- Author: Ben Faber
- Author: Craig Kallsen
- Author: Akif Eskalen
Dry Root Rot is a pretty fantastic disease symptom that is usually seen in lemon, but can be seen in orange, as well.
Craig Kallsen, UCCE Citrus Advisor in Kern Co. comments on a disease sample:
I have seen a lot of dry root rot over the years. It usually is something that damages or weakens the root system which allows a Fusarium species to colonize the rootstock. It is very common in older lemon groves that froze at some point in their past. Also common in groves that suffered a lot of gopher damage or where the wraps got too hot in the sun burning the bark and cambium. I have also seen it in cases of fertilizer or other soil-applied chemical burn. I have no doubt that graft incompatibility could do it too.
Akif Eskalen, UCCE Plant Pathology Specialist chimes in on a disease sample submitted:
As you can see from the attached picture there is a weird callus formation and symptoms of incompatibility at the graft union which I think is the primary cause of decline. We didn't observe any discoloration in the scion, however rootstock was completely discolored where we isolated Fusarium solani the causal fungus of Dry Root Rot. Dry root rot caused by either Fusarium solani and/or Fusarium spp. When there is a disconnection at the graft union, the phloem can not transfer enough carbohydrate to the rootstock to feed feeder roots. Fusarium fungal species are present in the soil and they can attack and easily colonize on starch depleted roots and cause DRR.
We still don't know what is causing the graft incompatibility on these plants. That needs to be investigated.
It's still not clear how and why citrus becomes affected.
- Author: Ben Faber
UC Riverside and the Citrus Research Board partner to provide:
UC Riverside Citrus Day for Professional Industry members
Thursday, February 20, 2014
8 a.m. to 3:00 p.m.
UCR Agricultural Operations, Riverside, California
For information: (951) 827-5906
Please join us for the 3rd Annual Citrus field day designed for citrus growers and citrus industry representatives. Pending approval, we will be offering 2.5 hours of California Continuing Education
Credit for Pest Control Advisers (PCA).
Presentations, field tours and topics of interest:
Pesticide safety – Vince Samons
Update on ACP and HLB in California – Joseph Morse
Phytophthora diseases of citrus – Jim Adaskaveg
Lemon Varieties for the Desert –Glenn Wright
Understanding factors that influence the eating experience in citrus – David Obenland and Mary Lu Arpaia
Citrus Variety Collection tours of new cultivars and “unforbidden” fruits – Tracy Kahn, David Karp, Tom Shea, and Robert Krueger
Update on Citrus Rootstock Field Demonstration – Mikeal Roose
Barbeque Lunch included.
Registration: $18. Deadline: February 14, 2014. There will be no walk-in registrations. We will email directions and updates to all who have registered.
Space is limited so please register early.
Please register online at
http://form.jotformpro.com/form/40196914861965
To make a tax-deductible contribution to the
Citrus Variety Collection Endowment fund or the Citrus Research Center & Agricultural Experiment Station support fund go to the following link and select College of Natural and Agricultural Sciences then select the specific fund:
https://advancementservices.ucr.edu/GivingForm.aspx
- Author: Ben Faber
Travel can be enlightening. In Turkey I learned that sour orange rootstock is routinely used with lemon and mandarin scions without any fear of tristeza virus, a formidable disease of oranges. When I heard this I asked Georgios Vidalakis in charge of the UC Clonal Protection Program and a virologist. And he said that it was true and the neat thing is that the rootstock can handle heavy, calcareous soils better than other citrus rootstocks. So we are doing a trial on rootstocks and sour orange is included.
Something else I “learned” was that if you girdle citrus at flowering, the fruit has few or no seeds. Well, I talked to many growers and scientists and they all said the same thing. I went through the citrus literature and I could find no mention of this. I emailed Carol Lovatt, the plant physiologist at UCR and she said that when you alter hormone flows by girdling, who knows what might happen. So we set up a little trial in lemon that flowers pretty much all year long on the coast. Every month we girdle branches with either a hand saw or a girdling knife which make different sized cuts, flagging the branches with different colored tape to identify the girdling date. Over an 18 month period we harvested fruit and cut it to count seeds. And………………………………………there was only a slight difference in seed numbers, a few less in the girdled trees.
The goal of this trial was to see if girdling worked and if so, what was being changed in the tree and if could identify that, then maybe we could develop a nutritional program that would do the same thing. That way we wouldn’t need to girdle. But not everything you hear turns out to be true.
- Author: Craig Kallsen
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 KernCounty 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 the University of California Research and ExtensionCenter experimental 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.
Fruit yield or quality was not different among topping heights in any of the four years of the experiment. Topping height did not affect yield, probably because of the wide spacing and tall trees in this orchard. The canopies of untopped trees had little fruit within 4 feet of the ground as a result of shading of the lower canopy by neighboring trees. Removing the top 4 feet from an 18-foot tall tree moved the fruit-bearing volume downward in response to greater light penetration into the lower canopy but did not decrease the volume of the tree that received sufficient light to produce fruit. This effect was in contrast to severe manual pruning, which reduced the volume of the unshaded canopy overall, limiting the volume available for fruit production. A highly significant positive-linear correlation was found in the data across the four years and treatments between the total numbers of fruit produced per acre versus the total number of fruit sized 88 to 48 per carton produced per acre. This functional relationship existed whether reductions in fruit numbers produced per acre were the result of severe pruning in March or from weather-related phenomena such as occurred in 2001, suggesting that anything that reduced fruit numbers below approximately 130,000 fruit per acre resulted in a decrease in the number of fruit sized 88 to 48 per carton in this orchard.
Of course, there are other reasons to manually prune orange trees, other than to improve fruit size. If certain insects, like California red scale or cottony cushion scale have been a problem, pesticide spray coverage may be improved by making the canopy less dense through pruning and fruit quality may be improved by making this investment. In general, what this pruning research has reinforced is the concept that growers should know why they are pruning orange trees and that manual pruning is unlikely to increase the number of fruit in the most valuable size ranges.
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1Fruit sizes refer to number of fruit that fit into a standard California 37.5 lb. carton. 2 The severe treatment refers to the treatment that included removal of two or more major scaffold branches in spring 2000.
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- Author: Craig Kallsen, Blake Sanden and Mary Lu Arpaia
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.