To maximize profits in the early navel orange market, growers need to have large fruit size and sufficient yellow-orange color and a high enough sugar-acid ratio to meet or exceed the legal minimum harvesting standards. Growers of early-maturing navel oranges in Kern County use different strategies to produce these oranges.  Some growers irrigate at full evapotranspiration rates nearly up to harvest with the belief this will maximize fruit size, while others begin deficit irrigating a month or two prior to harvest to maximize development of sugar and color to promote earlier maturity. Little information exists in the literature to assist growers in making decisions related to producing early maturing navels such as Beck,Fukumoto and Thompson Improved.  After three years of research, we have elucidated some of the trade offs that relate to irrigation strategies and early navel fruit production. 

Three different irrigation treatments, defined as low, mid and high, were developed based on the relative amounts of irrigation water applied to the test plots.  Each plot consisted of 10 trees in a central row, bordered by 10 similarly irrigated trees in the two adjacent rows. Each treatment was replicated 5 times. The same irrigation treatment was applied to the same plots for the first two years, while in the third year the low treatment was changed to the high treatment to provide information on how rapidly the trees would recover from stress.  The different irrigation treatments were administered by using irrigation emitters with different flow rates and by differentially shutting off water to some treatments as needed to achieve desired stress levels.  Between growing seasons, the top three feet of soil profile was refilled with water during the winter and differential irrigation began in early August.  Measurable differences in tree shaded stem water potential among treatment usually were noted by early September. In the second year of the experiment (2007), the low and mid-

irrigation treatments applied approximately 38 and 71 percent, respectively on average, of the water of the high treatment. Water potential measurements made mid-day on shaded, interior leaves demonstrated that good separation was achieved among the three  treatments.  In 2007,  for example, shaded stem water potential measurement in early September were about -9, -12, and -18 bars for the high, mid and low irrigation treatments, respectively and at harvest in mid October were -12, 18, -24, respectively.  Neutron probe measurements also demonstrated that trees differentially depleted available water stored in the soil as the season progressed (data not shown).  In 2007, differences in applied water among the treatments were large. Including the increased quantity of water applied to refill the soil profile in the winter, 3.55, 2.58 and 2.11 acre feet of water on a per acre basis, were applied to the high, mid and low irrigation treatments respectively, from October 30 2006 to harvest, October 15 2007.  Rainfall was minimal.                                                                                 

Again, using 2007 as an example, as the level of applied water decreased, soluble solids (i.e. sugars) and titratable acid, were greater at harvest, although the sugar acid ratio was not different (see Table 1).  Rows in the experimental orchard were oriented east and west.  Fruit on the south side of the tree had higher soluble solids concentration and sugar/acid ratio than fruit on the north side of the tree, regardless of irrigation treatment.  Fruit juiciness, either measured as weight of juice to weight of fruit (see Table 1) or volume of juice per weight of fruit (results not shown) were not different among irrigation treatments, suggesting the increase in sugars and acid was the result of osmotic adjustment and not fruit dehydration.  We were also interested in seeing if the differential irrigation treatments influenced eating quality of the fruit.  To test this idea, we provided fruit from the highest and lowest irrigation treatments of 2007 and 2008 to volunteer panelists at the UC Kearney Ag Center and asked if they could detect any differences between the fruit.  In both years the panelists could not detect differences between fruit from the two irrigation treatments, suggesting that the increase in soluble solids in the low irrigation treatment was not sufficient to influence eating quality. 

 In 2007, yield and grade decreased as the amount of applied water decreased (see Table 2).  Fruit in the high and mid irrigation treatments peaked on size 56 per carton and on size 72 per carton in low treatment (data not shown). The decrease in fruit grade at pack-out appeared to be largely due to a more oblong shape.  The negative yield, fruit size and grade effects measured in the low and mid treatments in 2007 were probably the cumulative result of deficit irrigation in Years 1 and 2 and not just Year 2 alone. Reduced rates of irrigation did increase the color in the fruit compared to the high irrigation treatment (see Table 3) and this occurred every year.

The deleterious effects on yield, and grade on the trees in the low-irrigation treatments suggested  that not much would be gained by continuing this level of stress for a third season in the same plots.  In 2008, the low irrigation treatment was replaced by a high irrigation treatment and, at harvest, yield by weight and fruit numbers were not different from the control high-irrigation treatment.  This observation demonstrated that the Beck navels rebounded quickly from the low irrigation stress of 2006 and 2007.  The mid level irrigation stress of 2006 and 2008 was less severe than that of 2007, and yield and fruit quality was not as adversely affected as in 2007.

This study provides information on some of the trade offs that might be expected among fruit yield, size, grade, sugar and color in relation to reduced irrigation as harvest approaches. Information from this study will be available in greater detail in the near future.  How growers respond to this information will depend on their approach to profiting in the early navel market and how much water will be available for irrigation.  If reducing water use, while minimizing effects on yield and fruit quality compared to fully irrigated orchards, is the primary goal of the grower, work by Dr. Goldhamer, UC irrigation specialist, demonstrated that regulated deficit irrigation in the mid-May through mid-July time period would be the best strategy.

USDA-ARS National Clonal Germplasm Repository for Citrus & Dates

‘Stubborn disease of citrus’ was first observed about 1915 in ‘Washington’ navel trees nearRedlands. The first report of stubborn from outside ofCaliforniawas fromPalestinein 1928. Stubborn is now known to be established in most warm, dry inland producing areas inCaliforniaandArizona, and is also a serious disease in most citrus-producing countries with suitable climates. These include countries with arid or semi-arid subtropical climates, but stubborn disease has not been reported from farther east thanIran. In addition, it has not been reported from countries or states with semi-tropical or tropical climates.

The classical concept of citrus stubborn disease involves symptoms expressed in the fruit and in the vegetative growth of citrus trees. Both vegetative and fruit symptoms are often variable and irregularly distributed in the tree.

Fruit symptoms are the most characteristic and the most useful for visual diagnosis in the field. Stubborn-affected trees often flower irregularly (usually in December) and so will have fruits of varying maturity and size present on the tree, although many of the fruit produced on stubborn-affected trees drop while very small. Fruit on stubborn-affected trees produce small fruit that are lop-sided or “acorn-shaped”. The skin of the stylar end of the fruit is thin and subject to early breakdown and split, or to stylar-end greening. Color development is often irregular, and frequently stubborn-affected fruit remain green or yellowish-green. Internally, the flesh is dry and the flavor bitter. In normally seedy varieties, there is extensive seed abortion and/or small, under-developed seeds. Another symptom seen less regularly, and generally only when cutting into the fruit, is a blue albedo.

Trees affected by stubborn have ”bunchy” growth, with shortened internodes and usually upright leaves. Leaves are often smaller and pointier than normal citrus leaves, and sometimes show a mottle resembling zinc deficiency. Stubborn-affected trees lack vigor and do not flush normally. When flushing does occur, it is often greater in the fall than in the spring. These patterns of vegetative growth result in the characteristic flattened top associated with stubborn-affected trees. In addition, there is often leaf drop and sometimes die-back associated with stubborn.

Trees infected with stubborn early in their life, particularly during the nursery phase, are often extremely stunted and may never attain more than 6 feet in height. This is the classical picture of stubborn disease, and veteran researchers have told us that they did not worry much about stubborn since they selected budwood from large, asymptomatic trees and once the trees were established, an infection with the stubborn pathogen did not have much effect. However, during our investigations the last several years, we have observed large, mature trees with sectors that show stubborn symptoms. These sectoral infections result in a decrease in the amount of salable fruit and thus of economic return. We have also observed trees that are smaller than normal having lower than normal production, but not showing the extreme stunting and negligible yield classically associated with stubborn.

Navel oranges and grapefruits are often severely affected.Valenciasgenerally (but not always) show less symptom development than navels but often drop fruit excessively when ripe. Mandarins seem to show vegetative symptoms more readily than they do fruit symptoms, possibly due to the more variable fruit in mandarins in general. Symptoms are harder to detect in lemons and limes.

Although symptoms and effects of stubborn were well established for many years, the causal agent was unknown and was thought to be a virus. However, in the late 1960’s to early 1970’s, the causal organism, Spiroplasma citri, was identified and characterized as a helical, wall-less bacterium motile in liquid and solid cultures. It has an optimum temperature for growth in culture of about 90 °F. S. citri has also been shown to cause various other diseases in crop plants and also affects a number of ornamental plants and several native or invasive species that have become established in California. S. citri is a simple organism with a reduced genome. Most published genes do not reveal any genetic diversity. This is true of the spiralin gene, which is used as the basis of detection by PCR. However, we have detected genetic diversity between stubborn isolates using AFLP.

Diagnosis based upon visual evaluation of symptom expression, particularly fruit symptoms, in severely stubborn-affected trees can be quite reliable. Visual diagnosis of field trees is most effectively done when temperatures are warm and particularly when fruit development is advanced enough that symptoms can clearly be seen (September - October).

In less severe cases of stubborn, positive diagnosis requires confirmation by controlled testing. Biological indexing for stubborn involves graft inoculation of tissue into sensitive varieties, such as ‘Madam Vinous’ or ‘Pineapple’ sweet orange, held at warm temperatures. The most definitive detection technique involves culturing the bacterium from vegetative or reproductive tissue, which requires a number of time-consuming and intricate steps. Growth of S. citri in culture usually takes 2 – 3 weeks and contamination can result in false positives.

Detection by serological techniques has not proven effective in California. Our recent work has resulted in easier and more reliable detection of S. citri using PCR. Initially, we were able to achieve more reliable results from field trees by first putting the appropriate plant parts into culture and then performing PCR on DNA extracted from the medium. We were later able to achieve satisfactory results by performing PCR directly from the culture medium. More recently, we have been able to detect S. citri directly from fruit or vegetative tissue by PCR. At this point, the PCR test is more sensitive than the traditional culture method. This is probably due to the fact that during seasons of low titer, growth of the organism in culture is very slow, whereas amplification of the DNA by PCR is less affected by the low titer.

No matter what actual assay is used, detection of S. citri is made more difficult by its irregular spatial and temporal distribution. All of the testing methodologies listed above are dependent upon the actual presence of the pathogen in the tissue sampled. In the case of S. citri, it cannot be assumed that the pathogen is present in symptomatic trees, nor in all parts of infected trees. Early work demonstrated that stubborn was most detectable during summer months in Riverside. In the San Joaquin Valley, the pathogen becomes routinely detectable slightly later in the year. This is most probably due to winter-time titers of S. citri being lower in trees in the San Joaquin Valley as compared to trees in Southern California due to the colder temperatures in the San Joaquin Valley. When we inoculated greenhouse-grown sweet orange indicators with tissue taken from the San Joaquin Valley in December, it took approximately 6 months until the titer had increased enough under these optimal conditions to be detected. S. citri is also irregularly distributed in infected trees and does not spread systemically with much efficiency. Therefore, if a tree is infected when it has already obtained some size, only the branch or area near the infection will become symptomatic. In randomly sampling symptomatic trees, we have found only about 15 – 20 % of the samples taken test positive. Because of these factors, a sampling strategy for stubborn is critical. Although not conclusive at this point, we recommend that trees should be sampled in the late summer through early fall (July through October). A fairly large sample of approximately 15 budsticks should be sampled, with samples being taken from symptomatic areas if possible.

Stubborn is a graft-transmissible disease, meaning that it can be spread via budwood. However, the graft-transmissibility of S. citri is low and often variable due to the irregular distribution of the pathogen in the tree and the low titers of the pathogen in infected budwood. Because of these factors, a significant proportion of the grafted progeny of an infected tree may be free of S. citri. S. citri has not been shown to be mechanically transmissible nor transmitted by seed. In the 1970s the natural spread of stubborn by insect vectors was confirmed. The beet leafhopper, Circulifer tenellus, was first confirmed to carry the S. citri pathogen. Later, S. citri was shown to be carried by two other species of leafhopper, Scaphytopius nitrides and S. acutus delongi, as well as several other species of insects.

In addition to citrus, various other plants have been shown experimentally to be hosts of S. citri; however, many of these experimental hosts do not appear to be hosts of S. citri in natural conditions. Some of the most important alternate hosts include various members of the Brassicaceae (mustard family), which are quite common in California as weeds, native species, or crops. Brassicaceae species are also hosts of the beet leafhopper, C. tenellus, the most important vector of stubborn disease in California despite citrus not being its preferred host. Most of the Brassicaceae that host S. citri are winter annuals and harbor the pathogen during the winter months. It appears that during the spring and early summer, C. tenellus migrates from the alternate Brassicaceae hosts, which are found in the foothill areas surrounding citrus production in the San Joaquin Valley, to the valley floor. The insects remain active as the season progresses and conditions for transmission remain suitable. As mentioned previously, it takes several months for initial infections of S. citri to build up in the plant. In the fall, C. tenellus migrates away from the valley floor towards the foothills. The foothills stay warmer than the valley floor during the winter months, and so provide a more suitable temperature for both C. tenellus and S. citri. So, as titer of S. citri decreases during the winter months, its perpetuation is assured by its presence in the foothills. Under experimental conditions, S. citri maintained in planta long-term has been demonstrated to lose its ability to be transmitted and apparently S. citri requires passage through the insect vector to retain its infectivity.

This implies that there would be little citrus-to-citrus transmission of stubborn by the leafhopper vectors under most circumstances, with the exception of young plantings. The spread of S. citri into citrus plantings apparently mostly comes from the alternate hosts in surrounding fields. Elimination of the pathogen from a grove or nursery would not prevent infection from an inoculum pool in the alternate hosts. The fact that the disease can apparently overwinter in areas removed from commercial production (ie, the foothills around the valley) and then be transported by the vector to the production area during the spring months means that elimination of alternate hosts or the vector from near the orchard would reduce but not eliminate the possibility of infection.

The much lower apparent susceptibility of older trees to infection is an advantage from the disease management standpoint. Orchards established with S. citri­-free trees in areas in which the populations of the beet leafhopper and the inoculum source in alternate hosts are consistently low for several years after establishment will greatly reduce the chances of stubborn infection and economic losses. The use of S. citri-free materials in areas with high S. citri presence in alternate hosts and high beet leafhopper activity would probably be of little use in the prevention of stubborn disease during years favorable to disease development. However, the use of pathogen-tested propagative material remains the first line of defense against all economically damaging citrus diseases. Maintenance of mother trees and/or production of young trees under screen provides protection against contamination with stubborn and other insect-vectored diseases and is a recommended practice.

Trap plants have in the past shown some usefulness in managing stubborn. Sugar beets are an attractant for the beet leafhopper but a non-host plant for S. citri. Sugar beets planted around a seedling planting of citrus reduced incidence of S. citri in periwinkle (an alternate host of S. citri) planted around the citrus seedlings by over 50 %. Although this did not provide complete control, the incidence was reduced enough that this strategy deserves consideration. Periwinkles planted around citrus would also provide an indication of the amount of S citri present as the periwinkle yellows symptoms develop. Vector control via the use of systemic insecticides has not been shown to be useful in the past in reducing the incidence of stubborn in either citrus seedlings or periwinkles. Antibiotics are effective against S. citri in vitro but apparently antibiotics injected into the xylem of large trees are not translocated into the phloem sieve tube elements in sufficient amounts for affected trees to improve.

Removal of stubborn-infected trees or branches is often practiced by growers. This is most effective in younger plantings, as it is at this stage that the trees are most susceptible to infection. Trees infected early in their life will never be productive, so, stubborn-infected trees in plantings less than 6 years old should be removed as soon as they are diagnosed. Trees that become infected after they are mature present only a small hazard to the rest of the grove since there is little tree-to-tree spread. It is mainly economic factors that will determine whether mature trees should be removed. Control of alternate hosts near these plantings will reduce (but not eliminate) the possibility of infection of re-planted trees.

In summary, the following are considered to be useful in lowering the incidence of stubborn or minimizing losses from it:

1. If possible, locate nurseries in areas where stubborn does not spread naturally and where the incidence of stubborn is low.

2. Use S. citri-free budwood for all propagations including topworking.

3. If possible, maintain mother trees under screen and if possible produce nursery trees in a protected environment (greenhouse or screenhouse).

4. Topworking should only be done on trees that are totally free of S citri.

5. On an annual basis, remove all stubborn-infected trees from orchards less than 6 years old.

6. All replants in orchards of any age should be removed if infected with stubborn and again replanted with stubborn-free trees.

7. Maintain a strict program of weed control in and around the planted orchards, particularly for the first 6 years.

8. In nurseries located in areas where S citri is endemic, substantial borders of attractive trap plants that are not hosts to S. citri can be used. The traps plants can also be treated with insecticides to kill the attracted leafhoppers.

9. Avoid the use of cover crops susceptible to S. citri in orchards less than 6 years old in areas with high populations of the insect vectors.

Infection by the stubborn disease pathogen, Spiroplasma citri, has caused the mature citrus fruit in the foreground to remain green, color unevenly, and remain smaller than normal.

A sure way to generate controversy among citrus growers is to initiate a discussion on navel orange tree pruning. Some growers maintain that yield and fruit size is best maintained by minimal pruning, while others believe that the number of large fruit is increased when trees are severely pruned. A ‘standard’ manual pruning for navel oranges does not exist, but the closest thing to it is a procedure that involves pruning from the tree; 1.) shaded, dead branches 2.) branches which cross from one side of the tree to the other and 3.) green, triangular, juvenile shoots from the tree.  This type of pruning commonly goes under the name of ‘deadbrushing’.  Deadbrushing is a relatively light form of pruning, and a trained crew usually spends less than 15 minutes per tree performing it.  In addition to any manual pruning, most navel orange orchards in California are mechanically ‘hedged’ and  ‘topped’ to provide continued access to trees and their fruit by equipment and people involved in orchard cultural and harvest activities. Although growers have been growing navel oranges in California for over one hundred years, surprisingly few experiments have been conducted to determine the effect of pruning on navel orange yield and quality.

 To assist in providing some guidance related to pruning and its possible effects on fruit yield and quality, an experiment was established in 2000 in northern Kern County in an orange orchard that was typically harvested in late December or in January.  In 2000, 2001, 2002 and 2003, yield, fruit quality parameters and manual pruning costs were compared among mature “Frost Nucellar” navel trees (90 trees/acre) having one of three topping-height treatments (14 ft, 16 ft, and untopped trees). In addition to a topping treatment, the experimental trees were given one of three levels of manual pruning 1.) removal of several large scaffold branches in March of 2000 followed by deadbrushing in 2001, 2002 and no manual pruning in 2003; 2. dead brushing only in 2000, 2001, 2002 and no manual pruning in 2003; or 3. no topping or deadbrushing).  Data were collected from experimental trees surrounded by similarly topped and manually pruned border trees. Fruit weight, numbers, size, grade and color were determined the day after harvest at theUniversityofCalifornia ResearchandExtensionCenterexperimental packline near Lindcove,California. The year, in this report, refers to the year that the crop bloomed and not to the year of harvest.

For the 2003 crop year, even after 4 years, trees that were severely pruned in the spring of 2000 produced less total yield and less fruit in the most valuable-size range (i.e. 88 to 48 fruit/carton) than trees that were deadbrushed or left unpruned.  In 2003, differences in yield among manual pruning treatments were greater than in 2002 probably because of the higher yield potential that appeared to exist across the industry in 2003. The canopy of the severely pruned trees in 2003 had not yet retained the size of the deadbrushed or unpruned trees after four years, which limited their potential fruit production.  In contrast, in 2001 only one year after the manual treatments were imposed and a year with high spring temperatures and very poor fruit set, no differences in yield were found among manual pruning treatments.

 When the data of average individual tree performance are summed over the four years that this experiment was conducted, the treatment that included removal of some major scaffold branches in March of 2000 with deadbrushing in 2001 and 2002, was inferior in terms of yield, fruit number, and number of valuable-sized fruit in the range of 88 to 48 per carton than to trees that were only deadbrushed or those that had no manual pruning. Most of the detrimental effects of severe pruning on yield (and on fruit quality) occurred at the December harvest following the severe pruning in March 2000.  Over the four years of the experiment, the trees that were not manually pruned produced equal or better cumulative yields of fruit, equal or more valuable sized fruit, and fruit with equal grade compared to deadbrushed or severely pruned trees. The percentage of the fruit on the tree larger than size 88 was greater in the severe pruning treatment, but because total fruit number per tree was less and more of this fruit was overly large (i.e. greater than size 48) the number of the most valuable-sized fruit/tree (sized 88 to 48) was less.  Obviously, the trees that were not manually pruned had no associated manual pruning costs when compared to the other two pruning treatments.  Manual pruning costs, from 2000 through 2003, not including stacking and shredding of pruned brush, were $8.50/tree for the deadbrushing treatment and $13.00/tree for the severe manual pruning treatment.