A general rule of thumb about pruning trees is that only healthy trees should be pruned. Pruning is a devitalizing practice that comes at the expense of the roots. If an avocado has root rot, make sure the tree has been treated with one of the phosphite products to get the root system healthy. A common pruning method is stumping to 3 feet and allowing regrowth to occur. A common phenomenon after stumping is that the tree puts on vigorous growth for two or three years and then collapses. All that canopy regrowth was coming from a large root system that was brought into balance with a smaller canopy. Energy is diverted from the root to fight off disease. Gradually the root system gets out of balance with a larger canopy that it can no longer support. Often when a severely impaired root system tree is pruned, it often does not have energy to push a new canopy and the tree dies. Make sure you only prune healthy trees.
- Author: Jim Downer
Jim Downer is the Environmental Horticulture Advisor in Ventura County Cooperative Extension
As growers of thousands of ornamentals we understand that minerals absorbed mostly by roots as ions are essential for plant growth and development. Some required in parts per hundred are macro-nutrients while others only required in ppm or ppb are considered micronutrients. As long as enough of the 18 essential minerals are available, plants grow and reproduce in a healthful way. When not enough of one of the essential elements is supplied, a deficiency occurs and plants present symptoms. Mineral nutrient symptoms are considered abiotic disorders. There are however cases where excess or deficiency of elements can be predisposing to disease caused by pathogens. While examples of this are not abundant, some mineral elements do have a role in the development of disease caused by some pathogens.
Soil-borne pathogens are affected by minerals dissolved in soil solution. Minerals can act in specific ways (specific ion effects) or total ion effects (osmotic strength/concentration) having direct effect on pathogenic propagules or on the host itself. If we utilize the plant disease tetrahedron and think of all the implications ions could have in a biological disease relationship there are several possibilities:
-Specific ions harm the pathogen
-Specific ions harm the host
-Ionic strength changes the root environment making the host weak and susceptible
-Ions change the pH of the soil solution making it more/less fit for a pathogen or the host
-Ions change the soil physical environment making plants conducive to disease
While it is often espoused that the well “fed” or fertilized plant it is resistant to disease it is rarely borne out in any research. Keeping a good nutritional level in nursery stock will not necessarily protect plants from many of the virulent pathogens that are capable of causing disease. Nitrogen fertilization can produce succulent growth that will lead to exacerbation of such diseases as powdery mildew (Powel and Lindquist, 1997). Too many mineral nutrients may only result in luxury consumption by the fertilized plant or may cause other problems. It has long been known that seedling disease caused by Rhizoctonia solani increases with increased salinity in media (Baker, 1957) and later discovered by Jim Macdonald and others (1984), that salinity increases susceptibility of ornamental plants to Phytophthora.
Two basic plant resistance mechanisms that mineral nutrition can affect in plants are: 1.) formation of mechanical barriers (cell wall strengthening) and 2.) synthesis of defense compounds that protect against pathogens (Spann and Schumann, 2010). The role of specific elements and their compounds is much more complicated. Certainly deficiencies of molecules such as Calcium and Potassium can interrupt either of these defense mechanisms.
Root rot is a disease of thousands of ornamental plants and a serious problem in most nurseries. Root rots caused by Phytophthora spp. occur in a range of nutritional environments and pH’s. While some studies have implicated nitrogen compounds in the control of Phytophthora these probably involve the release of ammonia which is also toxic to plant roots (Zentmeyer, 1963). Lee and Zentmeyer (1982) later showed that both ammonium and nitrate reduced disease caused by P. cinnamomi , and that low levels of nitrate stimulated production of sporgangia. Most studies have found no relationship of nitrogen source to root rot disease development. Zentmeyer’s early work also suggested a role for calcium in disease reduction caused by Phytophthora root rots. Calcium increases disease resistance to root rot in Avocado (Duvenhage and Kotze, 1991). While it is understood that calcium has direct effects on plant membranes, root cell membrane leakage, cell wall thickness and many other host factors, Messenger (2000), later showed that the calcium ion also has direct effects on Phytophthora, reducing its sporangia size and zoospore mobility. Soils and media low in soluble calcium or where calcium is easily precipitated out of solution, or where pH is high and limestone minerals decrease the availability of calcium, are conducive to Phytophthora root rots.
Wilt diseases have also been studied in relation to disease occurrence. Keim and Humphry (1984) showed that nitrogen source reduced the incidence of wilt cause by Fusarium oxysporum f.sp. hebe in Veronica. In their system ammonium sulfate promoted disease and calcium nitrate prevented fusarium infections. In later work on the Fusarium oxysporum wilt disease of Canary Island date palm, Downer and others (2012) found no effect of fertilizer source on disease development (2013 ). Every disease system must be considered independently to determine if nutrient relationships are part of that system.
While it is easy to see a role for essential elements in plant defense, non-essential elements may also play a role in some systems. Silicon increases resistance of plants to powdery mildew (Kauss and others, 2003) and root roots (Cherife et al., 1994) and to stress in general (Ma, 2011). Silicon is implicated not only in strengthening cell walls but in defense protein production in plants (Faufeux et al., 2006). Not all plants are capable of utilizing silicon, so its role in plant defense is limited to those species capable of metabolizing it. Much more study is necessary to understand silicon’s role with ornamental plant –pathogen systems.
Nutrient exchange in container media is complicated-- it is mediated by the substrate, water chemistry, temperature and the applied ion sources as well as by plants growing in the media. Growers are well served to apply fertilizers that can supply a constant nutrient charge. Supply of extra soluble calcium, may be helpful in managing root rots. Avoidance of salt built up or high salinity situations that can occur when plant dry out will also help keep plants from
Baker, K.F. 1957. The UC System Producing Healthy Container-Grown Plants. University of California Division of Agricultural Sciences Agricultural Experiment Stations Publication #23.
Cherif. M., A. Asselin, and R.R. Belanger. 1994. Defense responses induced by soluble silicon in cucumber roots infected by Phythium spp. Phytopathology 84:236-242.
Downer, A.J., D.R. Hodel, D.M. Matthews, and D.R. Pittenger. 2013. Effect of fertilizer nitrogen source on susceptibility of five species of field grown palms to Fusarium oxysporum f. sp. canariensis. Palms 57: 89-92
Duvenhage, J.A. and J.M. Kotze. 1991. The influence of calcium on saprophytic growth and pathogenicity of Phytopthora cinnamomi and on resistance of avocado to root rot. South African Avocado Growers Yearbook 14:13-14.
Faufeux, F., Remus-Borei, W., J.G. Menzies, and R.R. Belanger. 2006. Silicon and plant disease resistance against pathogenic fungi. FEMS Microbiology letters 249:1-6.
Kauss, H. Seehaus, K. Franke, R., Gilbert S., Dietrich R.A., and N. Kroger. 2003. Silica deposition by a strongly cationic proline-rich protein from systemically resistant cucumber plants. Plant J. 33:87-95.
Keim, R. and W.A. Humphrey. 1984. Fertilizer helps control Fusarium wilt of Hebe. California Ag. 38:13-14.
Lee, B.S. and G.A. Zentmeyer. 1982. Influence of calcium nitrate and ammonium sulfate on Phytophthora root rot of Persea indica. Phytopathology 72:1558-1564.
Ma, J.F. 2011. Role of silicon in enhancing the resistance of plants to biotic and abiotic stresses.
Macdonald, J.D., T.J. Swiecki, N.S. Blaker, and J.D. Shapiro. 1984. Effects of Salinity stress on the development of Phytophthora root rots. Calif. Agriculture 38:23-24.
Messenger, B.J., J.A. Menge, and E. Pond. 2000. Effects of Gypsum on zoospores and sporangia of Phytopthora cinnamomi. Plant Dis. 84:617-621.
Powell, C.W. and R.K Lindquist. 1997. Ball Pest and Disease Manual (2nd ed). Ball Publishing Batavia Publishing 426pp
Span, T.M . and A.W. Schumann. 2010. Mineral Nutrition contributes to plant disease and pest resistance. University of Florida document HS1181. Published on-line at http://edis.ifas.ufl.edu
Zentmeyer,G.A., 1963. Biological control of Phytophthora root rot of avocado with alfalfa meal. Phytopathology 53:1383-1387.
Early onset root rot in color or bedding plants./span>
There is a running debate about whether avocado canopies should be skirted up, raised up so that you can see under the canopy. In doing so, the tree’s tendency is to maintain its bearing volume by increasing a similar amount in height that is lost by removing the bottom layer of canopy. A tree with a full canopy is more cold resistant because it traps heat inside the canopy and is not so prone to cold winds. In an inversion freeze, though, warming air from irrigation, wind machines and orchard heaters is less likely to circulate when the skirts block air movement. A low skirt also impedes a uniform application of water from microsprinklers, and hence fertilizer distribution. A low skirt also has more fruit lying on the ground which is more uneven in coloration and more prone to disease and possibly food safety issues. A raised skirt also promotes more air circulation within the canopy which can reduce the incidence of some other diseases of both fruit, stems and branches.
A raised skirt, though exposes the base to light, and if there is no leaf mulch, there are more weeds to control. In the case of hillsides, because of gravity and wind exposure, leaves tend to blow away. The roots are now more exposed to drying because of increased evaporative loss. Loss of leaves is also a major disease problem, since leaves and organic matter are the first lines of defense (after proper irrigation management) against Phytophthora root rot. It is the microorganisms breaking down the leaves that create a hostile environment for the Phytophthora pathogen. In fact, in releasing enzymes to break down organic matter, the microorganisms also break down the cell walls of Phytophthora which are made of the same material as leaves. An orchard with no leaves is wide open to root rot infection.
So I propose something modest. On flat ground where trees are more prone to frost damage, and less subject to winds blowing away leaves that the trees are skirted. On slopes, though where winds blow away leaves and the trees are less subject to low lying cold, that the skirts are left. To maintain a more even water distribution, though, windows are cut into the canopy on the side facing the microsprinkler so that the canopy does not interfere with water spray.
Gary Bender has made his manual on avocado production available on his website. And it's free. Take a look at it to see if you might be missing something in your orchard:
- Author: Tracy Kahn
- Author: Georgios Vidalakis
Two programs at the University of California-Riverside are working together to provide information on new and existing varieties of citrus as well as information on other types of citrus diversity.
The University of California-Riverside Citrus Variety Collection (CVC) is one of the most diverse living collections of citrus varieties and related genera in the world. This collection encompasses varieties and species of genera in the Aurantiodeae subfamily of the Rutaceae (citrus). With two field trees of each of the over 1000 different types, this collection preserves valuable citrus types for research and to extend knowledge about citrus and citrus diversity. Although the main focus of the Citrus Variety Collection focus is to conduct research on new citrus varieties and facilitate the use of the collection for research by researchers, we also extend knowledge about citrus diversity by writing articles and providing a website with information about the different types of citrus in our collection: www.citrusvariety.ucr.edu Toni Siebert, David Karp and Tracy Kahn with the Citrus Variety Collection are in the process of “growing” our website to provide information and photographs of all of the different types in the collection. Please visit our website to learn more about other types citrus in the collection. Our website also has links to other citrus articles, references and citrus related information as well as a history of the collection and how you can help support the collection to preserve citrus diversity for the future.
The University of California-Riverside Citrus Clonal Protection Program (CCPP) is a cooperative program with federal, state, and citrus industry agencies and its purpose is to provide a safe mechanism for the introduction into California of citrus varieties from any citrus-growing area of the world. This procedure is highly regulated by federal and state law and only three programs in the whole country have authorization to introduce citrus varieties into the USA. The citrus variety introduction mechanism is complex and highly specialized and includes, disease diagnosis and pathogen elimination followed by maintenance and distribution of true to type, primary citrus propagative material. Currently the CCPP germplasm collection extents in more than 20 acres and contains over 1,200 trees representing over 350 different commercially important scion and rootstock varieties. The trees in the CCPP collection are constantly evaluated for several horticultural characteristics such as vegetative growth, fruit yield, and fruit quality. All the information related to the variety evaluation along with budwood distribution, citrus disease and citrus management issues is available to the public via the web site www.ccpp.ucr.edu.
The University of California-Riverside CVC and CCPP are in close collaboration and citrus variety information is exchanged between their websites. In the near future a dynamic data-based website will be launched enabling the users to query from the web pages of both the CVC and the CCPP at one time in order to answer specific questions and to compare variety data from either or both websites.