Citrus Research Report 1996
ABOUT THE COLLECTION
The Citrus Report, first published in 1978, is one of several commodity-based agricultural research reports published by the University of Arizona. The purpose of the report is to provide an annual research update to farmers, researchers, and those in the agricultural industry. The research is conducted by University of Arizona and USDA-ARS scientists.
Both historical and current Citrus Reports have been made available via the UA Campus Repository, as part of a collaboration between the College of Agriculture and Life Sciences and the University Libraries.
Contents for Citrus Research Report 1996
- Biology and Control of Coniophora Causing Decay and Decline in Arizona Citrus
- Analysis of Rootstocks and New Fungicides for Control of Phytophthora Root Rot and Gummosis in Arizona Citrus Groves
- Development of Control Measures for Alternaria Fruit Rot on Roanges in Arizona
- Relationship of Viroids to Macrophylla Decline
- Mycoplasma-like Organisms as the Causal Agent for Macrophylla Decline
- Nutrition Survey in Arizona Citrus
- Effect of Foliar Boron Sprays on Yield and Fruit Quality of Citrus
- Cultivar and Rootstock Research for the Arizona Citrus Industry
- Managing Vegetation on the Orchard Floor in Flood Irrigated Arizona Citrus Groves
- Efficient Irrigation and Nitrogen Management for Lemons: Results for 1993-1995
- Heat Unit Based Crop Coefficient for Grapefruits Trees
- Control of Citrus Nematode with Cadusafos
Control of Citrus Nematode with CadusafosGranular (Rugby 10G) and liquid (Rugby 100ME) formulations of Cadusafos were evaluated for the control of Tvlenchulus semipenetrans on mature lemon trees in a commercial citrus orchard at Yuma, Arizona. Three applications of cadusafos, with two months between applications, at the rate of 2 g a.i. /m2 reduced nematode populations to undetectable levels and increased the yield and rate of fruit maturity of 'Rosenberger' lemons. Yields were increased 12,587 kg per hectare with Rugby 100ME and 8,392 kg per hectare with Rugby 10G. Nematode populations were suppressed for at least 12 months after the last application.
Heat Unit Based Crop Coefficient for Grapefruits TreesUsing portable sap flow sensors, the onset and volume of sap moving up the branches of grapefruit (Citrus Paradisi Macfadyen) trees was monitored on an hourly basis. Reference evapotranspiration (ETo) data was obtained from The Arizona Meteorological Network (AZMET). Crop evapotranspiration (ETc) was estimated from soil moisture measurements using a neutron probe. The threshold temperature limits were delineated from an evaluation of the hourly sap flow responses to cold winter and extreme summer temperatures. The heat based crop curve was derived from a correlation of the crop coefficients with heat unit over one crop year. The heat based crop coefficients were found to be similar to crop coefficients derived by other researchers.
Efficient Irrigation and Nitrogen Management for Lemons: Results for 1993-1995Studies are being conducted which aim to improve the efficiency of irrigation and N fertilization for lemons produced on sandy soils in the low desert. The first experiment evaluates the response of 'Lisbon' lemons to various flood irrigation intervals. Irrigation intervals are based on soil moisture depletion (SMD) as calculated from frequent neutron probe soil moisture measurements. Individual treatments were irrigated when total SMD was 25 %, 40 %, 55 %, and 70 %, respectively. The second experiment compares the performance of young lemons produced under flood, trickle, and micro -spray irrigation systems. The third experiment evaluates the response of young lemons to water and N combinations (3 by 3 factorial) under micro -spray irrigation. The three irrigation rates were targeted for 30 cnbar, 20 cnbar, and 10 cnbar tension. The three N rates were 0.1, 0.2, and 0.4 kg N /tree. One flood irrigation treatment was added for comparative purposes. Overall, results obtained in experiment 1 during 1994 and 1995 indicate optimal fruit growth ,and yield is obtained at approximately 40% SMD. The results of experiment 2 show that after 18 months micro -spray irrigation produced significantly more tree growth than flood and drip irrigation methods. Additionally, first year fruit yields were significantly greater for pressurized irrigation compared to flood irrigation. Results from experiment 3 show a linear response in tree growth up to 10 cnbar soil moisture tension. Furthermore, tree growth at 10 cnbar tension was significantly greater than trees irrigated by flood. Yields were also increased to irrigation regime. There were no significant differences in tree growth to N fertilization rates. However, there was a yield increase to N fertilizer rate at the highest soil moisture regime.
Managing Vegetation on the Orchard Floor in Flood Irrigated Arizona Citrus GrovesSeveral orchard floor management strategies were evaluated beginning in the fall of 1993 in experiments on the Yuma Mesa in a Limoneira 8A Lisbon lemon grove and in a Valencia orange grove at the University of Arizona Citrus Agricultural Center (CAC) in Waddell, Arizona. On the Yuma Mesa, disking provided satisfactory weed control except underneath the tree canopies where bermudagrass, purple nutsedge, and other weed species survived. Mowing the orchard floor suppressed broadleaf weed species allowing the spread of grasses, primarily bermudagrass. Preemergence (Solicam and Surflan) and postemergence (Roundup and Torpedo) herbicides were used to control weeds in the clean culture treatment in Yuma. After two harvest seasons (1994-95 and 1995-96), the clean culture treatment resulted in greater yield than the other treatments. At the CAC, clean culture (in this location no preemergence herbicides were used), mowed resident weeds, and Salina strawberry clover orchard floor management schemes were compared. Again the clean culture treatment yielded more than the mowed resident weeds. The yield of the strawberry clover treatment was somewhat less than the clean culture yield but not significantly less. The presence of cover crops or weeds on the orchard floor were found to modulate tree canopy temperatures, and to have beneficial effects on soil nitrogen and soil organic matter content, but no effect on citrus leaf nutrient content. The decrease in yield in the mowed resident weed treatments compared to the clean culture treatment in both locations was attributed to competition for water.
Cultivar and Rootstock Research for the Arizona Citrus IndustryA lemon rootstock trial and a lemon scion trial were established in 1993. After two years growth, C. volkameriana and C. macrophylla rootstocks have begun to show significant growth and yield increases, compared with 'Swingle' citrumelo and 'Carrizo' citrange. These two rootstocks also have led to larger fruit size, especially early in the season. Trees on Rough lemon rootstock had equivalent growth, but less yield. 'Limoneira 8A Lisbon' scion cultivar had the greatest yield and largest early season fruit size, compared to 'Frost Nucellar', 'Corona Foothills' and `Prior Lisbon' lemons.
Effect of Foliar Boron Sprays on Yield and Fruit Quality of CitrusDeficiency of boron (B) in citrus has serious consequences for tree health and crop production. There is evidence that B deficiency may be a problem in Arizona citrus. Certainly, many symptoms of B deficiency are apparent, especially on the Yuma Mesa. A field trial was conducted at Yuma, Arizona to examine the effect of foliar boron application on fruit yield and quality of Citrus sinensis and C. limon. Boron was applied to 5 year old Citrus sinensis cv. Hamlin and C. limon cv. Rosenberger Lisbon trees at 5 different concentrations (0 ppm, 500 ppm, 1000 ppm, 2000 ppm, 3000 ppm) either before flowering or after flowering. At harvest, fruit yield and quality, and boron concentrations were determined. Foliar application appeared to increase leaf boron concentration (r= 0.50, p= 0.004). Fruit set was increased in Hamlin trees receiving bloom and post bloom applications of boron at the 1000 ppm level. Boron applications had no significant effect on lemon yield in these studies. This fruit set increase in Hamlin accounted for a 35% increase in overall yield relative to control trees. However, there was no significant difference in fruit weight, fruit pH, titratable acidity, peel thickness, juice volume, or soluble solid content of the fruits among treatments. Previous studies indicate that boron influenced in vivo and in vitro pollen germination in many crops. A plausible explanation for increased fruit yield may be that the applied boron was transported to the flowers where it exerted its influence of increased fruit set through an effect on pollen viability and/or pollen tube growth. However, clearly boron supplementation must be performed judiciously to avoid fruit drop from over -application of the element.
Mycoplasma-like Organisms as the Causal Agent for Macrophylla DeclinePrevious literature concerning citrus and other tree crops led us to ask if there was molecular evidence for mycoplasma -like organisms (MLOs) as the causal agent of Macrophylla decline and two other decline diseases, citrus blight and lemon sieve tube necrosis. We had molecular probes available to us that were either specific for MLOs of tree diseases and others that were universal for all known types of MLOs. We used a polymerase chain reaction (MLO) to determine if MLOs were present in the vascular tissues of decline and healthy citrus. I all trials performed, the trees were negative for MLO-PCR products. In addition, we attempted to transmit putative MLO 's from decline affected trees to Vinca rosea MLO-nurse plants. We were unable to affect this type of transfer. In addition, our attempts to identify MLO's in phloem tissue gave us negative results. We have since revised our hypothesis. We are currently pursuing the hypothesis that these decline disorders are the result of a rootstock scion incompatibility, that we may be able to avoid culturally, while maintaining these valuable combinations.
Relationship of Viroids to Macrophylla DeclineA physiological characterization has established that vascular changes in Macrophylla decline affected trees are not similar in character to xyloporosis affected trees. In addition, a survey of Macrophylla decline affected citrus did not establish any genetic similarity between Macrophylla decline and xyloporosis. We report diagnosis of either CCV or CEV by reverse transcription-polymerase chain reaction (RT-PCR), as well as diagnosis of Macrophylla decline or xyloporosis by Zn-distribution, water conductivity, accumulation of decline- specific proteins and examination of phloem morphology in lemon trees on the Macrophylla rootstock.
Development of Control Measures for Alternaria Fruit Rot on Roanges in ArizonaThe incidence and severity of Alternaria fruit rot on navel oranges has increased in Maricopa County. The objectives of this study were to test the efficacy of two fungicides, Kocide 101 and Rovral, for disease control and to determine an application schedule that will result in optimum control of disease. In 1994, compared to nontreated trees, a significant reduction in the number of dropped oranges occurred on trees sprayed one to five times with Rovral at monthly intervals from April through August. In the same year, monthly applications of Kocide 101 from April through August were no better than leaving trees untreated. In 1995, Rovral treatments from March through August provided no apparent beneficial effects on control of Alternaria fruit rot, while a single application of Kocide 101 in December, January, or February resulted in significantly less dropped oranges compared to nontreated frees. Because of the inconsistent activity of Kocide 101 and Rovral in these two studies, an additional trial is in progress during 1996. The additional data from 1996 may help identify when and what fungicide(s) could provide significant control ofAlternaria fruit rot of navel oranges in Arizona.
Analysis of Rootstocks and New Fungicides for Control of Phytophthora Root Rot and Gummosis in Arizona Citrus GrovesExperiments were initiated to evaluate potential new citrus rootstocks for their relative tolerance or resistance to root rot and gummosis caused by Phytophthora citrophthora and P. parasitica and to determine the efficacy of potential new fungicides for disease control. In greenhouse trials conducted in 1994 and 1995, the range of root loss due to Phytophthora in the 44 different rootstocks tested ranged from 26-96 %. Rootstocks sustaining 80% or less root loss will be evaluated further to identify those with superior tolerance to Phytophthora. In growth chamber experiments, the same rootstocks were inoculated on the stem to evaluate resistance to gummosis. The length of canker that developed on these test plants ranged from 1-25 mm. Rootstocks with canker development in the range of 1-10 mm in length will be tested further to identify the most resistant selections. Laboratory studies were conducted to determine the comparative activity of Aliette, Ridomil, Dimethomorph, Fluazinam, ICIA-5504, and SM-9 at concentrations of 1, 10, 100, and 1, 000 mg/l on sporulation and growth of P. citrophthora and P. parasitica. Each of the four new molecules was either comparable or superior to Aliette or Ridomil with respect to activity on at least one component of the life cycle of the Phytophthora species tested. The results presented in this report are preliminary in nature and will be validated in future studies.
Biology and Control of Coniophora Causing Decay and Decline in Arizona CitrusA field survey of mature lemon trees showed an average of 30% of trees with symptoms of brown heartwood rot caused by Coniophora sp. In vivo growth of Coniophora inoculated into branches of different types of citrus (Valencia orange, Marsh grapefruit, Orlando tangelo or Lisbon lemon) on rough lemon rootstock was significantly higher in lemon while Coniophora inoculated into Lisbon lemon wood branches on trees established on rough lemon, volkameriana, macrophylla, Cleopatra mandarin, sour orange or Troyer citrange rootstocks showed no significant differences in growth. Vegetative incompatibility trials from one mature orchard demonstrated that isolates from different trees are incompatible. In vitro fungicide trials showed that only NECTEC paste effectively reduced decay on lemon blocks 15 weeks after inoculation with Coniophora. Field fungicide trials showed that NECTEC P paste as well as the blank paste without fungicides, propiconazole at 10,000 μg /ml, imazalil at 20, 000 μg /ml or propiconazole plus imazalil in combination at 10,000 and 20,000 μg/ml, respectively, significantly inhibited the advance of fungus 7 mo. after inoculation. A second fungus isolated from brown rot in branches in younger orchards was identified as Antrodia sinuosa, a native decay fungus on conifers in Arizona.