Silicon is currently under consideration for elevation to the status of a "plant beneficial substance by the Association of American Plant Food Control Officials (AAPFCO). Silicon has been shown in university and field studies to improve cell wall strength and structural integrity, improve drought and frost resistance, decrease lodging potential and boost the plant's natural pest and disease fighting systems. Silicon has also been shown to improve plant vigor and physiology by improving root mass and density, and increasing above ground plant biomass and crop yields.
Silicon (Si) is the most abundant element (27.2%) present in the earth's crust following oxygen (45.5%). Silicon is known for a number of important chemical and physical properties, i.e. semiconductor property that are used in various scientific and technical applications. In most soils near a neutral pH, the composition is a complex of iron, aluminum, oxygen and silicon. Silicon is one of the most important constituents of dust, which is carried by winds all over the world. Geologists know silicon as the rock quartz and the many silicate materials, such as opal. Formally, silica (SiO2) is a silicic acid (H4SiO4), which is water soluble and stable in highly dilute aqueous solutions. There are many forms that silicon can take in the natural environment, often complexed with water. Plants take up a form of silicic acid and in highly leached, low pH environments, much of the silicon may have been removed over time.
It appears that grains, such as wheat and especially rice have an absolute need of supplemental silicon to improve plant growth. Few non-grass plants have shown this need other than cucurbits apparently. Much of the improvement typically is for improved disease control and improved stature (prevention of lodging).
Many of the studies showing benefits of silicon amendment have occurred in low soil pH environments or in solution culture where it has been possible to create low silicon growing media. Several years ago, potassium silicate was being promoted as a fungistat for controlling Phytophthora root rot in avocado. A number of field and greenhouse trials were tried in California during the early 2000s to assay its effect. For whatever reason, the material showed no effect on the disease. Potassium and calcium silicates are liming materials, raising soil pH. The effect that was noticed in its use in other countries may simply have been a soil pH effect on either the avocado tree, the Phytophthora or both.
TYPES OF NATURAL ENEMIES
Parasites, pathogens, and predators are the primary groups used in biological control of insects and mites (Table 1). Most parasites and pathogens, and many predators, are highly specialized and attack a limited number of closely related pest species. Learn how to recognize natural enemies by consulting resources such as the Natural Enemies Handbook and the Natural Enemies Gallery.
A parasite is an organism that lives and feeds in or on a host. Insect parasites can develop on the inside or outside of the host's body. Often only the immature stage of the parasite feeds on the host. However, adult females of certain parasites (such as many wasps that attack scales and whiteflies) feed on and kill their hosts, providing an easily overlooked but important source of biological control in addition to the host mortality caused by parasitism.
Although the term “parasite” is used here, true parasites (e.g., fleas and ticks) do not typically kill their hosts. Species useful in biological control, and discussed here, kill their hosts; they are more precisely called “parasitoids.”
Most parasitic insects are either flies (Order Diptera) or wasps (Order Hymenoptera). Parasitic wasps occur in over three dozen Hymenoptera families. For example, Aphidiinae (a subfamily of Braconidae) attack aphids. Trichogrammatidae parasitize insect eggs. Aphelinidae, Encyrtidae, Eulophidae, and Ichneumonidae are other groups that parasitize insect pests. It's important to note that these tiny to medium-sized wasps are incapable of stinging people. The most common parasitic flies are the typically hairy Tachinidae. Adult tachinids often resemble house flies. Their larvae are maggots that feed inside the host.
Natural enemy pathogens are microorganisms including certain bacteria, fungi, nematodes, protozoa, and viruses that can infect and kill the host. Populations of some aphids, caterpillars, mites, and other invertebrates are sometimes drastically reduced by naturally occurring pathogens, usually under conditions such as prolonged high humidity or dense pest populations. In addition to a naturally occurring disease outbreak (epizootic), some beneficial pathogens are commercially available as biological or microbial pesticides. These include Bacillus thuringiensis or Bt, entomopathogenic nematodes, and granulosis viruses. Additionally, some microorganism by-products, such as avermectins and spinosyns are used in certain insecticides; but applying these products is not considered to be biological control.
Predators kill and feed on several to many individual prey during their lifetimes. Many species of amphibians, birds, mammals, and reptiles prey extensively on insects. Predatory beetles, flies, lacewings, true bugs (Order Hemiptera), and wasps feed on various pest insects or mites. Most spiders feed entirely on insects. Predatory mites that feed primarily on pest spider mites include Amblyseius spp., Neoseiulus spp., and the western predatory mite, Galendromus occidentalis.
When resident natural enemies are insufficient, their populations can sometimes be increased (augmented) through the purchase and release of commercially available beneficial species. However, there has been relatively little research on releasing natural enemies in gardens and landscapes. Releases are unlikely to provide satisfactory pest control in most situations. Some marketed natural enemies are not effective. Many natural enemies are generalist predators and are cannibalistic and feed indiscriminately on pest and beneficial species, thereby reducing their effectiveness.
Only a few natural enemies can be effectively augmented in gardens and landscapes. For example, entomopathogenic nematodes can be applied to control certain tree-boring and lawn-feeding insects. Convergent lady beetles (Hippodamia convergens) purchased in bulk through mail order, stored in a refrigerator, and released in very large numbers at intervals can temporarily control aphids; however, lady beetles purchased through retail outlets are unlikely to be sufficient in numbers and quality to provide control.
Successful augmentation generally requires advanced planning, biological expertise, careful monitoring, optimal release timing, patience, and situations where certain levels of pests and damage can be tolerated. Situations where pests or damage are already abundant are not good opportunities for augmentation.
A classic example of poor timing for augmentative release of predatory mites for control of broad mite in coastal lemon or persea mite in avocado is right now. Pest populations for the most part have soared and releasing predatory mites is little help. Predatory mites need to be releases into a small growing population, so in both of these cases it would have been better to start small, frequent releases early and throughout the spring to knock their populations back.
Field topworking avocados and citrus can be a prolonged process that can take two years of maintenance before the trees are productive. Often, it's easier and cheaper to just start off with a new tree. Thegrafted tree needs to be pruned back and buds or bud sticks need to be inserted into the old trunk. Birds love to sit on these new perching spots, and can often damage the bud stick, so they need to be protected with some type of covering like a paper bag. The latent buds in the trunk need to be constantly removed to prevent the old scion from outgrowing the new one.
A nurse branch also needs to be maintained so that there is a steady flow of transpired water to cool the trunk. Bud sticks are leafless and when the tree is headed back to graft, the tree no longer has the leaf area to pump water through the trunk. The trunk can heat up and result in sun burn damage. This damage may not appear for a year or so after the incident has occurred. It normally will show up on the south or southwest side of the tree, which catches the afternoon sun. The damage may be so severe that the tree dies, or the tree may begin a healing process as with other wounds and start growing callus tissue along the margins of the sun burn damage. If the damage is nor too extensive, the callus growth can eventually cover over the damage with new bark. If the grafted tree has left some structural branches, damage can show up on upper surfaces of branches, as well as those sides facing the afternoon sun.
To avoid sunburn damage, retain the nurse limb for about a year after grafting and whitewash or cover the trunk with a protective paper. Managing irrigation is important, as well. If grafted trees are interplanted with trees with more mature canopies, the grafted trees should be on a separate irrigation system and schedule. This is to avoid water stress which can enhance sun burn.
Sun burn can happen to any grafted tree, especially those with thin bark, like citrus and avocado. Any time a portion of the bark is exposed to sun that has previously been in the shade, can also set up the tree for sun burn. So excessive pruning going into summer months can also cause significant sun burn.
Sunken area of trunk, showing sun burn damage and callus growth starting to grow along margins of damage. Grafted citrus with nurse limbs and whitewash.
Transpiration is essentially a function of the amount of leaves present. With no leaves, there is no transpiration and no water use. The extreme case is tree removal. If canopies are pruned there is reduced water use. The more canopy reduction, the more transpiration reduction. Most citrus produces terminal flowers, so there is also a reduction in yield, but there is also typically an increase in fruit size as competitive fruit growing points are removed. There is a balance between yield reduction and tree water use, but typically a 25% canopy reduction results in a 25% decrease in tree water use (Romero, 2006).
The severity of the drought will determine how drastic the canopy should be trimmed. The trees can be skeletonized so that only the main structural branches are left. The tree is whitewashed to prevent sunburn and the water is turned off. As the tree gradually leafs out, the water is gradually reapplied in small amounts. It's important to check soil moisture to make sure the tree do not get too much or too little water. The trees if pruned in the winter will often flower a year later in the spring, but normal production will often take three years for the trees to recover their previous yields.
Skeletonizing should first be practiced on orchards that are the poorest producing. In those areas that get too much wind and have lots of wind scarring or elevated water use, those areas that are most prone to frost damage, those areas that have been always problematic, such as fruit theft. In areas that are healthy and a new variety has been contemplated, this is the time to topwork and replace that old variety. In areas that have been poor producing from disease, this is the time to get rid of those trees.
Canopy sprays of kaolinite clay have shown some promise in reducing transpiration with negligible yield reduction (Skewes, 2013; Wright, 2000). If these are used, they should be done under the advisement of the packing house to make sure the clay can be removed in the packing house.
With a reduced canopy, there are often other benefits besides water reduction. There is better spray coverage for pest control. There is also reduced fertilizer use. New growth is normally coming from nutrients that are now being mined by a large root system and fertilizer applications can be significantly reduced or eliminated altogether for a year until fruit set recommences.
Kerns, D. and G. Wright. 2000. Protective and Yield Enhancement qualities of yield of kaolin on lemon. In: Eds. G. Wright and D. Kilby, AZ1178: "2000 Citrus and Deciduous Fruit and Nut Research Report," College of Agriculture and Life Sciences, University of Arizona. http://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1178_3.pdf
Skewes, M. 2013 Citrus Drought Survival and Recovery Trial. HAL Project Number CT08014 (16/12/2013). SARDI. http://pir.sa.gov.au/__data/assets/pdf_file/0004/238414/SARDI-Citrus-Drought-Survival-Recovery-Trial.pdf
Navel trees skelotinized and topworked, ready for rain and more profits in the future.
Citrus response to irrigation water deficits have demonstrated that sensitivity of yield to water stress is dependent on the phenological phase in which water stress was applied. Adequate water supply is of major importance during citrus flowering and fruit set. A second critical period coincides with the period when fruit growth is rapid (fruit set to harvest). Depending on the level of water stress developed, the abscission of flowers and young fruits will be affected in the first case, as will fruit size in the second case.
For navels and mandarins it is possible to identify these critical periods in the crop and possibly allow stress when the trees are not in those critical periods. Some varieties though are complicated by having overlap of critical periods when another crop is present at the same time. Valencias can have two crops on the tree at the same time in spring and into summer harvest and coastal lemons can have fruit in all stages from fruit set to mature fruit at all times of the year. In the case of navels, reductions of applied water by 25% or more have resulted in no fruit yield reductions, if those water reductions do not occur during critical periods (Goldhamer, 2006; Domingo, 1996; Hutton et al, 2007). Water reductions during the rapid expansion period can result in significant fruit size reduction, though, and this period should be avoided if fruit size is critical to marketing (Goldhamer, 2006; Hutton et al, 2007).
In the case of coastal lemons, the stress should be avoided when the period of the most profitable crop is in rapid expansion, this is normally the summer crop. Each grower would need to identify, when the most profitable fruit size is important. Growers in areas that have more summer heat than the coast might practice a ‘Verdelli' irrigation practice, where water is withheld for a period of time, in order to force flowering that can often result in more summer fruit being harvested the following year (Maranto and Hake, 1985).
Domingo, R., Ruiz-Sanchez, M.C., Sanchez-Blanco, M. J. and Torrecillas. A.1996. Water Relations, growth and yield of ‘Fino' lemon trees under regulated deficit irrigation. Irrig. Sci.16: 115-123 http://link.springer.com/article/10.1007%2FBF02215619#page-1
Goldhamer, D. and N. O'Connell. 2006. Using Regulated Deficit Irrigation to Optimize Fruit Size in Late Harvest Navels. Citrus Research Board. http://citrusresearch.org/wp-content/uploads/2006-GOLDHAMER1.pdf
Hutton RJ, Landsberg JJ, Sutton BG. 2007. Timing irrigation to suit citrus phenology: a means of reducing water use without compromising fruit yield and quality. Australian Journal of Experimental Agriculture (47): 71–80. http://dx.doi.org/10.1071/EA05233
Maranto, J. and K. Hake. 1985. Verdelli summer lemons: a new option for California growers. California Agriculture 39(5): 4. https://ucanr.edu/repositoryfiles/ca3905p4-62870.pdf
Phenological stages of navel orange.