Alternate bearing is typically initiated by adverse climate. Once initiated, in the absence of additional environmental constraints affecting crop load, the bearing status of an orchard alternates between ON and OFF years, with ON years exhibiting less vegetative growth than OFF years. This biennial cycle, however, can be reset by adverse environmental conditions affecting bloom and fruit set. Adverse conditions 8-10 weeks prior to bloom may cause abortion of female flower parts, resulting in a high proportion of staminate (male) flowers that do not give rise to fruit. Additionally, adverse weather conditions at bloom may impact pollination and subsequent fruit set. Any conditions resulting in loss of crop during an anticipated ON year may render the season an OFF year.
For a fuller discussion of this topic, see the full article by Carol Lovatt at:
The Topics in Subtropics Newsletter
- Author: Ben Faber
- Author: Jim Downer
Leaf analysis is the preferred method of guiding a fertilizer program for fruit tree crops. Soil testing is less important, since the tree has the capacity to store nutrients in its various parts – roots, trunk, stems and leaves. However, soil testing is a component of a plant nutrient management program and has been standard practice for growers to aid in adjusting fertilizer applications. Soil testing is performed not only to improve plant growth, but also to reduce over-application of fertilizers that may lead to nutrient toxicities, excessive leaching and consequent economic losses.
For maximum accuracy and benefit, soil testing must be conducted using reliable methods on correctly-sampled soils (if the user is not trained in obtaining representative soil samples, test results even from the same soil can vary greatly). Test results must also be properly interpreted for a specific crop. Interpretative guidelines are readily obtainable for many agronomic and horticultural crops, as well as landscape trees. Cost for laboratory analysis for pH, NO3-N, P2O5 (Olsen), and extractable K2O are typically under $20 per analysis, but frequently results take from 1-4 weeks to get back to the grower.
By contrast, many retail garden centers offer commercial test kits, ranging in cost from $10 to $50 for multiple tests, so that the cost per test can be relatively low. These commercial kits are also advantageous because results can be obtained within one to two days. Commercial kits typically use a colorimetric method for indicating macronutrient and pH levels. Soil is measured into a sample container, extractant is added, and after a specified time for the reaction, the user compares the color obtained to a color card corresponding to categorical nutrient and pH levels.
We have always wondered how well these kits performed, so we purchased five commercially-available test kits and compared their results to standard laboratory analysis of NO3-N, P2O5 (Olsen), extractable K2O and pH from the same soil type with three distinct cropping histories (Soils 1, 2, and 3). The objectives were to identify differences in accuracy, if any, among test kits and to suggest a kit that most closely corresponds to analytical lab results.
Four of the kits, “La Motte Soil Test Kit” (La Motte Co., Chesteron, MD); “Rapitest®” (Luster Leaf Products, Woodstock, IL); “Quick Soiltest” (Hanna,Woonsocket, RI); and “NittyGritty” (La Motte Co. Chesteron, MD) measured nitrate-N, P2O5, K2O and pH. “Soil Kit” (La Motte Co., Chesteron, MD) measured only nitrate-N, P2O5 and K2O. The kit results for macronutrients were categorical (high, medium, and low); pH results were numeric, rounding to half pH units for the Rapitest® and one pH unit for the other three kits. The manufacturers’ instructions for each kit were followed for soil testing.
Results show that pH measures from LaMotte Soil Test Kit and Rapitest consistently matched lab results. Soils 1 and 3 proved to be in the pH 6.5 range, but the pH of Soil 2 was 7.8, technically beyond the capacity of Rapitest (pH 4.5-7.5). NittyGritty did not match lab results at all. Quick SoiltTest generally indicated lower pH values than the analytical lab. Results from LaMotte Soil Test Kit, Rapitest, and Quick Soiltest consistently matched the analytical lab results for nitrate-N and P2O5, while Soil Kit and NittyGritty did not. Soil Kit and NittyGritty analyzed K2O content with greater accuracy than for the other nutrients; the commercial tests in total corresponded with the analytical lab 82% of the time for this test. For Soil 3; all the commercial test results matched the analytical lab results 100%.
Precautionary measures for these commercial kits may increase their accuracy. For Soil Kit and Nitty Gritty, the extracting powders that came with the kits dissolved poorly; these kits generally yielded inaccurate results, but pulverizing the tablets or powders may increase extraction potential. Interpretation of color development should be made only within the time specified by the kit instructions because color intensity could vary within minutes. Also, interpretation can occasionally vary depending on the user. In this study, the observers independently interpreted the same result for 91% of the tests; this would probably be an acceptable proportion for a home gardener or farmer individually conducting tests, but occasional independent interpretation by another source may change the result.
La Motte Soil Test Kit results corresponded to those from the analytical lab for pH and all nutrients (86% of the tests matched). This kit is suitable for growers because it proved to be very accurate even over a range of pH values and is housed in a hard-sided, padded container. Rapitest yielded accurate results 92% of the time for all nutrients and pH less than 7.5, and was comparatively easy to use and interpret. Quick Soiltest matched the analytical lab results only 64% of the time because pH and K2O values were inaccurate. Interpretation of values from this kit may have resulted in application of potassium in excess of the needs of Soils 1 and 2.
An important limitation of all commercial test kits is the approximate or categorical value of nutrient content (i.e., low, medium, high). Analytical labs must be used when precise values are required. Nevertheless, commercially-available kits such as Rapitest and La Motte Soil Test Kit have shown to provide accurate, fast, and economical results and can help growers improve nutrient management.
- 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: Neil O'Connell
Dry root rot has been a problem in citrus orchards for many years. Although generally a problem in coastal and northern California counties it has been reported in other citrus producing areas of the state. When present it generally occurs as a chronic problem affecting only a few trees in the orchard. Trees may be invaded at any time from planting to maturity; frequently mature, good producing trees are invaded. Once infection has occurred, it may be several years before any symptoms are visible in above-ground portions of the tree. Symptoms may be a gradual leaf drop and twig dieback or a sudden death of leaves which dry and remain in place. The tree rapidly collapses as a critical mass of roots is damaged or the crown area is girdled. Investigations of declining trees in the past revealed decaying bark in the root system and/or crown area of the tree which was thought to involve brown rot gummosis caused by Phytophthora invasion. The decaying bark area eventually dried and cracked. No gumming was observed, however, as is typical of brown rot gummosis. A grey staining of the woody portion of root or crown tissue was observed which is not seen with Phytophthora where only the cambium tissue is affected. Further investigations by researchers revealed that in affected tissue in these declining trees Fusarium solani could be isolated. Other organisms including bacteria and weak parasites and saprophytes could be isolated as well. Tissue samples from affected trees have consistently yielded Fusarium spp. Microscopic examination of affected areas revealed a plugging of the water conducting xylem tissue. During high temperatures, this plugging could result in slight wilt or rapid collapse of the tree depending upon the percentage of water conducting elements affected in the roots or crown area. Early investigations in declining orchards identified stress factors which seem to predispose the tree to invasion by the organism which is not possible without one or more of these agents. Stress factors identified included environmental factors such as drought, cultural such as damage from fertilizer, herbicide, nematicide or waterlogging, and damage from rodents such as gophers. Chemical agents applied at critical periods or in excessive amounts appeared to be stressful to affected tissue thus rendering it susceptible to invasion. Water ponding next to the trunk of the tree or waterlogging of the roots was associated with invasion of root or crown tissue and later colonization by this wood rotting organism. Stress produced in the tree together with the presence of the dry root rot organism is thought to predispose the tree to invasion of the organism. Research involving the mechanisms of invasion of Fusarium involved exposure of seedlings to hot water and then the dry root rot organism which resulted in invasion where exposure to the organism without previous exposure to high temperature did not result in invasion. It was hypothesized that high temperatures may have interfered with natural defense mechanisms allowing invasion. Research has identified a relationship between Phytophthora and the vascular wilt causing Fusarium spp. Phytophthora lesions on roots favored the invasion of the Fusarium. Seedlings exposed to only the wilt causing organism were not invaded, but were invaded if exposed to Phytophthora and then the wood rotting organism. A relationship was established between temperature and invasion of Phytophthora. Seedlings were not invaded by Phytophthora in a medium at 75 or 65 degrees but were at 55 degrees. Results suggested that the seedling formed scar/callus tissue capable of excluding the organism at higher temperature but was unable to do so at the lower temperature. While most commercial rootstocks possess a moderate to high degree of tolerance to Phytophthora invasion, all rootstocks are thought to be susceptible to the dry root rot organism.
- Author: Craig Kallsen
Not too many years ago, most growers and pest control advisors were unaware that earwigs were a potential pest problem in citrus. Earwigs simply were not often found in large numbers in citrus orchards. Earwigs’ increasing pest status is probably related to advances in integrated pest management techniques and attendant reductions in use of broad-spectrum organophosphate and carbamate insecticides for control of common citrus pests. On the plus side, fewer toxic, broad-spectrum pesticides treatments reduced the safety hazard for pesticide applicators, field workers and the environment and biologically integrated pest management has been effective for controlling most pests. However, once general broad-spectrum pest suppression was removed by significant reductions in these insecticides, some secondary pests, or insects that were not known to be pests, began to do serious economic damage to citrus under some conditions.
In April 2000, samples of earwigs collected in the act of chewing on citrus fruit by Robert Walther, private pest control advisor in Kern and Tulare Counties, were sent from the University of California Cooperative Extension Office in Bakersfield to the California Department of Food and Agriculture for identification. These earwigs were identified as the European earwig (Forficula auricularia). The adult European earwig is about ¾ inch long, with a reddish brown head and darker body. A distinctive feature of the adult earwig is a pair of prominent appendages that resemble forceps at the tail end of its body. These forceps are straighter in the female and more curved in the male. The European earwig has wings hidden under short, hard wing covers. Earwigs are capable of flight, but when disturbed during daylight hours, usually scurry and hide under any available cover. Immature insects look like adults except are smaller and lack wings. Females lay eggs in the soil and produce a single, if somewhat extended, generation per year.
Earwigs are active and feed mostly at night, especially during hot days in spring, summer and fall. They prefer to inhabit cool, moist and dark places. Generally, earwigs will return to the ground before daylight after feeding in citrus at night. During the day they are often found in tree wraps commonly placed on the trunks of young citrus for frost protection and under heavy leaf litter adjacent to irrigation emitters in mature orchards. High populations of earwigs do not normally develop in citrus unless protective, shaded habitat is present. Earwigs damage citrus leaves and small-diameter developing fruit. Often growers and pest control advisors do not correctly identify earwig damage as such, and snails, citrus cutworms, leaf rollers, katydids, other chewing insects or wind damage are often incorrectly blamed. Examples of earwig, citrus cutworm, katydid and similar scarring can be viewed in the University of California Agriculture and Natural Resource publication #8090 “ Photographic Guide to Citrus Fruit Scarring” that is available at UC Cooperative Extension Offices and downloaded at
For earwigs to be an economic problem in citrus, they usually have to be present in large numbers. Fifty earwigs in a tree wrap is not an unusual find in infested young orchards. In young trees, earwigs are capable of causing severe defoliation. Buds, newly expanded leaves and soft, fully expanded leaves are all susceptible. Earwigs gouge leaves, and chew irregular holes in leaves and around the edges of leaves. Recently expanded spring flush leaves can be chewed down to the midrib Heavy infestations of earwigs in newly planted trees may require treatment, in that severe defoliation may result from their feeding activities.
In mature orchards the principal damage results from the earwigs chewing newly developing fruit in April and May. This damage is typified by holes gouged at the base of the fruit near the attachment to the stem or shallow crescent or star shaped slashing marks across the fruit. Badly damaged fruitlets will fall from the tree, but the scars on fruit that remain on the tree continue to expand as the fruit grows, and the fruit will not be marketable. Earwigs usually stop feeding on fruit larger than about an inch in diameter.
Pruning citrus so that branches do not contact the ground and blowing or raking leaf litter from under the tree into the row middles away from the wetted irrigation pattern can reduce earwig populations in mature orchards. In young orchards, simply removing trunk wraps can remove the earwig problem. Finding pesticides specifically labeled for control of earwigs in citrus may be difficult. Some growers have observed that after treating an ant infestation with an appropriately labeled chlorpyrifos formulation, that earwigs are effectively controlled as well.