Recently, I read an article in "Fresh Plaza" about the arrival of large amounts of 'Kinnow' mandarin fruit from Pakistan. http://www.freshplaza.com/article/117470/Discover-Pakistani-kinnow-mandarins-at-Fruit-Logistica-2014#SlideFrame_1
This is a country that is surrounded by countries with huanglongbing. It's also the country where Mark Hoddle, Biocontrol Specialist from UC Riverside, collected Tamarixia radiata , the tiny wasp that is helping control Asian Citrus Psyllid. Pakistan has invested heavily in juice plants just for this industry which was established with this selection that came out of the Citrus Research Station breeding program in the 1930's. It makes me wonder if there might be a significant tolerance to this citrus disease in this mandarin variety. There has been a lot of work by both USDA and U. of Florida evaluating citrus varieties for tolerance to HLB. A wide range of tolerances have been noted. Fred Gmitter along with others are involved with this work and find that under different climatic conditions, resistance can vary. In the 'Kinnow' variety, it looks like there is hope in finding a variety that can be used to breed tolerance into other varieties. The fruit itself is noted for its juiciness and sweetness. But as you can see from the photo, it's got a lot of seeds.
University of Florida research Jude Grosser has been working with a new breeding technique that creates tetraploid rootstocks that are showing significantly improved resistance to Huanglongbing. This is done with conventional breeding and is not based on genetic engineering. He takes citrus rootstocks that have shown some resistance but because of their genetic makeup, it has not be possible to interbreed them. This new technique permits these crosses that were before not possible. He and his group have created new rootstocks that are now being field trialed.
Also on another front, Richard Lee and Manjunath Keremane at the USDA Citrus Germplasm Repository in Riverside have been working with University of Hawaii and a private company – Diagenetix – to develop a field test for identifying HLB infected psyllids. LAMP (loo-mediated isothermal amplification) is a faster, cheaper method than the traditional PCR (polymerase chain reaction) method. It would allow for rapid identification of infected psyllids and a more rapid identification of a potential quarantine area. Conventional PCR would still need to be performed to legally identify infected insects. The technology has been used on other disease organisms such as powdery mildew in grape and bacterial infections in stone fruit trees. Literally anything that carries DNA can be identified by this new technique.
- Author: Craig Kallsen
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
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