I had a chance to talk to PCA Dennis Seaton a few weeks back. During the farm call, he mentioned that he was making a survey of leaf ‘yellowing’ in his clients’ citrus orchards. He explained that he thought this would help him in the future, should HLB disease show up, in distinguishing the yellowing associated with HLB, from the other things, both biological and abiotic, that currently cause leaves to yellow in Kern County. This project sounded like a good idea to me. Currently, every time a new article appears in our local Bakersfield paper related to ACP and HLB, my phone is busy with homeowners concerned with yellow leaves. Unfortunately, we have always had a lot of ‘yellow’ leaves on Kern County citrus trees. A list of a few things (not complete) that can cause leaf yellowing is as follows:

Citrus stubborn disease - caused by a mycoplasm – also produces small, hard, green, lopsided fruit

Root rots (e.g.Phytophthora species, Fusarium species)

Alkaline soil - iron deficiency

Excessive irrigation

Gas leaks, oil-field waste oil in soil

Sunburn, heat stress (tends to be on older leaves)

Nutrient deficiency – iron, zinc, manganese,

Nutrient toxicity – boron, arsenic, sodium, chloride

Trifoliate and citrange tree decline

Pre-emergent herbicide uptake

Mite feeding, citrus leaf miner feeding, citrus thrips feeding

Chemical toxicity from foliar sprays (e.g. Biuret toxicity)

Tristeza disease – caused by a virus

For those who really want to try to diagnose HLB from yellow leaves, pictures of leaves with HLB can be found at the following webs address; http://www.californiacitrusthreat.org/huanglongbing-citrus-greening.php .

If we mentally divide a leaf with HLB symptoms in half, longwise, along the midrib, the yellowing is more asymmetric than is the case with other factors that can yellow leaves. That is, one half of the leaf will have yellow areas in different locations than the other half. Yellowing from zinc deficiency, on the other hand, looks pretty similar between halves. HLB leaf yellowing often affects individual branches or shoots more than neighboring branches or shoots. If you see leaf yellowing, always looks for the presence of ACP nymphs on new, young leaves. The nymphs, about the size of aphids, produce distinctive waxy tubules. See the website at www.californiacitrusthreat.org for pictures of the nymphs and adults of ACP. The insects themselves are much better indicators of possible HLB infection than leaf yellowing. The production of small, hard, greenish-yellow, sour fruit is another indication of HLB infection (but not definitive).

The weather is perfect for looking for Asian Citrus Psyllid.  There is new flush and that is where the adults go to lay their eggs and feed.  This is also the tissue where aphids and scale crawlers will be found, as well.

ACP has been active all this winter, but now is when they are going to be the most noticeable.  Get out there and look and alert the CDFA Exotic Pest Hotline to confirm a find, 1-800-491-1899.

ACP adult and nymphs with waxy exudate from nymphs.

At a recent conference on Postharvest Technology Advances, Cristina Davis from the UC Davis Department of Mechanical and Aerospace Engineering presented information on the development of a device that can smell out trees infected with Huanglongbing (HLB).

Scientists at UC Davis are refining a mobile chemical sensor that can detect diseased citrus trees by sniffing their volatile organic compounds (VOCs). VOCs are emitted by all types of plants and contribute to their distinctive odors—such as the perfume of orange blossoms and pungent scent of garlic in the air. VOCs must exist at very high levels for humans to smell them, and there are some VOCs people cannot smell at all.  The machine is able to figure out the signature smells of HLB infected trees, sort of the way people evaluate wines with terms like “grassy”, “plum”, or “austere”, and distinguish them from  healthy or trees infected by other diseases.

Finding HLB-infected trees and eliminating them before Asian Citrus Psyllid (ACP) picks up the disease and spreads it to neighboring trees is a major challenge. The pathogen in the tree cannot be detected by leaf testing for three to nine months after infection, and the symptoms don’t show up in the tree for a year or more after infection. Meanwhile, the disease can be spread by ACP. Research is under way to develop early HLB detection so that infected trees can be rapidly removed.  Early detection will also allow researchers to more rapidly assess treatment programs for controlling not only the spread of the disease, but also possible cures or rootstocks or scion varieties that might have some resistance to the disease.

HLB infected tree showing mottling in one part of the canopy

This February there was a four day international conference in Orlando, FL that attracted 467 people from 21 countries, including about 20 from California.  There were 87 oral presentations and over 80 posters that covered all aspects of Huanglongbing, the insect vector (Asian Citrus Psyllid or ACP), disease detection, insect control and monitoring and a whole lot of information on the genome of the bacteria, how it compares to other infectious bacteria and what can be done to exploit its genetics to control the disease.  To learn more, the proceedings and agenda can be found at: http://irchlb.org/hlb/schedule.aspx.

To start off, this is an amazing example of coevolution among a plant, a bacteria and an insect.  It appears that the citrus tree may give off an odor which at low concentrations acts as an attractant, but at higher concentrations is a repellant.  The infected tree also gives off volatile organic compounds (VOCs) that can be used to identify affected trees.  These chemicals are not pheromones which are social odors emitted by some insects which act to affect insect behavior, such as mating or causing aggregation. Being able to use these new odors will allow for better methods of monitoring the insect with lures.  We currently use blunder traps which are not a very good indication of whether there are ACP present.

Currently the most commonly used technique for identifying infected trees is the use of the polymerase chain reaction (PCR) method which is a biochemical technology that identifies the presents of the bacterial DNA.  This method was used on the leaves of trees since that is what the psyllid feeds on and where the bacterial infection starts.  Results have been erratic and inconsistent and often would not give results until many months after the infection started. 

The bacteria clog the phloem tissue which carries sugar to the roots and on which the psyllid feeds.  The higher concentration of sugar accelerates the development of the insect and it can lay more eggs.  The bacteria travel down the stem at measured rates of one centimeter per day and accumulate in the root system.  The root tips are the growing points and where the plant directs sugar to feed the new root growth.  It is from the roots that the bacteria go out to all points of the canopy where it can then be picked up by uninfected psyllids which can then go on to find a new tree to feed on.  By measuring root tips, rather than leaves, the detection is more rapid and much more consistent, since the bacteria levels are higher in the tips than the point of infection. Disease detection is now being improved by better understanding of the biology of the infection process and thereby allowing faster determination of whether a treatment works.

The ability to evaluate what treatments work has allowed researchers to determine such things as what rootstocks and scions might be most vulnerable, what spray programs are the most successful and how better to lay out trials.  It turns out that there is an edge effect during a psyllid invasion and that the first affected trees are right on the perimeter.  By focusing spray programs in this area can slow the movement of the insect to the middle of the orchard.

Actual economic control of this pest and disease are still some time away, but with this new understanding of the disease process we will be closer to finding a solution.

Fruit drop and defoliation associated with HLB