
And when freeze damage gets extreme
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=16448
- Author: Ben Faber
So, there are peach trees right now that are flower-less and leaf-less. Wisteria which should have flowered in February is bare. The Royal apricot next door has no flower. The Lombardy poplars and birch in Oxnard don't have leaves. Spring is here, grass is growing, but many deciduous trees are still leafless and haven't flowered? What is going on?
Deciduous fruit trees and many landscape trees like poplar, birch, willow and sycamore, etc. must go through a dormant period each winter in preparation for producing fruit and leaves the following spring and summer. This rest period, also known as a chilling period, is directly related to winter temperatures. For many varieties of trees, the most efficient temperature for chilling is 45°F, with little additional chilling effect at temperatures below 32°F. Brief warm spells in winter have a negative effect — temperatures above 70°F for four or more hours offset any chilling that happened in the previous 24-36 hours.
Once chilling is complete, the trees prepare to wake up from dormancy and bud after a certain amount of warming takes place. The amount of required warming is cumulative, measured by counting the number of degrees each day above a threshold temperature, usually 40°F. This cumulative warming, combined with how well the tree met its chilling requirement over the winter, determines whether a tree buds early or late in the spring.
Tables and charts have been developed for different chilling requirements of fruit trees. The number of hours needed at or below 45°F varies with the type of tree:
- Peach: 400 to 1050 hours
- Apple: 800 to 1100 hours
- Cherry: 1000+ hours
But in coastal Southern California, those hours are never achieved. You need to go to Santa Ynez to get close to those hours. More typically in the Santa Paula, Santa Barbara, San Luis Obispo area, the chilling hours below 45 are close to 200 and lower in many years. Some years there is more, some years less. How temperatures above 70 affect chilling aren't always clear. As a result there are several different ways to account for chilling and none of them work very well for the coast./
Fortunately, we have low-chill varieties of many fruit trees that will produce with lower chilling. So ‘Anna' apple and ‘Royal' apricot do well, and many landscape trees are adapted to the lower winter chilling along the coast and do well. Low chill blueberries thrive to the point that in many years, they don't even go dormant.
This year the warm, cold, warm, cold pattern has mixed trees up. There are some deciduous trees that are doing fine, while others still have not flowered and leafed out. The mix of temperatures is not following the traditional patterns used to calculate chilling requirement. The trees are following their own pattern.
In the last several years in Southern California, winters have seemed shorter and milder, resulting in earlier springs. Trees that have flourished in a location could have decreasing yields in the future, and the favorable locations to grow these fruit trees could shift.
The Fruit and Nut Center at UC Davis has a link to the CIMIS system operated by the CA Dept of Water Resources. The site has various methods of calculating chilling hours, none of them seem adequate though for describing what is happening in the landscape today in Southern California
http://fruitsandnuts.ucdavis.edu/Weather_Services/chilling_accumulation_models/
Look at it to see if you can see a new way of understanding deciduous tree response to the weather.
Just got a call recently that 'Valencia' oranges are splitting. This normally happens to ripe 'Navels' that are over mature and get erratic winter irrigation, especially during drought. In this case, the 'Valencias' are advanced in maturity because of the warm winter, probably advanced by two months. Again erratic watering has probably lead to this splitting.
What a crazy winter.
Splitting coastal 'Valencia' oranges in April, 2018
photo: Peaches without adequate chilling.
- Author: Ben Faber
It is that time of year and we should be alert to threat of freezing weather and damage to trees. Last winter was one of the warmest on record, but there was still a sneak cold blast around the New Year that caused some problems in some areas. Wet winters tend to have lower frost threats, and even though wet is forecast for this winter, that rain is not forecast until late in January. That still leaves December and early January which historically when most of our damaging frosts occur. Fox Weather on the CA Avocado Commission is forecasting some cold weather coming up, so growers need to be prepared for the worst.
Here are some links to frost information, preparing for frost and managing frost damage to trees.
A Frost Primer
http://ceventura.ucanr.edu/Com_Ag/Subtropical/Publications/Frost/A_Frost_Primer_-_2002_/
Protecting Avocados from Frost
Rehabilitation of Freeze-Damaged Citrus and Avocado Trees
The forecast is for north winds, which often means cold, dry air and often with winds. Winds mean no inversion and no warm air that can be introduced at ground level to warm trees. If this occurs, running a wind machine can make the damage worse. Wind machines and orchard heaters work on the principle of mixing that warmer air higher up – 20-100 or so feet higher than ground level which has colder air. When temperatures drop, the air is dry (wet-bulb temp below 28 deg F) and there is no inversion, running a wind machine can just stir up cold air and cause worse conditions (freeze-drying). It's better to not run the machine. The only thing left to do is to run the microsprinklers during the day so that the water can absorb the day's heat. Then turn the water off before sunset so that evaporative cooling from the running water isn't accentuated. Then when temperatures drop near 32 at night and the dewpoint is much below that, it's time to start the water again and let it run until sunrise (when risk is less). Running water works even if the water freezes. This is due to the release of heat when water goes from liquid to frozen state. This 1-2 degrees can mean the difference between frost damage and no damage. Also, ice on fruit and leaves can insulate the fruit. As the ice melts at the surface of the plant, it releases heat, protecting the plants. If there is not sufficient water to run the whole orchard, it's best to pick out the irrigation blocks that are the coldest or the ones you definitely want to save and run the water there continuously. Running the water and turning it off during the night to irrigate another block can lead to colder temperatures in both blocks.
Keep warm this winter.