- Author: Ben Faber
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.
- Author: Jim Downer
All the media are full of stories about the impending El Nino effects and possible flooding rains in California. As I write this, it is the day before our first significant storm (Jan. 4) with a 100% chance of rain throughout Ventura county for tomorrow. Still long range forecasts for Jan and Feb show near normal or less than normal rainfall possibilities for the Avocado growing areas of California. The “real rain”, they say, will come in March. Although nobody can accurately predict the occurrence of rain, we are certainly due for an increased rain year based on statistical models and estimates of the El Nino effect by NOAA, JPL and NASA. If heavy rains are in the near future, nurserymen and growers will need to take measures to protect their operations from the many and disastrous effects of downpours. Physical displacement of soil as erosion and loss of soil are common during heavy rains. While runoff water should not leave agricultural properties, it has to move somewhere when rains come, so ditches should be cleared of weeds and other obstructions to permit efficient flow of water. Most operations will have already done this by now. So what can be expected when the rains come and after they leave? It will likely be a banner year for diseases both biotic and abiotic.
Many pathogens are splash inoculated from plant to plant or soil to plant, so it is imperative to prevent the development of flooded or puddled ground near growing areas. Now or between storms would be a great time to lay down additional gravel under container beds or other outside nursery areas. Keeping containers off soil, either with a gravel or fiber mat and gravel system is imperative when trying to control Phytophthora in nurseries. Compacted walkways and beds may become saturated this winter and create ideal sporulation conditions for Oomycetes or water molds which may then move in water flows to new areas of the nursery causing infections where never seen before. Consider boardwalks or additional gravel in known low spots and walkways so that workers don't move infested mud from one part of a nursery to another. For citrus and avocado growers it is vital to give water a place to go on flat or low lying areas. When we get the expected deluges, trees can suffocate from extended soil saturation and defoliate rapidly due to anoxic conditions.
For Phytophthora sensitive crops, it may be wise to increase the calcium by adding additional gypsum now to reduce sporulation and potential spread of disease. It is also wise to use preventative fungicides such as mefenoxam, and phosphorous acid to increase plant readiness for Phytophthora increases following wet weather.
This is also a great time for woody plant growers to prune any diseased or dead materials from plants ahead of winter rains because many Ascomycete canker fungi that cause disease in woody plants will have inoculum in dead twigs. This has been a banner year for Botryosphaeira fungi which have caused canker diseases in citrus, avocado and ornamentals at record levels due to drought stress. When rain comes, spores are splashed to new plants and can cause new infections. Since this is an El Nino year, it is warm, and warm rains are best for disease promotion. Remove inoculum now, cull and remove weak, diseased or dead plants ahead of the rains to cut down on disease spread. Even though it may be inappropriate to plant some crops now, it is always a good time to remove weak trees, plow diseased row crops and chip up the waste to be used as mulch.
With rains often come strong winds. Greenhouse and tunnel growers should consider the effects of wind this winter on their operations and possible crop loss from this damage. Tunnel houses used in berry and other production are at risk but other greenhouse materials such as polycarbonate sheeting can be detached by wind. Now is a great time to inspect and repair these structures or apply new sheeting as necessary. Wind can also move woody plants to rub against each other, causing injury to the main stem or fruit if tightly spaced. Trees that are blown over due to high winds can be damaged and devalued. Spend time now inspecting trellis systems and staking of woody plants to minimize damage that may be coming.
Outdoor nurseries that have planting media storage piles should start now to downsize these piles or provide new tarps in advance of wet weather. Greenhouse operations with media stored outside should ensure that bales are properly covered with new tarps to prevent saturation of the media. Media bales should be stored off the ground on raised pallets to avoid contamination with soil or mud flows.
The challenge of a wet and potentially stormy winter is to envision what excess water can do in your operation and then try to prepare. Flooding conditions create a time of potential pathogen movement and the best protection for plants is to keep them elevated above the mud and keep workers from spreading it with the movement of machinery or foot traffic. It is also useful to imagine invasion of soil from adjacent land owners who may have diseases or weeds not on your own property. Money spent now on infrastructure will prevent disease loss later this spring or summer.
- Author: Ben Faber
Years of drought, and a stressed tree are a perfect set up for navel oranges and fruit splitting.
The days have turned cooler and suddenly out of nowhere there is rain. That wonderful stuff comes down and all seems right with the world, but then you notice the navel fruit are splitting. Rats! No, a dehydrated fruit that has taken on more water than its skin can take in and the fruit splits. This is called an abiotic disease. Not really a disease but a problem brought on by environmental conditions.
Fruit splitting is a long-standing problem in most areas where navel oranges are grown. In some years, the number of split fruit is high; in other years it is low. Splitting in navel oranges usually occurs on green fruit between September and November. In some years, splitting may also occur in Valencia oranges but it is less of a problem than in navel oranges.
Several factors contribute to fruit splitting. Studies indicate that changes in weather including temperature, relative humidity and wind may have more effect on fruit splitting than anything else. The amount of water in a citrus tree changes due to weather conditions and this causes the fruit to shrink and swell as water is lost or gained. If the water content changes too much or too rapidly the rind may split. In navel oranges the split usually occurs near the navel, which is a weak point in the rind.
Proper irrigation and other cultural practices can help reduce fruit splitting. Maintaining adequate but not excessive soil moisture is very important. A large area of soil around a tree should be watered since roots normally grow somewhat beyond the edge of the canopy. Wet the soil to a depth of at least 2 feet then allow it to become somewhat dry in the top few inches before irrigating again. Applying a layer of coarse organic mulch under a tree beginning at least a foot from the trunk can help conserve soil moisture and encourage feeder roots to grow closer to the surface.
If trees are fertilized, apply the correct amount of plant food and water thoroughly after it is applied. If the soil is dry, first irrigate, then apply fertilizer and irrigate again.
- Author: Mary Bianchi
Capturing Precipitation - How much rainfall do I need to capture?
Managing precipitation to your advantage is really a three step process (Lal and Stewart, 2012).
ü Step 1 - maximize preciptitation captured in the soil
ü Step 2 - minimize the evaporation of the stored soil moisture
ü Step 3 - maximize plant water use efficiency
The first step of the process is often thought of as “effective rain”. Effective rainfall refers to the percentage of rainfall which becomes available to plants and crops. It considers “losses” due to runoff, evaporation and deep percolation (Klein, 2011). In the past we might have considered deep percolation as a loss. We now know that percolation “losses” may be a vital resource in sustaining our groundwater basins. As we move into the fall of 2015, we have the opportunity to plan for effective rainfall by managing the orchard floor for maximum capture of precipitation. This will help provide stored soil moisture for plant growth as well as deep percolation of water to groundwater
The following figure illustrates some of the important points about effective rainfall and reminds us of what we can do to maximize capture of precipitation (1). We want to maximize 2 (infiltration during a rain event), 3 (surface capture), 6 (infiltration from surface capture), 7 (percolation to ground water), and 8 (rootzone storage for use by the crop). We want to minimize 4 (runoff) and 5 (evaporation).
When rain water ((1) falls on the soil surface, some of it infiltrates into the soil (2), some stagnates on the surface (3), while some flows over the surface as runoff (4). When the rainfall stops, some of the water stagnating on the surface (3) evaporates to the atmosphere (5), while the rest slowly infiltrates into the soil (6). From all the water that infiltrates into the soil ((2) and (6)), some percolates below the rootzone (7), while the rest remains stored in the rootzone (8). From FAO Irrigation Water Management 1985 http://www.fao.org/docrep/r4082e/r4082e05.htm#4.1.4 effective rainfall
Larry Stein from Texas A&M wrote a very good basic explanation “So What Constitutes an Effective Rain Event ?” (Stein, 2011) We can use his approach to look at managing precipitation in the Central Coast. Understanding these concepts can help you manage precipitation in your operation.
For example, the majority of olive roots are in the top 18 inches of soil. So how much rainfall do we need to capture to refill the rootzone of an olive grove in Paso Robles? We need to know:
ü The amount and intensity of rainfall
ü The infiltration rate of the soil (how fast the soil takes in water). Sandy soils take water in more quickly.
ü How much water the soil will hold in the rootzone of the grove
Average rainfall for Paso Robles in January is about 2.75 inches. Table 1 shows that olives on a sandy loam soil might be able to infiltrate 1 to 1.5 inches per hour. If all that rain comes in one storm then as much as 1.25 inches may either run off (4) or pond (3) in the low spots until it can infiltrate.
Average rainfall in Paso Robles in January would be adequate to refill the rootzone of olives (8) on a sandy loam soil, IF all of the rainfall infiltrates (2), and none is lost to evaporation (5) or runoff (4).
Table 1. General soil water storage and depletion characteristics for three different soil types (Klein, 2011)
|
Soil Texture |
||
|
Sands |
Loams |
Clays |
Water infiltration rate (inches / hour) |
2.0 – 6.0 |
0.6 – 2.0 |
0.2 – 0.6 |
Available water (inches / foot) |
1.0 – 1.5 |
1.5 – 2.5 |
2.5 – 4.0 |
Days to depletion when ET – 0.2 inches / day |
5 – 7.5 |
7.5 – 12.5 |
12.5 – 20.0 |
Amount of water to wet to 18 inches in a dry soil (inches) |
1.5 |
2.25 – 3.0 |
3.75 |
Cover crops help keep the soil surface from crusting as well as protecting the soil surface from erosion. Their roots provide channels for water to infiltrate into the soil. Remember that cover crops may also be using water stored in the rootzone (8). When facing drought conditions, it may be advantageous to manage with low residue cover crops to reduce the amount of water extracted from the rootzone. Here's a link to a video on low residue cover crops and their impact on runoff from work by UC Cooperative Extension Advisors in Monterey County https://www.youtube.com/watch?v=k0oVVJ_BA7s
Klein, L. 2011. So What Constitutes an Effective Rain Event? http://aggie-horticulture.tamu.edu/earthkind/drought/drought-management-for-commercial-horticulture/so-what-constitutes-an-effective-rain-event/ .
Lal. R.and, B.A. Stewart. 2012. Soil Water and Agronomic Productivity
/h3>- Author: Ben Faber
Soil likes to be covered at all times. It doesn't “want” to be exposed to the elements, so you either cover it (plants, asphalt, paper etc.) or it will cover it for you with plants (weeds). If it can't be covered fast enough, it disappears – erodes. This can be from wind or rain or just natural movement down slope. Plants that are managed for other than their agricultural return are called cover crops, although they can also have a crop that is saleable. Often weeds can be managed to be a cover crop, as well.
We are looking at a possibly wet winter and many tree crops grown on hillsides and sloping ground are prone to soil erosion. Covers can be grown year round, but that usually means they require water all year round. That means they need an irrigation system dedicated to their needs. It also means having extra water which may be limiting.
A winter cover crop that grows out in the winter, does its thing (although that “thing” can include lots of other things, e.g. insectary, nutrients, water retention, etc.), and then dies or goes dormant, can be ideal. It also requires less water than a permanent cover.
But there is a big problem here. Establishment of an introduced cover still requires water. Rainfall in Southern California is erratic and there may be early rains to germinate seed, but it may not be consistent enough to get the plants established. In fact, they may die for lack of further rain or be delayed.
Delayed germination means that soil is cooler and there is less growth. The real growth may occur after there has been sufficient rainfall by January and February when the soils are cooler and there is even less chance for growth
So when rainfall is doing its worst, there's no effective cover. Or what cover there is, is what has germinated from “native” seed. It may not have the characteristics you want for management: low stature, low entanglement with the trees, low water use, holds the soil without holding up harvest, etc.
So what do you do? There are several approaches. You can move the sprinklers out into the middles and irrigate up the seed. If you are in a limited water situation, you can do alternate middles, not cover cropping the whole area, or every third middle. Whatever it takes to break the surface flow of water. Or you can turn to mulching. Put down sufficient mulch in a middle or every other middle to break overland water flow.
Cover cropping is easier than mulching, but it takes water and timing.
Below are two websites with descriptions of cover crops and how to distinguish them from “weeds”. Often a good cover can be the residential weeds. A low–lying cover allows pickers in to get lemons without mess and fear of snakes. It also means that it can be more easily treated (mowed, weed whipped) at the end of the rainy season to reduce fire hazard.
1) Characteristics of different cover crops
http://asi.ucdavis.edu/programs/sarep/research-initiatives/are/nutrient-mgmt/cover-crops-database1
2) Weed identification from the UC IPM