- Author: Ben Faber
At a recent Fresh Index-sponsored meeting, David Crowley recently of UC Riverside talked of a five year-long study that assessed nutritional status and yield. This has been a study area that has long been confused by the problems of alternate bearing, weather-dependency of the avocado, soil variability, root rot, etc. etc. etc. that we all know about. There are nutrient interactions that confound results, as well. High phosphorus affects micronutrient uptake of zinc, copper and others. Zinc impedes copper uptake. Loss of roots from Phytophthora especially affects micronutrients. Irrigation and aeration again affects nutrient uptake, and especially micronutrients.
The elements coming from the soil are divided into primary nutrients, secondary nutrients and micronutrients. This grouping is based on the relative amounts required by plants, but all are essential. Crowley describes the relative need for each element being based on the “Law of the Minimum”; if only one element is deficient it eventually affects growth and yield of the entire plant in a negative manner. It doesn't matter how much the other nutrients are raised, if one is limiting, growth is limited by that one. The primary nutrients required by avocados are nitrogen, phosphorus and potassium. The secondary nutrients required are calcium, magnesium and sulfur. The micronutrients are zinc, iron, manganese, copper, boron, molybdenum, nickel and chlorine.
The Law applies not just to nutrients but to light, temperature, water, disease, pests – anything that affects growth. The limiting input needs to be fixed before the others can boost growth to whatever the biological maximum might be in that environment. In irrigated agriculture, water is the most common limiting input.
So, it is complex. Really complex. But with computers and different techniques of analysis and just looking at nutrients, Crowley has been able to get a better handle on what could be limiting growth in an individual grove. This applies not to what is lacking, but what might be in excess – too much chloride, too much nitrogen, too much…………….
So, in the case of all this data collection the Crowley team has done, something unusual has popped up. Copper deficiency.
Copper deficiency is not commonly recognized as a problem in California avocado groves, but occasionally a grower will report a leaf analysis showing less copper than the 5ppm recommended by Embleton (http://ucavo.ucr.edu/General/LeafAnalysis.html). Typical copper deficiency was reported by Barnard and others (http://www.avocadosource.com/Journals/SAAGA/SAAGA_1991/SAAGA_1991_PG_67-71.pdf). They reported the symptoms of copper deficiency as follows: • Dull appearance of older leaves • Prominent leaf veination • Reddish-brown leaf color • Premature defoliation and twig. This is an extreme case, and Crowley is suggesting there may be some low, chronic level that limits avocado. His final report can be found at:
Of course, why copper might be limiting is another question. Is it due to root rot? Interaction with other applied materials like phosphorus (not phosphite, phosphorous, phosphonate) fertilizers? With irrigation management? Something(s) to think about.
And citrus in California is a different beast. It can commonly show copper deficiency and be a limiting nutrient. We apply copper as a frost/brown rot/septoria spray and as a result don't often see deficiency in citrus.
Image
Liebig's Barrel. Optimum production occurs when all the barrel staves are as high as they can be. When one element is low, that becomes the limiting factor for production. Increased production doesn't occur until that uptake is improved and then the next limiting input restricts production. When that next one is corrected, then some other input then limits production. Correction keeps improving production until the biological limit is reached.
Have any readers actually seen a wooden barrel?
- Author: Ben Faber
There are 4,000 species of earthworms grouped into five families and distributed all over the world. Some grow uo to 3 feet long, while others are only a few tenths of inches. We call them nightcrawlers, field worms, manure worms, red worms and some people call them little diggers.
In California, we have some native species of earthworms, but in many cases non-native introduced species have come to dominate. The predominant native species belong to the Argilophilus and Diplocardia while many of the non-native are of European in origin in the Lumbricidae family. Many of these non-natives were probably introduced by settlers bringing plants from home, which had soil containing the worms. A survey of California earthworms by the US Forest Service can be found at:
https://www.fs.fed.us/psw/publications/documents/psw_gtr142/psw_gtr142.pdf
This is a wonderful description of earthworm biology and their occurrence in the landscape.
When digging in citrus orchards, it is common to find earthworms in the wetted mulch under tree canopies. Many of our citrus orchards were initially established by “balled and burlap” nursery trees that brought worms along with the soil. In the case of many avocado orchards, on the other hand, it can be rare to find earthworms in orchards. Most avocado orchards have been established since the 1970s when potting mixes and plastic liners were the standard practice and worms were not part of the planting media. Even though there is a thick leaf mulch in avocado orchards, the worms have not been introduced, and it is rare to find them.
Numerous investigators have pointed out the beneficial effects of earthworms on soil properties. One of the first of these observers was Charles Darwin who published Earthworms and Vegetable Mould in 1881. He remarked on the great quantity of soil the worms can move in a year. He estimated that the earthworms in some of his pastures could form a new layer of soil 7 inches thick in thirty years, or that they brought up about 20 tons of soil per acre, enough to form a layer 0.2-inch-deep each year.
Earthworms, where they flourish, are important agents in mixing the dead surface litter with the main body of the soil. They drag the leaves and other litter down into their burrows where soil microorganisms also begin digesting the material. Some earthworms can burrow as deeply as 5 to 6 feet, but most concentrate in the top 6 to 8 inches of soil.
The worm subsists on organic matter such as leaves and dead roots near the soil surface. The earthworm ingests soil particles along with the organic matter and grinds up the organic matter in a gizzard just as a chicken does. This is excreted in what we call worm casts. The castings differ chemically from the rest of the soil, as they are richer in nitrogen, potassium and other mineral constituents.
Castings are a natural by-product of worms. When added to normal soils in gardens or lawns, they provide the same kinds of benefits as other bulky organic fertilizers. Castings today are not commonly used as fertilizer by commercial plant growers because of their cost relative to other fertilizers. However, castings are used by some organic growers and are sold commercially as a soil amendment or planting medium for ornamental plants grown in pots.
The physical soil churning process also has several important effects:
-Organic residues are more rapidly degraded with the release of elements such as nitrogen, sulfur and other nutrients.
-Some of the inorganic soil minerals tend to be solubilized by the digestive process.
-Extensive burrowing improves soil aeration.
-Burrowing can improve water penetration into soils
-The earthworm carries surface nutrients from the soil surface and imports them into the root zone of the plant.
Although earthworms are considered beneficial to soil productivity, few valid studies have been made to determine whether their presence will significantly improve plant growth. This may seem odd since many of us have learned from childhood that worms are good. It is something like the chicken and the egg analogy. The conditions that are conducive to earthworms are also ideal for plants. Both plants and worms need temperatures between 60 and 100 degrees F for good growth; both need water, but not too much or little; they both require oxygen for respiration; and they do not like soils that are too acid or basic or too salty. By correcting soil conditions that are unfavorable for one will also improve the outlook for the other. The earthworm is a natural component of the soil population. If the soil is properly managed this natural population will thrive. In this sense, the presence or absence or earthworms can be an indicator of the "fertility" of one's soil.
- Author: Ben Faber
This is a reminder of the complexity of huanglongbing and the bacterial infection it causes. This abstract is from the HLB Conference in Florida last fall.
4.a.5 Symptom variations and molecular markers that illustrate the HLB complexity
Yongping Duan, Marco Pitino, and Cheryl Armstrong
USHRL-ARS-USDA, Fort Pierce, FL 34945, USA
Huanglongbing (HLB) is a devastating bacterial disease of citrus worldwide due to its intracellular and systemic infection. Various HLB symptoms are observed on different species/varieties of citrus plants: from yellow shoots to blotchy mottle on the leaves, from vein yellowing/vein corky to mosaic/green islands similar to zinc deficiency on the leaves, from whitish discoloration to stunted green leaves, etc. These variations of symptoms, which result from a combination of biotic and abiotic stresses, are not only present on individual plants from a variety but also exist on individual branches of an infected plant. Our results indicated that the adaptation of the bacterial populations, such as the dynamics of ‘Candidatus Liberibacter asiaticus' (Las), plays an important role in the induction of various symptoms and that Las mutations as well as the number and recombination events of Las prophages/phages affect this phenomenon. In addition, the selection of the host plants (resistance/tolerance) for the bacterial populations is also critical for symptom expression during disease progression. Based on severity, we divided HLB symptoms into four grades. It is worth noting that the grades of HLB symptom severity show a positive correlation with our newly identified biomarkers from host plants, and that gene expression profiling of different grades of infected leaves rationalized the differentiation based on the dynamics of these biomarkers. Because of these findings, we propose new approaches that allow for rapid selection of variant citrus plants, including bud sports with greater HLB resistance/tolerance.
Non-Technical Summary: Various symptoms of citrus huanglongbing display in different species/varieties of infected citrus plants. These variations of symptoms are not only present on individual plants from a variety, but also exist on individual branches of an infected plant. We have identified some molecular markers from the citrus plants and Las pathogen that illustrate the HLB complexity. Therefore, we propose new approaches that allow for rapid selection of variant citrus plants, including bud sports with greater HLB resistance/tolerance.
http://www.icc2016.com/images/icc2016/downloads/Abstract_Book_ICC_2016.pdf
- Author: Sonia Rios
Dr. Gary Bender, UC Cooperative Extension (UCCE) Farm Advisor Emeritus, is the lead instructor for a six-week course entitled “Avocado Production for New Growers.” Co-instructor, Sonia Rios, current subtropical Farm Advisor, Riverside/San Diego Counties will also be teaching in the course. The course is designed for new avocado growers, as well as those interested in learning more about avocado production best practices and meeting fellow growers.
The six-week course consists of six, two-hour sessions and will be held in Fallbrook, CA this year. The fee for the course is $105 and includes two avocado books, an IPM book and a post-harvest handbook. Final dates and the location will be announced soon. The always fills up, so please register A.S.A.P.
Topics include:
- Introduction to Agriculture in San Diego County, History of Avocado Production in California
- Botany, Flowering, Varieties, Harvest Dates, Rootstocks
- Irrigation Systems, Irrigation Scheduling, Salinity Management
- Fertilization, Organic Production
- Weed, Insect and Mite Control, Disease Control
- Ag Waiver Water School Training (Dr. Loretta Bates)
- Canopy Management, Tree Spacing, Frost Management
- Field trip to High Density Trial grove and a commercial grove
For more information, contact Erin Thompson at 858.822.7919 or enthompson@ucanr.edu.
- Author: Patrick Moran
Editor: Guy B Kyser
The giant invasive grass arundo (Arundo donax), one of the weeds targeted under the USDA-ARS-funded Delta Region Areawide Aquatic Weed Project (DRAAWP), has been re-acquainted with one of its natural enemies imported from arundo's native range. A tiny insect called the arundo armored scale (Rhizaspidiotus donacis) has been successfully released in the Sacramento River watershed and in the Delta.
Arundo forms dense stands across at least 10,000 acres in California, and over 100,000 acres in other arid riparian areas such as the Lower Rio Grande Valley of Texas and Mexico. Other control methods such as herbicide application, mechanical removal, mowing or burning have been used to reduce arundo populations in California, costing tens of millions of dollars. However, arundo is a tough plant and takes advantage of human disturbance and fire in riparian habitats along creeks, sloughs, rivers and reservoirs. Arundo populations in California thus exceed the capacity of these other control methods. In the absence of control, arundo consumes and wastes scarce water – a single plant can consume as much water as corn when growing in moist soil under hot, sunny conditions. Dense arundo stands block access to water for irrigation and recreation, and also obstruct flood control structures such as drainage ditches. Arundo also displaces native plants and animals and alters geomorphology and water flow dynamics in riparian habitats in ways that make it difficult for the natives to come back even if the arundo is controlled.
Biological control of invasive weeds focuses on the characterization, release and evaluation of insects (or plant pathogens) from the weed's native range into areas where the weed is non-native. The arundo armored scale was collected originally in southern France, Spain, and Italy. Studies by Spanish collaborators showed that, even in its native range with its own natural enemies, the arundo armored scale reduces shoot growth and rhizome size by 50%.
Biological control agents undergo rigorous testing to ensure they are not a threat to native plants or crops. After a permit review process, the USDA granted a permit for field release of arundo armored scale in 2010. (This is one of two insects that have been released for biological control of arundo in North America.) Since 2011, this biological control agent has been released in the Lower Rio Grande Basin. Initial releases of this agent in California began in 2014, and it was found that the scale insect had become established at one site in the northern Sacramento Valley by November of that year.
Armored scales are small insects that spend most of their lives in an immobile state, covered by their waxy secretions (‘armor'). Adult females produce ‘crawlers' that disperse locally (typically just a few feet) to find new buds coming up from the arundo rhizomes or lateral shoot buds above ground. The crawlers then lose their legs and antennae and insert their stylet-like mouthparts into the arundo tissue to feed on the fluids in the plant's vascular system. Crawlers molt to a second immature phase, and about six to eight weeks later, short-lived adult males emerge from their armor and mate with the immobile adult females. The females continue to feed and slowly develop embryos. A new generation of crawlers then emerges from the females. The life cycle takes four to six months.
Top row, left to right: Tiny (0.5 mm) crawlers emerge from females and settle on rhizomes or lateral shoots. Second-instar immature scales continue to feed and expand. Winged adult males emerge from their oyster-like scale covering. Females (armored scale cover removed) are shriveled and skinny right after molting. Mature females are plump and turn a darker color when they are full of crawlers ready to emerge. Bottom row, left to right: Adult females form aggregations on arundo rhizomes and shoots. The presence of armored scale populations causes shoot distortion and reduces both shoot growth and rhizome size.
In 2015, we tested a new release technique using arundo ‘microplants'. We soaked arundo shoot fragments in water for one month, then planted them in pots where they produced new shoot buds and roots. Armored scale crawlers were released onto the microplants. After about six months, we planted the infested microplants at field sites in the Delta – Andrus Island on the Sacramento River, and at Big Break near Oakley – and along Stony Creek in Glenn and Butte Counties north of the Delta. We established the microplants adjacent to large arundo shoots, and we cut off some of the established shoots to encourage production of new rhizome buds and lateral shoots. We watered the microplants as needed to keep them alive for about 6 months.
Left to right: Microplant with gelatin capsules used to isolate scale crawlers from females (capsules had been opened and crawlers poured onto the base of the plant). Greenhouse bench with arundo microplants. Field plot with arundo shoots cut back to promote new shoot and rhizome growth. Base of an arundo shoot at the field site (arrow indicates location of adult female scales that developed from crawlers that had previously came out of the females on the microplants.
Almost one year after planting, in November 2016, we sampled arundo rhizomes and shoots from the areas where the now-dead microplants were placed. At the Sacramento River site, 150 females were found, and at a site on Stony Creek in Butte County, 72 females were found. The females were placed in gelatin capsules to capture crawlers. A total of 1,668 crawlers emerged by early January 2017. Since there are still many more arundo shoots around the microplant sites, these results indicate that the arundo armored scale has established reproductive populations at three sites in California. This is the first establishment of this biocontrol agent in the state. Additional releases are planned throughout the Delta and surrounding watersheds. Along with the arundo wasp (Tetramesa romana), the arundo armored scale is expected to significantly reduce the potential for arundo to grow, disperse and form damaging populations that threaten water resources.
This work is conducted under the USDA-ARS-funded Delta Region Areawide Aquatic Weed Project (DRAAWP). This portion of the project is led by Dr. Patrick Moran (Patrick.Moran@ars.usda.gov) of the USDA-ARS Exotic and Invasive Weeds Research Unit, Albany, CA. Dr. Moran is working with a postdoctoral researcher, Dr. Ellyn Bitume (Ellyn.Bitume@ars.usda.gov), on this project. Contact us if you have questions. The Sacramento-San Joaquin Delta Conservancy (Beckye Stanton) is collaborating with USDA-ARS to identify field sites in the Delta, connect with landowners, and integrate biological control with their chemical arundo control program. Dr. Moran cooperates with landowners and local Reclamation Districts to obtain access to field sites.