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.

http://ashs.org/abstracts/2013/abstracts13/abstract_id_14201.html

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.

http://www.agnet.org/htmlarea_file/library/20110706164834/ac2007k.pdf

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).

It shouldn’t be news to local citrus growers and industry people that the Asian citrus psyllid (ACP) is being found with increasing frequency in the southern San Joaquin Valley. If this is news to you please follow the website at http://www.cdfa.ca.gov/plant/acp/ and/or sign up for U.C. entomologist Dr. Beth Grafton-Cardwell’s blog at;

http://ucanr.edu/blogs/ucanrorgblogscitruspest/.

Go to Beth’s website and in the upper right corner you can subscribe and receive the blog each time she sends one out. She also has a twitter account ‘ucanrbethgc’ that you can follow. This blog covers more than just ACP and is a great source of information on citrus IPM and citrus entomology. At the CDFA website, or through links to the site at Dr. Grafton-Cardwell’s blog, you can find maps delineating quarantine areas around new ACP finds, such as the one surrounding the recent find in the Wasco area (see attached map as an example). Regulations related to what needs to be done related to harvesting fruit and selling nursery trees growing within the quarantine zone can be found at:

http://www.cdfa.ca.gov/plant/pe/interiorexclusion/acp_quarantine_sjv.html

So far, there is no sign of HLB disease in the San Joaquin Valley, but that can change on short notice. Where ACP shows up, HLB disease (spread by a bacterium) is usually only a few years behind.