- Author: Ben Faber
This February there was a four day international conference in Orlando, FL that attracted 467 people from 21 countries, including about 20 from California. There were 87 oral presentations and over 80 posters that covered all aspects of Huanglongbing, the insect vector (Asian Citrus Psyllid or ACP), disease detection, insect control and monitoring and a whole lot of information on the genome of the bacteria, how it compares to other infectious bacteria and what can be done to exploit its genetics to control the disease. To learn more, the proceedings and agenda can be found at: http://irchlb.org/hlb/schedule.aspx.
To start off, this is an amazing example of coevolution among a plant, a bacteria and an insect. It appears that the citrus tree may give off an odor which at low concentrations acts as an attractant, but at higher concentrations is a repellant. The infected tree also gives off volatile organic compounds (VOCs) that can be used to identify affected trees. These chemicals are not pheromones which are social odors emitted by some insects which act to affect insect behavior, such as mating or causing aggregation. Being able to use these new odors will allow for better methods of monitoring the insect with lures. We currently use blunder traps which are not a very good indication of whether there are ACP present.
Currently the most commonly used technique for identifying infected trees is the use of the polymerase chain reaction (PCR) method which is a biochemical technology that identifies the presents of the bacterial DNA. This method was used on the leaves of trees since that is what the psyllid feeds on and where the bacterial infection starts. Results have been erratic and inconsistent and often would not give results until many months after the infection started.
The bacteria clog the phloem tissue which carries sugar to the roots and on which the psyllid feeds. The higher concentration of sugar accelerates the development of the insect and it can lay more eggs. The bacteria travel down the stem at measured rates of one centimeter per day and accumulate in the root system. The root tips are the growing points and where the plant directs sugar to feed the new root growth. It is from the roots that the bacteria go out to all points of the canopy where it can then be picked up by uninfected psyllids which can then go on to find a new tree to feed on. By measuring root tips, rather than leaves, the detection is more rapid and much more consistent, since the bacteria levels are higher in the tips than the point of infection. Disease detection is now being improved by better understanding of the biology of the infection process and thereby allowing faster determination of whether a treatment works.
The ability to evaluate what treatments work has allowed researchers to determine such things as what rootstocks and scions might be most vulnerable, what spray programs are the most successful and how better to lay out trials. It turns out that there is an edge effect during a psyllid invasion and that the first affected trees are right on the perimeter. By focusing spray programs in this area can slow the movement of the insect to the middle of the orchard.
Actual economic control of this pest and disease are still some time away, but with this new understanding of the disease process we will be closer to finding a solution.
Fruit drop and defoliation associated with HLB
- Author: Elizabeth Fichtner and Rachel Elkins
Lime-induced Iron Chlorosis: a nutritional challenge in the culture of several subtropical perennial crops in California
Elizabeth Fichtner, UCCE Tulare County and Rachel Elkins, UCCE Lake and Mendocino Counties
Spring, and new leaves are coming out, but this could, but yellow could be a sign of iron chlorosis, as well.
Although iron (Fe) is the 4th most abundant element in the lithosphere, Fe deficiency is among the most common plant micronutrient deficiencies. Fe deficiency in plants is common in calcareous soils, waterlogged soils, sandy soils low in total Fe, and in peat and muck soils where organic matter chelates Fe, rendering the element unavailable for plant uptake. In California, lime-induced Fe deficiency is often observed in soils and irrigation water containing free lime, and is exacerbated by conditions that impede soil drainage (ie. compaction, high clay content), resulting in reductive conditions. Given that over 30% of the world's soils are calcareous, lime-induced Fe deficiency is a challenge in numerous perennial cropping systems including: grapes, pears, apple, citrus, avocado, pecans, and stone fruit (prune, almond, apricot, peach, nectarine, cherry).
In most soils, Fe oxides are the common source of Fe for plant nutrition. Solubility of Fe oxides is pH dependant; as pH increases, the free ionic forms of the micronutrient are changed to the hydroxy ions, and finally to the insoluble hydroxides or oxides. In calcareous soils, the bicarbonate ion inhibits mobilization of accumulated Fe from roots to foliage and directly affects availability of Fe in soil by buffering soil pH. When irrigation water is also high in bicarbonate, probability of Fe deficiency is enhanced because bicarbonate is continuously supplied to the soil, and more importantly, the roots may become crusted with lime as water evaporates, thus inhibiting root growth and function. Inside the plant, bicarbonate inhibits nutrient translocation from roots to aboveground plant parts. The adverse effects of high bicarbonate levels are exacerbated in very saturated, very dry, or compact soils, where bicarbonate levels increase concurrent with diminished root growth and nutrient uptake.
Symptoms of Fe deficiency in plants
Fe is immobile in plants; therefore, symptoms appear in young leaves. Interveinal chlorosis (Figure 1) is the main symptom associated with Fe deficiency, followed by reduced shoot and root growth, complete foliar chlorosis, defoliation, shoot dieback, and under severe conditions may result in tree mortality. Overall productivity (yield) is reduced, mainly from a reduced number of fruiting points.
Plant species and cultivars vary in their sensitivity to Fe deficiency, and are categorized as either "Fe-efficient" or "Fe-inefficient". Fe-efficient plants have Fe uptake systems that are switched on under conditions of Fe deficiency. Fe-inefficient plants are unable to respond to Fe deficient conditions. All Fe-efficient plants, except grasses, utilize a Fe-uptake mechanism known as Strategy 1. Strategy 1 plants decrease rhizosphere pH by release of protons, thus increasing Fe solubility. Some plants may excrete organic compounds in the rhizosphere that reduce ferric iron (Fe3+) to the more soluble ferrous (Fe2+) forms or form soluble complexes that maintain Fe in solution. Additionally, roots of Strategy 1 plants have specialized mechanisms for reduction, uptake, and transfer of Fe within the plant. Strategy 2 plants (grasses) produce low molecular weight compounds called phytosiderophores which chelate Fe and take up the chelated Fe with a specific transport system.
Amelioration of Fe chlorosis
Planting sites in calcareous soils should be well drained to provide optimal conditions for root growth and nutrient uptake. Waterlogged and compact soils contain
more carbon dioxide, which reacts with lime to form even more bicarbonate. These conditions, as well as very dry soils, also inhibit microbial activity which aids in
solubilization and chelation of Fe. Prior to planting, soils and water should be tested to determine the pH, lime equivalent, and bicarbonate concentration. Bicarbonate concentrations greater than 3 meq/L in irrigation water increase the hazard of lime accumulation on and around roots. If high bicarbonate water must be used, the pH must be adjusted to 6.0-6.5 to dissolve the bicarbonate and prevent it from negating the effects of soil-based treatments. In microsprinker and drip systems, acidification of irrigation water will also reduce the risk of emitter clogging, a common problem at bicarbonate levels over 2 meq/L. The cost of reducing the pH of irrigation water will more than compensate for the savings incurred from avoiding wasted investment in failed soil- and plant-based remedies. Systems can be set up to continuously and safely inject water with acids such as sulfuric, urea-sulfuric, or phosphoric during irrigations. Specific choice and rate will depend on crop, soil type, other nutrient needs, availability, and cost. Downstream pH meters are available to continuously adjust rate of acid use. Acetic and citric acid can be utilized by organic growers.
Soil based pre-plant treatments to reduce pH include elemental sulfur (S) and acids as mentioned above. It is only necessary to treat a limited area near the root zone to ameliorate symptoms because the tree only needs to take up a small amount of Fe. Material can be shanked in or banded and incorporated in the prospective tree row. One ton of elemental sulfur per treated acre is needed to mitigate three tons of lime, and may need to be re-applied every 3 to 5 years after planting. The addition of organic matter such as well-composted manures will benefit poorly drained or compact soils by increasing aeration for better root growth, fostering chelation of nutrient cations, and reducing pH (depending on source material).
If possible, choose a Fe efficient species or cultivar. In perennial systems, lime-tolerant rootstocks may be the first line of defense in combating Fe deficiency. Some rootstocksmentioned are peach-almond and Krymsk-86 for stone fruit, Gisela 5 for cherry, and Pyrus communis for pear. Ongoing research studies in Europe focus on screening rootstocks of grape and olive for lime tolerance.
Once soil and water quality improvements are made, post-plant management strategies may also be implemented to ameliorate lime-induced Fe chlorosis in the short term. Soil can be acidified as described above. Individual trees can be treated by digging four to six 12-24 inch
holes around the drip line and burying a mixture of sulfur and Fe fertilizer. Historically, two principal methods have been utilized: 1) foliar application of inorganic Fe salts (ie. ferrous sulfate), and 2) soil or foliar application of synthetic chelates. Application of Fe salts to foliage may have mixed results due to limited penetration of Fe into leaves and inadequate mobilization within the plant. Use of Fe chelates may be of benefit; however, they are expensive and pose an environmental concern due to their mobility within the soil profile. Because soil lime interferes with Fe mobility with the plant, repeat application of inorganic Fe salts or Fe chelates may be necessary throughout the growing season.
Choice of nitrogen (N) fertilizer may also influence solubility of rhizosphere Fe. When N is applied in the ammonium form (NH4+), the root releases a proton (H+) to maintain a charge balance, thus reducing rhizosphere pH. Alternately, fertilization with nitrate (NO3-) results in root release of hydroxyl ions (OH-), resulting in an increase in rhizosphere pH. Solubility of Fe3+ increases 1000 fold with each one unit decrease in pH; therefore, fertility-induced rhizosphere pH changes may significantly influence Fe availability.
New methods for amelioration of Fe chlorosis are under investigation. For example, container studies have demonstrated that inter-planting sheep's fescue, a Strategy 2 plant, with a Fe-inefficient grape rootstock may ameliorate Fe chlorosis in grape. In this system, the grass produces a phytosiderophore that enhances Fe availability to the grape. Additionally, soil amendment with Fe3(PO4)2• 8H2O), a synthetic iron(II)-phosphate analogous to the mineral vivianite, has been effective at preventing Fe chlorosis in lemon, pear, olive, kiwi, and peach. Vivianite has a high Fe content (~30%) and serves as a slow release source of Fe in calcareous soils.
Figures below: 1) Shoot dieback in citrus, 2) Interveinal chlorosis in citrus and 3) Various stages of iron chlorosis in avocado.
- Author: Brad Hanson
Today I thought I'd share a recent research report on the the phenomenon of "enhanced" degradation of the herbicide simazine in citrus orchard soils. Click here for a link to the publication in the open-source journal, Air, Soil, and Water Research (Abit et al. 2012. Air Soil and Water Research 5:69-78). The lead author was a UC Davis post doctoral researcher and her coauthors include UC Davis, USDA-ARS, Fresno State, and UC Cooperative Extension folks.
This work was started several years ago in response to some questions from San Joaquin Valley orchard and vineyardists poor weed control with simazine. They suspected herbicide resistance, which is certainly a possibility given the number of weed biotypes resistant to photosystem II inhibitors reported worldwide (69 biotypes at latest count according to http://www.weedscience.org). However, when we visited some of the fields, it just didn't look like resistance or poor applications - too many species affected, good early control but poor residual performance, affected some orchards but not others, no clear application problems.
Some colleagues in Colorado and Mississippi were conducting research on faster-than-normal degradation of atrazine, a related triazine herbicide. We decided to conduct laboratory studies to determine if the poor residual performance in our orchard systems could also be due to the so-called "enhanced degradation".
First, some background. To a large degree, all herbicides degrade is soil - a complex molocule is eventually broken down into component elements; however the rate of degradation can vary greatly. The two biggest contributors to herbicide degradation are "chemical processes" that include all sorts of ways that chemical bonds are broken and "microbial processes" in which soil microorganisms use the molocule as a carbon source (ie "food"). Generally, microbial processes are the most important but are usually not specific to a particular herbicide. Sometimes, however, a microbial population with a special ability to degrade a particular herbicide (or chemically related compounds) can build up in the population and very rapidly degrade all of the herbicide. [analogy: a bowl of candy on your desk that lasts for weeks if you eat one or two per day but lasts only a few seconds when your hungry teenager and his friends show up!].
To determine if there were differences in simazine degradation rates among orchards, we collected soil from 27 citrus orchards in Tulare and Ventura counties. The orchards had a range of simazine use histories ranging from being treated every year for decades to having no simazine for at least the past 15 years.
1. In one set of experiments, we found that the soils that had not been exposed to simazine for at least 15 years degraded the herbicide much more slowly and that annual use (for the past 5 years, or longer) tended to be accociated with faster degradation.
2. In a second set of experiments, we autoclaved (heat sterilized) soil to kill any microbes present to determine if the herbicide degradation was due to microbial activity. This study showed that the degradation was almost completely due to microbial degradataion - simazine degradation was almost non-existent in autoclaved soils. Some soils have the the specialized microbial population while others do not.
The conclusions of the study were that repeated use of simazine can lead to more rapid degradation of the herbicide but that the correlation was not perfect. The correlations were better in the Tulare County sites than in the Ventura County soils. Some soils with long-term use of the herbicide continued to degrade simazine at a "normal" rate while other degraded it faster than predicted.
Overall, this is an interesting phenomenon that growers and pest control advisors should be aware of when making weed control decisions. Simazine can provide very effective and economical weed control in many situations but, as always, we recommend monitoring orchard conditions and adjusting weed control strategies accordingly.
This research was partially supported by funding from the California Citrus Research Board and USDA-ARS.
A couple of my previous posts on herbicide degradation at the UC Weed Science Blog can be found at: http://ucanr.org/blogs/blogcore/postdetail.cfm?postnum=5929 and http://ucanr.org/blogs/blogcore/postdetail.cfm?postnum=3923
- Author: Neil O'Connell
Wrap removed - whitewashed
Author - Neil O'Connell
Installing tree wraps on young trees provides protection to the trunk from applications of herbicides during weed management operations. Additionally, the wraps minimize light interception by trunk tissue thereby reducing sucker growth. During hot weather tree wraps provide shade to the trunk and reduce the incidence of sunburn. With the increasing incidence of earwigs, damage to young trees and the tendency for the insect to congregate under the wraps, tree wraps are being removed in some cases. Recent laboratory data from Dr. Beth Grafton-Cardwell (confirmed by observations in the field) suggests that as the season progresses adults become less and less interested in feeding on leaves (peak of feeding in March and April, declines to next to nothing by June). So, a management consideration would be to check the earwigs in the wraps in the summer, and if there are only adults present there is less concern than if there are immatures present. Another point would be to consider treatment with an insecticide such as Lorsban if wraps are to be left on for sunburn protection.
If wraps are removed a uniform coating of sun protective material should be applied to the trunk to protect against sun damage. Trunk surfaces should be monitored to ensure that a uniform coating is in place. Sun damage to unprotected trunk tissue can result in partial or complete girdling of the tree.
- Author: Ben Faber, University of California Cooperative Extension
In both avocado and citrus there can be a rapid collapse of tissue brought on by a host of related fungi. The pathogen was once lumped as Dothiorella, but lately University of California extension plant pathologist Akif Eskalen has been able to tweeze out more species which mainly belong to the Botryosphaeria genus. The collapse can be quite rapid, so fast that the leaves continue to hang on to the tree. This disease is more common in years of low rainfall, where inadequate water is being applied (especially when Santa Ana winds are blowing), and where salinity build up has occurred. In the last 2 months, I have been called out to diagnose this problem five times. In each case, they were trees that had been sidelined and neglected or the grower was trying to save money by saving water. Luckily for a mature tree, there can be recovery as long the tree is protected from sunburn that occurs with defoliation. White wash the exposed parts, and wait for recovery. When it is clear what part is recovering, cut into fresh wood to remove the dead parts. For a more detailed discussion of this blight, see our 2009 Topics in Subtropics.