- Author: Ben Faber
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.
- Author: Ben Faber
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:
http://ucanr.edu/sites/alternativefruits/?story=237&showall=yes
- 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.
Valentine pummelo
- Author: Ben Faber
In many ways our pest and disease management of fruit tree crops are exacerbated by our cultural practices. Avocado and citrus offer some very clear demonstrations of how we manage our trees can lead to reduced pesticide use. From the beginning, our selection of rootstock and scion can help lessen pest and disease problems. In both avocado and citrus we have good rootstocks which can handle problems, such as root rot more effectively than seedling rootstocks. So it is imperative that if you know that drainage will be a problem, starting off with the right, healthy rootstock helps. Also scion selection can have a major impact, as well. For example, ‘Lamb’ avocado is much less prone to persea mite than is ‘Hass’. This pest can significantly impact a spray program and planting ‘Lamb’ could mean virtually no sprays for this pest. There are similar examples in citrus where one variety is more prone to a pest or disease than another.
Irrigation is probably the most important cultural factor in managing tree disease. Over, under and improperly timed irrigations are the conditions necessary for many root diseases. The Phytophthora spp. fungi are looking for distressed root systems brought on by waterlogging and other stressful situations. Other conditions, such as wetted trunks can also bring on some trunk diseases, like gummosis in citrus and crown rot in avocado. Simply preventing irrigation water on the trunks can limit these diseases. Other diseases, such as black streak, stem blight and bacterial canker in avocado are bought on by soil moisture stress.
Nutrients, especially nitrogen management, has been long known to affect levels of insects, such as scale, mealy bug and aphid. Encouraging lush growth helps sustain these insects, so reducing this growth tends to lower their numbers. Managing when canopy growth occurs can affect pest severity. Avocado thrips build their populations in the spring and moves easily from leaf to fruit causing significant scarring. By promoting leaf growth at flowering time with a nitrogen application, keeps the insect on the leaves and reduces fruit scarring. This also promotes growth that replaces leaves that have been damaged by persea mite. Likewise the incidence of citrus leaf miner damage can be reduced if spring pruning is avoided so that a flush of growth does not occur at the same time as the population is building. Timing of pruning is important in lemons to avoid wet periods of rain and fog to reduce the spread of hyphoderma wood rot fungus when its fruiting bodies are active.
Pruning can change pest pressure by changing the humidity in the canopy, introducing light and changing the climate supporting disease and pests. By making spray coverage more thorough, it also makes for a more effective application. Modified skirt pruning can have significant effects on mealy bug and scale control, fuller rose weevil incidence, ant colonization and snail damage. It’s important that the trunk be protected as an avenue of movement for snail and ant control to get the best effects of this pruning. Skirt pruning also reduces problems with such weeds as bladder pod and the ladder effect of brown rot in citrus – fungal propagules splashed from the ground onto low-hanging fruit, which in turn is splashed to higher fruit.
Keeping a canopy clean of dust and fire ash also makes for more efficient biological control. Because predators are slowed in their search, they are less efficient. They also spend more time grooming their sensory organs, and this also slows them down. Parasites such as wasps are actually slowed by the physical abrasion to their tarsi. Dust also creates a drier environment, which is more hospitable to our pest mites. Watering picking rows, roads and even the trees themselves can lessen mite populations. Use of cover crops can also reduce dust and potentially provide pollen and nectar for predators and parasites. Of course cover crops create a whole new set of management issues, such as colder winter orchards and snails
Finally harvest timing to avoid pest and disease is often overlooked. In avocado, fruit is often set in clusters. Greenhouse thrips love the microclimate created, and if in a size-pick the cluster is reduced, greenhouse thrips will often not be a problem. Harvest timing is also important in citrus. Fruit left too long on the tree can often develop septoria fungal spot. Picking in a timely manner reduces the incidence of this disease.
These are just a few examples of how cultural practice at the right time can reduce pest and disease problems.
- Author: Ben Faber
Stem and leaf blights are symptoms that appear for various reasons – high rainfall or humidity, spray burn, chewing insect infestation. Here in California we can add other causes, such as drought and salinity burn. These conditions can cause wounding of leaf and stems allowing entry of fungal spores that can cause leaf and stem dieback. This condition is most common near the coast where weather conditions can change from mild and low temperatures to extremely high temperature with winds, such as the Santa Anas or the Sundowners in Santa Barbara. Leaves suddenly dry out, causing cracking either at that time or when they are rehydrated with irrigation. This allows spore entry into the wounds and permits the pathogen to grow in the dead tissue. Symptoms appear 7 – 10 days after the stress. These are decay fungi that create these spores and they are the ones that cause decay of dead tissue on the ground. So their spores are everywhere.
The greater part of a tree is dead – the woody part of the branches and trunk. And it is dead tissue that these fungi are feeding on. Most trees will limit the growth of the fungus by sealing off the infection with gums of various sorts. In that case, the disease is limited and you may only see a leaf or small branch dying back. In mature trees it is possible to see a small branch here and there that has died back, but the bulk of the canopy is still green. It has been called “salt and pepper syndrome”, because of that speckled appearance. In the case of young trees with their smaller root systems and a lesser ability to seal of the disease process, a whole tree can die.
Since this is a severe water stress or salt stress induced problem, the most important management issue is to watch the weather forecasts predicting unusual hot, dry weather and make sure the trees are adequately irrigated going into the stressful period. Shallow rooted trees like avocados are more prone to dry out rapidly in these high water demand situations, but it can be occur in other trees (citrus, apple, peach) and shrubs if the weather conditions are severe enough. With poor leaching due to low rainfall, this can be more of a problem
The only solution to the symptoms is to cut out the diseased parts to prevent its further spread. Once the disease starts spreading, the fungus can produce copious amounts of spores, which in the case of avocado can cause cankers and rots on the fruit.
Figure. In the case of young trees, the whole tree may die from blight.