- Author: Rock Christiano
Citrus Clonal Protection Program Lindcove Research & Extension Center, University of California
When I started working at the Citrus Clonal Protection Program (CCPP) in 2012 (Fig. 1), citrus budwood was distributed three times per year (i.e. January, June and September) and there was a minimum order limit for 36 buds per order. In July of 2013, CCPP began monthly budwood distribution and essentially removed the budwood order limit offering as little as one budstick (6 buds). This was a game changer. In the following three years of monthly budwood distribution, the amount of requested buds has increased by almost 50% (Fig. 2), and most importantly, the orders placed by citrus hobbyists has increased by almost 80%. Citrus hobbyists are growing citrus for non-commercial purposes. Many of the hobbyists have a small “citrus forest” in their backyards, typically of diverse varieties, and they are very proud of their trees. They are typically not interested in purchasing grafted citrus trees, they want to graft their own citrus trees. It is hard to understand the citrus hobbyists' deep affection for their trees! I grew up in a citrus family farm and for me, citrus was as any other crop, a plant for profit. However, after interacting with the citrus hobbyists over the past several years, I have gained a level of respect and understanding that the passion citrus enthusiast have.
I had always been taught that citrus hobbyists are a threat to the citrus industry and their capacity to propagate citrus should be limited or denied. This line of thinking has resulted in some citrus production areas to restrict citrus budwood access for non-commercial use. Today, I see that ignorance is the true threat to the citrus industry. People usually don't understand or comprehend that smuggling plants or plant parts can disseminate diseases and cause severe economic damage to the farmers. Restricting the desire to propagate a citrus variety may force someone to smuggle it. A sad example is Huanglongbing (HLB) in California. This imminent threat to the California citrus industry was first found in a back yard citrus tree that had 23 grafts of unknown budwood origin.
The California HLB/Asian Citrus Psyllid prevention campaign is doing an excellent job of educating the public. Also the University of California Cooperative Extension is teaching Master Gardeners about the dangers of smuggling plants. On the Internet, there are individuals, such as the pomologist-writer “Fruit Detective” and the citrus hobbyist-blogger “Fruitmentor”, educating people on the correct way to propagate citrus and providing information regarding the threat of importing budwood that that may contain pathogens that could effects citrus production locally. Thanks to this multi educational effort, many citrus hobbyists are now part of the solution and they are actively engaged in the effort to protect the California citrus.
CCPP has over 300 citrus varieties available to anyone interested in propagating citrus trees for commercial or personal use. Orders can be as small as 6 buds (one budstick) per variety at $ 4.50. Therefore, the CCPP offers the incentive to use inexpensive-easily purchased- tested budwood over smuggled or exchanged “dirty” citrus budwood.
Despite all of the above, I still strongly recommend to purchase grafted trees at local stores or online (e.g.
www.fourwindsgrowers.com). Grafting citrus is not as easy as it may look. It requires skill, another citrus tree to be used as a rootstock, and a controlled environment (especially
temperature and water). However, if you are going to do it yourself, make sure the material your using is disease free!
To learn more about the CCPP, go to www.ccpp.ucr.edu and remember: CCPP is the place for starting citrus correctly.
Panoramic view of the Citrus Clonal Protection Program foundation block operations at the Lindcove Research and Extension Center, Exeter, California. Photo: E. Grafton-Cardwell.
Beth Grafton-Cardwell our IPM Specialist who is Lindcove Research and Extension Director and a UC Riverside Entomologist recently gave a talk on the different approaches being taken to confront Asian Citrus Psyllid and Huanglongbing. A summary of a lot of her points is available at:
She brought up some points that I think need to be better known. Some early detection techniques are being developed so that infected trees can be quickly identified and removed so that they do serve as a reservoir of inoculum that can increase the spread of the bacterial disease. These techniques are based on measureable levels of different chemicals.
Sick trees produce different volatiles (Volatile Organic Compounds can be “sniffed” by machines or trained dogs)
Sick trees produce proteins that can be measured
Trees produce small RNAs in their defense response that can be measured
The bacteria produce proteins that can be measured
The micro-organisms associated with sick trees are different than those associated with healthy trees and these can be measured.
All of these techniques are being tested out right now and being refined. It will mean faster diseased tree identification and removal. This will still mean chemical control of the psyllid to control disease spread.
Early detection is just one of the techniques being employed to fight this insect/disease complex. Every conceivable possibility is being explored, including:
Psyllid traps – attract and kill
Psyllid deterrents – chemicals that would drive psyllids away from citrus
Antibiotic treatments to control the tree infection
Heat treating trees to destroy the bacteria in the tree
Resistant rootstocks and scions (traditional breeding and genetic engineering)
Utilize an altered citrus tristeza virus to introduce anti-HLB genes into plants (Genetically Engineered)
Altering the psyllid so it can't vector the disease and releasing the ‘nupsyllid' to replace the wild ones (GE)
Treat trees with chemicals (interference RNAs) that prevent the psyllid from picking up the disease
Wow. Many of these are a long way off, but some might be coming out soon. We still need to deal with the psyllid so that when infected insects become more widespread, the disease will not spread as fast as it has in Florida and other citrus growing areas.
In a recent meeting the topic of where to go for irrigation information came up. Well there's no substitute for attending a class in irrigation, such as offered at Cal Poly SLO (http://www.itrc.org/classes/iseclass.htm ,
but here's some written sources to get you started thinking.
It's possible to grown many other tree crops along the coast other than avocado and citrus. When speaking of other perennial evergreens like avocado, trees that don't lose their leaves but retain a canopy year round, we want trees that can handle the occasional cold periods that happen in winter. In fact, some trees like 'Hass' avocado need something like 50 hours of chilling – hours below 45 degree Fahrenheit. When trying to grow ‘Hass' in the tropics, the tree gradually loses vigor because the cold need to break dormancy of buds isn't there. So they grow tropical varieties in Florida and the Dominican Republic and consumers get used to the less oily flavor of the tropical varieties. Low or no chilling requirement fruit trees like durian, jackfruit and other tropical fruit can't handle the cold we get in coastal California. There are many different unusual fruits that can be grown, like cherimoya, sapote and mango throughout the southland.
However, if we try to grow low chill varieties of deciduous fruit that can handle winter cold, like apple and peach varieties, they often have insipid in flavor. Here's a list of fruit trees that are adapted to coastal southern California. And if you choose any of the deciduous varieties, make sure you plant them bare root next winter. They are cheaper and transplant better with less transplant shock than planting them when they are in leaf.
Get ready for more rotting avocado fruit if you have leaf blight showing up in your tree canopy. The fungal spores (one of the Botryosphaerias we once lumped as Dothiorella) that create the infection spread in an irregular pattern over the leaf and down the stem (then called “stem blight”). This is often confused with salt or tip burn. The two conditions are caused by the same problem, water and or salt stress. However, in the case of leaf blight, this is a pathogen that can pass to neighboring fruit and begin the process of rot. This starts happening when the fruit starts ripening and softening, so it's often not seen in the orchard, but the packhouse or in the market.
Control is basically gaining control over the soil moisture and salinity in the root zone and when the leaf blight starts showing up in the canopy, cutting as much out back to green tissue as is economically possible.
Leaf blights from this group of fungi have also been reported as infecting other fruits, such as citrus, apple, peach and grape among others. The solution is the same - water right and cut the stuff out when and if it shows up.
Rot spreading to flesh