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Abstract
‘Star Ruby’ grapefruit (Citrus paradisi Macf.) grew well on Texas sour orange and bittersweet orange (C. aurantium L.), Cleopatra and Sunki mandarin (C. reticulata Blanco), Colombia sweet line (C. limettioides Tan.), and Troyer citrange [Poncirus trifoliata (L.) Raf. × C. sinensis (L.) Osb.] rootstocks. Growth was most vigorous on Texas sour orange and Troyer citrange. Trees on Columbia sweet lime and Sunki mandarin developed problems with Phytophthora foot rot. Most of the trees on Christiansen trifoliate orange (P. trifoliata) and Swingle citrumelo (P. trifoliata × C. paradisi) developed severe chlorosis and died. Trees on Morton citrange were slow growing, somewhat chlorotic, and slightly affected by foot rot.
Abstract
Vesicular–arbuscular mycorrhizal (VAM) development of Glomus intraradices Schenck & Smith and growth response of container-grown rough lemon (Citrus jambhiri Lush.) were compared in a phosphorus-deficient (2 μg P/g) sand-soil and peat-perlite, soilless mix, both fertilized with 3 levels of superphosphate (SP) and rock phosphate (RP). Mycorrhizal development and growth response were affected primarily by initial P availability in the media. In both soil and peat-perlite, VAM formation was greatest at 8 μg available P/g medium and maximum growth responses occurred over a range of 4 to 8 μg P/g medium. Available P from SP greater than 20 μg P/g medium inhibited mycorrhizal colonization and eliminated growth response. Less VAM development in peat-based media compared to soil suggested an effect of organic matter. In peat-perlite, moderate levels of colonization and a growth response equal to that of nonmycorrhizal plants grown in soilless media with high levels of available P were attained if RP was used to provide a low but sustained level of available P. In soilless media, mycorrhizae prevented P-induced, copper-deficiency symptoms in rough lemon.
Abstract
Vesicular-arbuscular mycorrhizal (VAM) development of Glomus intraradices Schenck & Smith and growth response of Carrizo citrange [Poncirus trifoliata (L.) Raf. × Citrus sinensis (L.) Osbeck] and sour orange (C. aurantium L.) seedlings were examined in peat-perlite medium fertilized with 4 levels of superphosphate (SP) and rock phosphate (RP). Loss of P from SP-amended medium was exponential, whereas release of P was linear from RP-amended medium. With SP, root colonization by G. intraradices and growth response of both citrus rootstocks were reduced compared to RP-amended medium. VAM colonization and growth response were less with SP-amended medium than RP, probably because of the initially high available P from SP which inhibited VAM development, and because P later declined to levels insufficient to support maximum growth. When RP was used as a controlled-release source of P, VAM colonization was comparable to that observed in mineral soils. Growth of VAM plants exceeded that of uninoculated plants fertilized with increased levels of soluble P because of P-induced Cu deficiency in absence of VAM. An additional advantage of RP was long-term availability of P, compared to SP which leached within weeks after application to peat-perlite. No further P fertilization may be necessary if RP and VAM inoculum are incorporated into soilless media before planting, whereas repeated application of soluble P would be required for slow-growing woody plants like citrus.
Abstract
Stunting of sour orange seedlings was observed in seedbeds which had been fumigated chemically, heat-treated, or planted in soil-less media. Mycorrhizae were consistently absent from stunted seedlings and present in vigorous seedlings in seedbeds and nurseries and also present in orchards. Inoculation with Glomus fasciculatus (Thaxter) Gerd. & Trappe increased seedling height more than 300% and increased foliar phosphorus concentrations in greenhouse experiments on sour orange seedlings grown in sand culture with complete nutrient solution. Preplant application of P at 200 mg/liter did not increase growth or foliar P concentrations under the same conditions. Inoculation of seedlings receiving 0 to 800 mg P/liter with G. fasciculatus increased growth where 0 or 100 mg P/liter was added, but not where higher rates of P were used. Application of high rates of P induced copper deficiency symptoms, which were more severe on non-mycorrhizal than on mycorrhizal plants.
Abstract
Preplant fumigation with methyl bromide, sodium N-methyl dithiocarbamate, and heat treatment temporarily eliminated Phytophthora parasitica Dast. from most seedbeds, prevented damping-off and reduced root rot on sour orange (Citrus aurantium L.) seedlings. Stunting of seedlings due to elimination of mycorrhizal fungi occurred frequently following these treatments. Seedlings in non-fumigated, captan-treated seedbeds were not affected by damping-off or lack of mycorrhizae, but growth was reduced compared to that in fumigated beds because of severe root infection by P. parasitica. Application of 500 to 750 kg P/ha to fumigated seedbeds temporarily stimulated the seedling growth, and larger seedlings resulted when mycorrhizae subsequently became reestablished in the beds. Application at these rates did not overcome the stunting problem if mycorrhizae failed to become re-established. Application of 1900 to 2000 kg P/ha stimulated early growth of seedlings in fumigated seedbeds but inhibited mycorrhizal formation and induced copper deficiency symptoms. Fumigation programs which are effective in eradicating P. parasitica also eliminate mycorrhizal fungi and induce severe stunting symptoms which are partially reversible by application of P, but completely reversible only by inoculation with mycorrhizal fungi.
Abstract
Citrus blight is a vascular wilt disease of unknown etiology. Blight-affected ‘Pineapple’ sweet orange [Citrus sinensis (L.) Osb.] trees on rough lemon (C. jambhiri Lush.) rootstock in early stages of decline were treated with benzimidazole fungicides or with oxytetracycline (OTC) by trunk injection, soil drench, or injection plus drench. The distribution and persistence of the materials were monitored by bioassay using Bacillus cereus var. mycoides for OTC and Penicillium expansum L. for benzimidazoles. Both materials were well-distributed in the canopy following trunk injections, in the root systems following soil drenches, and though most of the tree after injection plus drench treatments. Benzimidazoles and OTC persisted within the trees for several months and relatively high levels of activity were maintained for over a year by 3 applications. OTC persisted in soil for more than 6 months; whereas, benomyl disappeared from the soil in 3 months or less. Although high levels of bactericidal or fungicidal activity were maintained in the treated trees, most treated trees declined as rapidly as the untreated control trees. The injection plus drench group treated with OTC showed a slight improvement after 2 years. None of the treatments increased tree growth or resulted in increased water uptake. High levels of zinc in trunk wood, an internal symptom of blight, were unaffected by the benzimidazole treatments, but injection plus drench treatment with OTC significantly reduced zinc levels in trunk wood. Since neither OTC nor benzimidazoles completely reversed symptoms of blight, we were unable to conclusively confirm or refute proposed bacterial or fungal etiologies for citrus blight.
Abstract
Blight-affected sweet orange [Citrus sinensis (L.) Osb.] trees on rough lemon (C. jambhiri Lush.) rootstock were injected with high concentrations of oxytetracycline (OTC) (10–30 g a.i./tree) under high pressure (1300–1700 kPa) in an attempt to induce remission of blight symptoms. OTC activity, determined by bioassay with Bacillus cereus var. mycoides, was distributed in 70–95% of the twigs sampled 3 weeks after injection. Activity persisted in the twigs and mature leaves for 3 to 5 months and in a few twigs as long as 7 to 8 months after a single injection. No activity was detected in small roots and downward movement appeared to be limited. Distribution and persistence of OTC in healthy trees was similar to that in blight trees. Trees with mild blight symptoms injected once with OTC continued to decline and had moderate to severe blight 2 years later. Of 16 healthy trees injected with OTC 4 times over 2 years, 1 became severely blighted and several others developed mild symptoms. Our trunk injection work with OTC does not support the hypothesis that blight is caused by a tetracycline-sensitive organism, but does not eliminate that possibility.
The inheritance of resistance to a host-specific isolate (Shinn) of Alternaria alternata (Fr.:Fr.) Keissl. from `Minneola' tangelo (a cross between Citrus paradisi Macf. `Duncan' and C. reticulata Blanco `Dancy') was shown to be controlled by a single recessive allele, aaM1, within the citrus genome. A backcross between resistant `Clementine' mandarin (C. reticulata) and susceptible LB#8-10 (a hybrid of `Clementine' mandarin and `Minneola' tangelo) resulted in 61 resistant (R) and 58 susceptible (S) plants (χ2 = 0.0756, P ≥ 0.05), but the reciprocal cross deviated from the expected 1R:1S ratio (87 R and 36 S plants (χ2 = 21.1463, P ≥ 0.05). A dominant allele, AaM1, of this resistance gene was found in a loose coupling phase linkage with two RAPD markers, P12850 (15.3 cM) and AL31250 (36.7 cM), after JOINMAP computer analysis.
Colletotrichum acutatum J. H. Simmonds infects citrus flower petals, causing brownish lesions, young fruit drop, production of persistent calyces, and leaf distortion. This suggests that hormones may be involved in symptom development. To identify the types of hormones, cDNA clones encoding proteins related to ethylene and jasmonate (JA) biosynthesis, indole-3-acetic acid (IAA) regulation, cell-wall modification, signal transduction, or fruit ripening were used to examine differential gene expressions in calamondin (Citrus madurensis Lour) and/or `Valencia' sweet orange (Citrus sinensis Osbeck) after C. acutatum infection. Northern-blot analyses revealed that the genes encoding 1-aminocyclopropane-1-carboxylate (ACC) oxidase and 12-oxophytodienoate required for ethylene and JA biosynthesis, respectively, were highly up-regulated in both citrus species. Both gene transcripts increased markedly in petals, young fruit and stigmas, but not in calyces. The transcripts of the genes encoding IAA glucose transferase and auxin-responsive GH3-like protein, but not IAA amino acid hydrolyase, also markedly increased in both species 5 days after inoculation. The expansin and chitinase genes were slightly up-regulated, whereas the senescence-induced nuclease and ß-galactosidase genes were down-regulated in calamondin. No differential expression of transcripts was detected for the genes encoding expansin, polygalacturonase, and serine-threonine kinase in sweet orange. As compared to the water controls, infection of C. acutatum increased ethylene and IAA levels by 3- and 140-fold. In contrast, abscisic acid (ABA) levels were not significantly changed. Collectively, the results indicate that infection by C. acutatum of citrus flowers triggered differential gene expressions, mainly associated with IAA, ethylene, and JA production and regulation, and increased hormone concentrations, consistent with the hypothesis of the involvement of phytohormones in postbloom fruit drop.
Postbloom fruit drop (PFD) of citrus is incited by the fungus Colletotrichum acutatum J. H. Simmonds and may result in young fruit drop and severe yield losses. Previous studies suggested that imbalance of growth regulators such as auxin, ethylene, and jasmonic acid (JA) plays an essential role in young fruit abscission. In this work, we determined the factors associated with fungal-induced fruit drop by testing compounds inhibitory to hormonal transport or biosynthesis. As assessed on sweet orange (Citrus sinensis Osbeck) and grapefruit (C. paradisi Macf.) for 4 years, we found that many auxin transport and action inhibitors such as 2,3,5-triiodobenzolic acid (TIBA), 2-(4-chlorophenoxy)-2-methyl-propionic acid (clofibrate), or quercetin and JA biosynthesis inhibitors such as salicylic acid (SA) and aspirin (methyl-SA) applied 7 d after C. acutatum infection resulted in higher percentages of young fruit retention compared with the water controls. The commercial products ReZist and Actigard, widely used as systemic acquired resistance (SAR) agents, also improved fruit retention. Furthermore, application of gibberellic acid (GA3) on sweet orange, regardless of C. acutatum infection, significantly increased fruit retention. These commercial products may be very useful in managing this destructive disease of citrus in the field.