'Bitters', 'Carpenter' and 'Furr' trifoliate hybrids are three new citrus rootstocks released in August 2009 by the University of California Riverside. These three rootstocks are hybrids of Sunki mandarin x Swingle trifoliate orange. As rootstocks for citrus, they produce good quality fruit on small ('Bitters') or medium-large ('Carpenter' and 'Furr') trees. They all show good tolerance to Citrus tristeza virus (CTV), but vary in their tolerance of Phytophthora and nematodes. They all produce uniform seedlings due to high levels of nucellar embryony, and are graft compatible with sweet orange, Lisbon lemon and grapefruit. They also appear compatible with mandarins based on observation of 11 year-old satsuma and ‘Pixie’ trees. We are comparing their performance compared to ‘C-35’ as a standard and with ‘sour orange’.  These are being grown with ‘Pixie’ and ‘Lisbon’ lemon scions. Sour orange grows well on calcareous, heavy soils and are less prone to nematodes.  It turns out that mandarin and lemon are not prone to tristeza virus when grown on sour orange rootstock.  Trees will be planted this coming spring.  Stay tuned.

Alternate bearing is typically initiated by adverse climate. Once initiated, in the absence of additional environmental constraints affecting crop load, the bearing status of an orchard alternates between ON and OFF years, with ON years exhibiting less vegetative growth than OFF years.  This biennial cycle, however, can be reset by adverse environmental conditions affecting bloom and fruit set.  Adverse conditions 8-10 weeks prior to bloom may cause abortion of female flower parts, resulting in a high proportion of staminate (male) flowers that do not give rise to fruit.  Additionally, adverse weather conditions at bloom may impact pollination and subsequent fruit set.  Any conditions resulting in loss of crop during an anticipated ON year may render the season an OFF year. 

For a fuller discussion of this topic, see the full article by Carol Lovatt at:

http://ceventura.ucanr.edu/newsletters/Topics_in_Subtropics45348.pdf

The Topics in Subtropics Newsletter

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)

http://ucce.ucdavis.edu/files/repositoryfiles/ca3909p13-62889.pdf

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.

Lime-induced Iron Chlorosis:  a nutritional challenge in the culture of several subtropical perennial crops in California

Elizabeth Fichtner, UCCE Tulare County and Rachel Elkins, UCCE Lake and Mendocino Counties 

Spring, and new leaves are coming out, but this could, but yellow could be a sign of iron chlorosis, as well.

Although iron (Fe) is the 4^th most abundant element in the lithosphere, Fe deficiency is among the most common plant micronutrient deficiencies. Fe deficiency in plants is common in calcareous soils, waterlogged soils, sandy soils low in total Fe, and in peat and muck soils where organic matter chelates Fe, rendering the element unavailable for plant uptake. In California, lime-induced Fe deficiency is often observed in soils and irrigation water containing free lime, and is exacerbated by conditions that impede soil drainage (ie. compaction, high clay content), resulting in reductive conditions. Given that over 30% of the world's soils are calcareous, lime-induced Fe deficiency is a challenge in numerous perennial cropping systems including:  grapes, pears, apple, citrus, avocado, pecans, and stone fruit (prune, almond, apricot, peach, nectarine, cherry).

In most soils, Fe oxides are the common source of Fe for plant nutrition. Solubility of Fe oxides is pH dependant; as pH increases, the free ionic forms of the micronutrient are changed to the hydroxy ions, and finally to the insoluble hydroxides or oxides.  In calcareous soils, the bicarbonate ion inhibits mobilization of accumulated Fe from roots to foliage and directly affects availability of Fe in soil by buffering soil pH. When irrigation water is also high in bicarbonate, probability of Fe deficiency is enhanced because bicarbonate is continuously supplied to the soil, and more importantly, the roots may become crusted with lime as water evaporates, thus inhibiting root growth and function. Inside the plant, bicarbonate inhibits nutrient translocation from roots to aboveground plant parts. The adverse effects of high bicarbonate levels are exacerbated in very saturated, very dry, or compact soils, where bicarbonate levels increase concurrent with diminished root growth and nutrient uptake.

Symptoms of Fe deficiency in plants

Fe is immobile in plants; therefore, symptoms appear in young leaves. Interveinal chlorosis (Figure 1) is the main symptom associated with Fe deficiency, followed by reduced shoot and root growth, complete foliar chlorosis, defoliation, shoot dieback, and under severe conditions may result in tree mortality. Overall productivity (yield) is reduced, mainly from a reduced number of fruiting points.

Plant Adaptation

Plant species and cultivars vary in their sensitivity to Fe deficiency, and are categorized as either "Fe-efficient" or "Fe-inefficient".  Fe-efficient plants have Fe uptake systems that are switched on under conditions of Fe deficiency. Fe-inefficient plants are unable to respond to Fe deficient conditions.  All Fe-efficient plants, except grasses, utilize a Fe-uptake mechanism known as Strategy 1.  Strategy 1 plants decrease rhizosphere pH by release of protons, thus increasing Fe solubility.  Some plants may excrete organic compounds in the rhizosphere that reduce ferric iron (Fe³⁺) to the more soluble ferrous (Fe²⁺) forms or form soluble complexes that maintain Fe in solution.  Additionally, roots of Strategy 1 plants have specialized mechanisms for reduction, uptake, and transfer of Fe within the plant.  Strategy 2 plants (grasses) produce low molecular weight compounds called phytosiderophores which chelate Fe and take up the chelated Fe with a specific transport system. 

Amelioration of Fe chlorosis

Planting sites in calcareous soils should be well drained to provide optimal conditions for root growth and nutrient uptake. Waterlogged and compact soils contain

more carbon dioxide, which reacts with lime to form even more bicarbonate. These conditions, as well as very dry soils, also inhibit microbial activity which aids in

solubilization and chelation of Fe. Prior to planting, soils and water should be tested to determine the pH, lime equivalent, and bicarbonate concentration.  Bicarbonate concentrations greater than 3 meq/L in irrigation water increase the hazard of lime accumulation on and around roots.  If high bicarbonate water must be used, the pH must be adjusted to 6.0-6.5 to dissolve the bicarbonate and prevent it from negating the effects of soil-based treatments. In microsprinker and drip systems, acidification of irrigation water will also reduce the risk of emitter clogging, a common problem at bicarbonate levels over 2 meq/L. The cost of reducing the pH of irrigation water will more than compensate for the savings incurred from avoiding wasted investment in failed soil- and plant-based remedies.  Systems can be set up to continuously and safely inject water with acids such as sulfuric, urea-sulfuric, or phosphoric during irrigations. Specific choice and rate will depend on crop, soil type, other nutrient needs, availability, and cost.  Downstream pH meters are available to continuously adjust rate of acid use.  Acetic and citric acid can be utilized by organic growers.

Soil based pre-plant treatments to reduce pH include elemental sulfur (S) and acids as mentioned above. It is only necessary to treat a limited area near the root zone to ameliorate symptoms because the tree only needs to take up a small amount of Fe. Material can be shanked in or banded and incorporated in the prospective tree row. One ton of elemental sulfur per treated acre is needed to mitigate three tons of lime, and may need to be re-applied every 3 to 5 years after planting. The addition of organic matter such as well-composted manures will benefit poorly drained or compact soils by increasing aeration for better root growth, fostering chelation of nutrient cations, and reducing pH (depending on source material).

If possible, choose a Fe efficient species or cultivar. In perennial systems, lime-tolerant rootstocks may be the first line of defense in combating Fe deficiency. Some rootstocksmentioned are peach-almond and Krymsk-86 for stone fruit, Gisela 5 for cherry, and Pyrus communis for pear. Ongoing research studies in Europe focus on screening rootstocks of grape and olive for lime tolerance. 

Once soil and water quality improvements are made, post-plant management strategies may also be implemented to ameliorate lime-induced Fe chlorosis in the short term. Soil can be acidified as described above. Individual trees can be treated by digging four to six 12-24 inch

holes around the drip line and burying a mixture of sulfur and Fe fertilizer. Historically, two principal methods have been utilized:  1) foliar application of inorganic Fe salts (ie. ferrous sulfate), and 2) soil or foliar application of synthetic chelates.  Application of Fe salts to foliage may have mixed results due to limited penetration of Fe into leaves and inadequate mobilization within the plant.  Use of Fe chelates may be of benefit; however, they are expensive and pose an environmental concern due to their mobility within the soil profile.  Because soil lime interferes with Fe mobility with the plant, repeat application of inorganic Fe salts or Fe chelates may be necessary throughout the growing season.

Choice of nitrogen (N) fertilizer may also influence solubility of rhizosphere Fe.  When N is applied in the ammonium form (NH₄⁺), the root releases a proton (H⁺) to maintain a charge balance, thus reducing rhizosphere pH.  Alternately, fertilization with nitrate (NO₃^-) results in root release of hydroxyl ions (OH^-), resulting in an increase in rhizosphere pH. Solubility of Fe³⁺ increases 1000 fold with each one unit decrease in pH; therefore, fertility-induced rhizosphere pH changes may significantly influence Fe availability. 

New methods for amelioration of Fe chlorosis are under investigation.  For example, container studies have demonstrated that inter-planting sheep's fescue, a Strategy 2 plant, with a Fe-inefficient grape rootstock may ameliorate Fe chlorosis in grape.  In this system, the grass produces a phytosiderophore that enhances Fe availability to the grape. Additionally, soil amendment with Fe₃(PO₄)₂• 8H₂O), a synthetic iron(II)-phosphate analogous to the mineral vivianite, has been effective at preventing Fe chlorosis in lemon, pear, olive, kiwi, and peach.  Vivianite has a high Fe content (~30%) and serves as a slow release source of Fe in calcareous soils. 

Figures below: 1) Shoot dieback in citrus, 2) Interveinal chlorosis in citrus and 3) Various stages of iron chlorosis in avocado.