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
This has been a hot time. Look for irrigation problems, but also look for other weather related problems. This is possibly the most shocking.
If you see your citrus tree suddenly collapse. It happens on a weekend or in a few days, and it has suddenly turned hot with a Santa Ana or Sundowner, it is quite likely "Dry Root Rot". This is a situation where the water conducting tissues, the xylem, have been clogged by fungi and when a sudden demand for water comes from the weather, the tree collapses because it can not support the movement of enough water to satisfy that demand. It is more common along the coast than the Valleyk, but it does occur there, as well.
It starts with some type of wound to the crown roots or stem - gophers or field mice nibbling away at the roots or crown. Or damage from weed whipping or some sort of mechanical damage. This allows entry of the Fusarium fungus that causes the xylem clogging. You can usually see the damaged tissue just below the bark, near the base of the tree. The damage has usually occurred several years prior and takes awhile for the fungus to do its clogging. There's no know cure for this situation. Cut the tree out. It's not particularly contagious as far as we know. The fungus is cosmopolitan, meaning it is found in most orchards, and it just waiting for a wound to invade. You can replant into the orchard, but just avoid causing wounding.
Dry root rot affects all citrus species and varieties.
Collapsed canopy, grapefruit
Dead tissue below the bark