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
Potassium deficiency in avocado and citrus leaves often looks like salt stress and more specifically sodium toxicity. Plants will often look wilted with curled leaves, yellow areas between leaf veins and dead areas along the margins of the leaves. Salt stress refers to the excessive amount of soluble salts in the root zone which induce osmotic stress (appearance of lack of water) and ion toxicity (growing problems and often symptoms) in the growing plant. Among toxic ions, sodium (Na+) has the most adverse effects on plant growth by its detrimental influence on plant metabolism in inhibiting enzyme activities. An optimal potassium (K+) : Na+ ratio is vital to activate enzymatic reactions in the cytoplasm necessary for maintenance of plant growth and yield development These enzymes control such functions as the stomata which regulate water and photosynthesis control in the plant. Although most soils have adequate amounts of K+, uptake is exacerbated under sodic or saline-sodic soil conditions as a consequence of K+-Na+ antagonism. Here K+ uptake by plants is severely affected by the presence of Na+ in the soil. Due to its similar chemical properties, Na+ competes with K+ in plant uptake It would seem a reasonable assumption therefore that an increase in the concentration of K+ in salt-affected soils may support enhanced K+ uptake. And that has been noted in many plant species including citrus and avocado.
But aside from the role of potassium in drought tolerance there are many functions of potassium in plants:
• Increases root growth and improves drought resistance
• Activates many enzyme systems
• Maintains turgor; reduces water loss and wilting
• Aids in photosynthesis and food
• Reduces respiration, preventing energy losses
• Enhances translocation of sugars and starch
• Produces grain rich in starch
• Increases protein content of plants
• Builds cellulose
• Helps retard crop diseases
In the case of avocado and citrus there is about twice the amount of potassium as nitrogen harvested in the crop, yet many growers do not consider potassium in their normal practices, much less when drought has increased salt stress on the trees. The end of August through September is when leaf analysis is best used to adjust a fertilizer program.
Sodium toxicity and Potassium deficiency in avocado
Water moves in a wetting front. When irrigation water hits the soil it moves down with the pull of gravity and to the side according to the pull of soil particles (more lateral with more clay). Soil is a jumble of different sized soil particles, from clay to silt to sand sizes and then often intermixed with stones of different sizes from gravels to boulder. The different textures determine how water moves. It moves fastest through coarse textures and slowest through finer ones – the clays, the ones with the smallest pores. But soils are a jumble of particle sizes and pores.
Water first moves down the larger pores and then it slowly moves through the larger ones. As water moves through the soil it carries salts that have accumulated in the soil. At the wetting front is where the salt accumulate. As the water moves through the larger pores, salts migrate/diffuse from the small pores to the larger ones. This diffusion takes a bit of time, so typically the small pores have a larger salt concentration than the larger ones.
So an initial application of water will carry the salts from these large pores and if the irrigator were to stop in mid-application, it allows time for the salts to move out of the small pores into the larger ones. Then when the irrigation recommences, it will carry more of the salts out of the wetted area – the root zone. This technique is called “bumping” where an irrigation is stopped and then restarted in order to improve not only leaching, but also reduce runoff.
This principle also is at play when there are two or more sources of water quality. Soil salinity can be no lower than the irrigation water that is applied. Then as the soil water is removed through plant absorption or evaporation, the salinity increases. The soil salinity can easily be two to three times higher than the irrigation water.
If there are two sources of water, the initial application can be with the poorer quality water, and once that has reduced the soil salinity, then the better water quality can be applied which will then bring the soil salinity closer to that of the better quality water. By doing this two part leaching, the amount applied of the better quality water can be significantly reduced. This is a type of “bumping” to improve leaching.
Watch this U-Tube video on how water moves through soil, thanks to the work at Walla Walla Community College.
Thank you Walla Walla Community College for the video
Events | Information | Experts | Media Coverage | Story Highlights
This new online seminar series from the University of California, Agriculture and Natural Resources, with support from the California Department of Water Resources, brings timely, relevant expertise on water and drought from around the UC system and beyond directly to interested communities.
I've been getting calls and have gone out to see several avocado orchards in the Ventura/Santa Barbara area and the comments are that the trees look worse this year than they normally do. When I see the trees, it's clear that they don't look in the best shape. This time of year, when there are old leaves that have accumulated salt all through the irrigation period and the trees are getting ready to flower, the leaves just don't have much energy. Also, two years of drought with no rain, means that salts have probably accumulated more by this time of year, than in a year when we do have “normal” rain to leach the soil. The accumulated salts can lead to water stress which also brings on stem and leaf blight, along with salt burn. With the bicarbonates in the water, the pH may have rise as well, inducing some iron chlorosis. Compounding the leaf damage is some frost burn, which was not cold enough to kill the leaves. Any dead tissue, also makes the leaves look more ratty, because with Santa Ana winds earlier, those dead areas have often blown out, making them look like they have been nibbled on by insects. Further adding to the stress was a huge crop year that put a lot of stress on the tress. When clearing the leaves to look at roots, it has often been hard to find viable roots. All these stresses are going to make the trees more susceptible to root rot. So it's going to be necessary for growers to keep their eyes out for further disease symptoms and to be ready to treat with phosphites when the soil warms up enough for the roots to start growing again.