'Pixie' mandarin is a very vigorous, upright tree. Although the fruit is small, hence its name, it can produce fruit on the ends of long branches which deform the canopy structure, making it hard to pick. The sweet, seedless fruit is worth picking, though. The rootstock standards for this small industry are ‘Citrumelo' and ‘C-35' citrange. The industry is looking for alternatives to these choices, especially those that reduce the vigor of the trees.
There is no one ideal rootstock at this point and growers have the option of a wide range of choices. The search includes those that are resistant to Citrus Tristeza Virus (CTV), Phytophthora, calcareous soils and ideally one that is resistant to the bacteria that causes Huanglongbing.
In many California coastal growing areas, land is expensive, water scarce and costly and prone to calcareous soils that are derived from marine sediments which can bring on iron chlorosis. Growers are also looking for smaller trees that will give early economic returns, are easier to prune and pick, and may be more compatible with the economics driven by Huanglongbing.
‘Citrumelo' citrange yields a large tree with good quality and quantity of fruit. It is tolerant of CTV (Citrus tristeza virus) and Phytophthora spp, but is susceptible to iron chlorosis in high pH soils. ‘C-35' citrange is a smaller tree than Citrumelo, also has resistance to Phytophthora spp and CTV, and is more tolerant of high pH soils.
The USDA had a breeding program in California which was taken over by the University of California. Out of this breeding project, the university selected three rootstocks for release in 2009 because of their horticultural characteristics, such as dwarfing, although not as much as ‘Flying Dragon' trifoliate, resistance to CTV and tolerance of calcareous soils. These three rootstocks also show good tolerance to Phytophthora parasitica and nematodes.
Pixie growers have been looking for a more compact tree, easier to handle and not need so much pruning. They funded a long-term project to see how these newer selections of rootstock performed in their area which has a hot summer/cool winter. A 2014 planting of ‘Pixie' has been evaluating the size reducing effects of the relatively new rootstocks ‘Bitters' citrange, ‘Carpenter' citrange and ‘Furr' citrange. After two years, ‘Pixie' on ‘Citrumelo' is the largest tree. Of the new rootstocks, ‘Furr' is the largest and ‘Bitters' the smallest. The trial was replicated at two sites with two different pH soils. At one site with the highest soil pH, ‘Bitters' showed iron chlorosis.
Photo: long whip growth on 'Pixie'
The rootstocks ‘Bitters', ‘Carpenter' and ‘Furr citrandarins were developed at the USDA Date and Citrus Station in Indio, California. Having mandarin genetics with different horticultural properties and being more tolerant of calcareous soils than some other commonly used rootstocks, their effect on ‘Pixie' mandarin is being evaluated. These three are being compared to the mildly dwarfing ‘C-35' rootstock and to the standard sized ‘Citrumelo' to see how their growth might be used to control tree size, also to see how well they do in an alkaline soil. In 2014, five of each of the rootstock/'Pixie' combinations were planted in randomized blocks at two different sites on mildly alkaline soils (pH 7.3 -7.8) in the Ojai, CA area. Trees were monitored for growth on a yearly basis. At both sites ‘Citrumelo' is the largest in height with the greatest shoot length. All three of citrandarins are smaller than ‘C-35” at both sites. Shoot length is the shortest for ‘Bitters', ‘Carpenter' and ‘C-35' at both sites. At the site with the highest soil pH (7.8), two of the five ‘Bitters' show iron and zinc chlorosis. The only trees to do so. This trial will be monitored for another five years to evaluate their performance. Growth characteristics on other varieties of citrus, such as orange and lemon will probably be the same.
Photo: A young Pixie on Bitters.
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.
I got a call yesterday about yellowing avocados. They are on Dusa rootstock which is very tough when it comes to root rot, but cannot handle wet feet. It turns out that this area is one where the grower cannot order water on demand, but must take it when the water company will deliver. So the poor trees get drowned and then go through drought. This sensitive rootstock is not the right one for this area. Many of the root rot resistant rootstocks have some sensitivity to salt, wet, crown rot, whatever, which brings up the question, if we have phosphites, what about the old rootstocks? Barr-Duke, Duke 7 and Borchard have some very attractive qualities: they are tough and can handle things like salinity and drought and are good producers in the absence of root rot. With phosphites now, they can muscle through root rot. Should we reconsider them? Of course, if you are an organic grower, you need to stick with the most root rot resistant selections. And it is also important to remember that the best line of defense against root rot is proper irrigation, which is the leading problem for root rot.
- Author: Craig E. Kallsen
In 1985, H. Schneider and J. Pehrson published an article documenting a decline of Frost Nucellar navel oranges on a number of trifoliate rootstocks (H. Schneider and J.E. Pehrson, Jr. 1985. Decline of navel orange trees with trifoliate rootstocks. California Agriculture. Sept. – Oct. 1985 p. 13-16)
In this decline, which began in the 1970s, trees began demonstrating symptoms when they were 15 to 20 years-old. Affected trees showed leaf discoloration, some defoliation, twig dieback and subnormal growth. They describe how some declining orchards were removed and, in others, individual trees were removed and replanted. Schneider and Pehrson concluded the following: “disorganized phloem and cambial tissues at the budunion proliferate into a tongue like wedge that protrudes from the inner side of the bark. Affected tissue acts as a girdle and is presumed to be responsible for the decline of the trees.” In this article, Schneider and Pehrson provide excellent micrographs illustrating what was occurring at the budunion. However, the actual cause of this aberrant growth pattern was not described.
If we fast-forward to 2012, citrus growers in Kern and Tulare County, and presumably in other counties of the San Joaquin Valley, are experiencing similar tree symptoms to those described and pictured by Schneider and Pearson. The problem has been observed with blood oranges; navel oranges, including Fukumoto, Earli-Beck, Newhall, Atwood, and Powell; and on Satsuma and Page mandarins; on trifoliate and citrange rootstocks such as C-35 and Carrizo. This decline has not been reported, to our knowledge, in California outside of the Central Valley. There are similar reports of bud union disorders in Florida. We are observing symptoms much earlier in orchards than did Schneider and Pehrson. Decline is present in one two-year old blood orange orchard and in several navel orange orchards that are 7 years-old or less. This decline is not common, but can be devastating in a particular orchard, with most trees within an affected orchard showing decline or evidence of the disorder of the graft union. In some orchards only a few trees may initially demonstrate symptoms.
Normally, the scion of a navel orange tree grafted onto trifoliate or citrange rootstock will grow more slowly than the rootstock and a ‘bench’ will form at the graft union. This bench begins to form when a tree is six or seven years old. Conversely, the growth of the scion and rootstock are more similar in affected trees when young and the scion will usually show a slight overgrowth of the rootstock. In Fukumoto, the graft union is an area of intense suckering, and the graft union can become much distorted. As described by Schneider and Pehrson, a groove containing a light brown gum is apparent at the graft union of affected trees. In young trees only staining may be present at the union. The groove does not always traverse the entire circumference, especially in the early stages. This groove is associated with the decline and death of trees.
The cause of the decline is not known. No pathogen has been identified, consistently, in affected trees. If the decline is a result of incompatibility between the scion and rootstock, there must be an additional stimulus, as the decline is not common and trifoliate and citrange rootstocks are the preferred rootstocks in this citrus growing area. In some young affected orchards, most trees show the groove at the graft union, and it seems unlikely that this uniformity was the result of tree-to-tree transmission of a pathogen. Currently, we have no suggestions on how to prevent this problem or alleviate the symptoms once found. Causes of the problem are being investigated.