Oviposition behaviour and larval development of Anastrepha fraterculus from Argentina in citrus

Citrus peel physicochemical attributes are considered the main components conferring partial or even total resistance to fruit fly (Diptera: Tephritidae) infestation. Fruit fly females adapt their ovipositional strategies to overcome such resistance. Here, we explored the effects of citrus species (Rutaceae) on the ovipositional behaviour of the South American fruit fly, Anastrepha fraterculus (Wiedemann), and on its immature development. Particularly, we investigated the effects of (1) citrus species on oviposition behaviour and immature development, (2) citrus species on oviposition preference and on the location of the eggs at different depth in the citrus peel, and (3) harvest season and post‐harvest storage time on oviposition behaviour and immature development in lemon. Citrus species influenced ovipositional behaviour and affected survival of immature stages. Females laid eggs in lemon [Citrus limon (L.) Burm.], orange [Citrus sinensis (L.) Osbeck], and grapefruit (Citrus paradisi Macfadyen). In orange and lemon, larvae were found dead close to the oviposition areas, suggesting chemically mediated resistance mechanisms. Under choice conditions, females preferred grapefruit over lemon and bigger clutches were found in the layers where embryonic development is favoured. Unsuitability of lemon as a medium to complete development was neither affected by harvest season nor by storage time of the fruit after harvest. The physical and chemical characteristics of the peel were distinctive to each citrus species and may have affected the specific levels of resistance of these citrus species to infestation by A. fraterculus.     

Growth promotion and inhibition by antibiotic-producing fluorescent pseudomonads on citrus roots

Summary Forty-three strains of feeder root colonizing fluorescent pseudomonads from rough lemon (Citrus jambhiri Lush.) roots were examined for effects on rough lemon and sweet orange (Citrus sinensis Osbeck) seedlings. Plants inoculated with a single bacterial soil-drench had, after 10 months, a range of stimulatory (to 116%) and inhibitory effects (to 52%). Stimulatory bacteria particularly increased growth of root systems. Cultivar-specific inhibition and stimulation was evident in inoculations of rough lemon and sweet orange seedlings. Populations of fluorescent rhizobacteria on inoculated and noninoculated, as well as on stimulated and nonstimulated seedlings, did not differ significantly (10.8×10⁶ to 30.3×10⁶ CFU/g root). Population of fluorescent rhizobacteria on seedlings were higher than populations on feeder roots from grove trees (2.8 to 5.7×10⁶ CFU/g). Ninety-four and 81% of 251 fluorescent strains produced antibiotics against the fungusGeotrichum candidum and the bacteriumErwinia stewartii, respectively. Antibiotic activities of 90% of the antibiotic producing strains were repressed by Fe³⁺, indicating siderophore production. In comparison, only 9.6 and 15% of 94 randomly selected nonfluorescentPseudomonas strains were antibiotic producers. Differences between stimulatory and inhibitory or neutral bacteria were not apparent from antibiosis tests. On the basis of physiological tests,Pseudomonas putida was the most abundant (>62%) pseudomonad species on rough lemon roots. Growth stimulating strains appeared to be in bothP. putida andP. fluorescens groups. FewP. aeruginosa strains were identified on citrus roots.Florida Agricultural Experiment Stations Journal Series No.     

Biology of Diaphorina citri (Homoptera: Psyllidae) on four host plants

The biology of the citrus psyllid Diaphorina citri Kuwayama was studied at 25 degrees C on four commonly grown citrus and related plants [rough lemon, Citrus jambhiri Lush; sour orange, C aurantium L.; grapefruit, C. paradisi Macfadyen; and orange jessamine, Murraya paniculata (L.) Jack] in the laboratory. The biological characteristics of each life stage are described. The average egg incubation periods on orange jessamine, grapefruit, rough lemon, and sour orange varied very little (4.1-4.2 d). The average nymphal developmental periods on these four host plants were essentially the same except the fifth stadium. Survival of immatures on orange jessamine, grapefruit, rough lemon, and sour orange was 75.4, 84.6, 78.3, and 68.6%, respectively. Female adults lived an average of 39.7, 39.7, 47.6, and 43.7 d on these respective host plants. The average number of eggs laid per female on grapefruit (858 eggs) was significantly more than those on other hosts (P < 0.05). The intrinsic rate of natural increase (r(m)) for D. citri on grapefruit was highest. Jackknife estimates of r(m) varied from 0.188 on grapefruit to 0.162 on orange jessamine and rough lemon. The mean population generation time on these hosts ranged from 31.6 to 34.1 d. The continuous flushes produced by orange jessamine could play an important role in maintaining high populations of this vector when the new flushes are not available in the commercial citrus groves.     

Influence of Metalaxyl on Three Nematodes of Citrus

Metalaxyl significantly reduced population of Pratylenchus coffeae, Radopholus similis, and Tylenchulus semipenetrans in roots of Citrus limon (rough lemon) under greenhouse conditions. Postinoculation treatment of rough lemon seedlings was not as effective i n reducing nematode populations as was treatment before inoculation. Fewer nematodes infected metalaxyl-treated roots than nontreated roots. However, incubation of nematodes in metalaxyl did not inhibit nematode motility or their ability to locate and infect roots. Cellular responses to nematode injection differed between treated and nontreated tissues. Metalaxyl appeared to confer nematode contraol by modifying citrus roots such that a normally susceptible rootstock became tolerant.     

A zinc-binding citrus protein metallothionein can act as a plant defense factor by controlling host-selective ACR-toxin production

Metallothionein is a small cysteine-rich protein known to have a metal-binding function. We isolated three different lengths of rough lemon cDNAs encoding a metallothionein (RlemMT1, RlemMT2 and RlemMT3), and only RlemMT1-recombinant protein had zinc-binding activity. Appropriate concentration of zinc is an essential micronutrient for living organisms, while excess zinc is toxic. Zinc also stimulates the production of host-selective ACR-toxin for citrus leaf spot pathogen of Alternaria alternata rough lemon pathotype. Trapping of zinc by RlemMT1-recombinant protein or by a zinc-scavenging agent in the culture medium caused suppression of ACR-toxin production by the fungus. Since ACR-toxin is the disease determinant for A. alternata rough lemon pathotype, addition of RlemMT1 to the inoculum suspension led to a significant decrease in symptoms on rough lemon leaves as a result of reduced ACR-toxin production from the zinc trap around infection sites. RlemMT1-overexpression mutant of A. alternata rough lemon pathotype also produced less ACR-toxin and reduced virulence on rough lemon. This suppression was caused by an interruption of zinc absorption by cells from the trapping of the mineral by RlemMT1 and an excess supplement of ZnSO₄ restored toxin production and pathogenicity. Based on these results, we propose that zinc adsorbents including metallothionein likely can act as a plant defense factor by controlling toxin biosynthesis via inhibition of zinc absorption by the pathogen.     

Improved Transformation Efficiency in Citrus by Plasmolysis Treatment

Procedures for high efficiency production of transgenic citrus plants using an Agrobacterium tumefaciens system with plasmolysis treatment were developed. Longitudinally cut epicotyl segments of eight different citrus species [’Milam’ Rough lemon (Citrus jambhiri Lush), ‘Volkamer’ lemon (Citrus volkameriana L), Rangpur lime (Citrus limonia L), ‘Hamlin’ sweet orange (Citrus sinensis L Osbeck), ‘Duncan’ grapefruit (’Citrus paradisi’ Macf), Sour orange (Citrus aurantium L), ‘Cleopatra’ mandarin (Citrus reticulata Blanco) and Carrizo citrange (Citrus sinensis L Osbeck x Poncirus trifoliata L Raf) ] were plasmolyzed in different concentrations of sucrose and maltose [0, 3, 6, 8, 9, 10, 12 % (w/v) ] prior to Agrobacterium inoculation. Plasmolyzed epicotyl explants were cocultivated with either the hypervirulent Agrobacterium tumefaciens strain, the EHA-101 (harboring a binary vector pGA482GG) or Agl-1 (carrying pCAMBIA1303 vector). Both binary vectors contained neomycin phosphotransferase II (NPT II) and β-glucuronidase (GUS) genes. The binary vector, pCAMBIA1303 also contained a fused mGFP5 gene at the 3’ end of GUS gene as a reporter. Epicotyl explants of Rangpur lime, Rough and ‘Volkamer’ lemons plasmolyzed in 9–12 % maltose showed transient GUS gene expression comprising up to 95 % of the cut surface of explants, while Carrizo citrange showed 80 % expression when they were plasmolyzed in 6–10 % sucrose. On the other hand, epicotyl explants of ‘Hamlin’ sweet orange, Grapefruit, Sour orange and ‘Cleopatra’ mandarin showed transient GUS expession in 80–90 % of explants with 6–10 % sucrose. Basal portions of the regenerated putative transgenic shoots harvested from the cut surface of epicotyl explants within 2–3 months, were assayed for GUS, and apical portions were shoot-tip grafted in vivo for the production of whole plants. The transformation efficiencies in different species obtained are the highest so far reported for citrus.     

Identity and Behavior of Xylem-Residing Bacteria in Rough Lemon Roots of Florida Citrus Trees

An aseptic vacuum extraction technique was used to obtain xylem fluid from the roots of rough lemon (Citrus jambhiri Lush.) rootstock of Florida citrus trees. Bacteria were consistently isolated from vascular fluid of both healthy and young tree decline-affected trees. Thirteen genera of bacteria were found, the most frequently occurring genera being Pseudomonas (40%), Enterobacter (18%), Bacillus, Corynebacterium, and other gram-positive bacteria (16%), and Serratia (6%). Xylem bacterial counts fluctuated seasonally. Bacterial populations ranged from 0.1 to 22 per mm³ of root tissue (about 10² to 2 × 10⁴ bacteria per g of xylem) when bacterial counts were made on vascular fluid, but these numbers were 10- to 1,000-fold greater when aseptically homogenized xylem tissue was examined similarly. Some of the resident bacteria (4%) are potentially phytopathogenic. It is proposed that xylem bacteria have an important role in the physiology of citrus.     

Comparative sensitivity of 'Prior Lisbon' lemon and 'Valencia' orange trees to foliar sodium and chloride concentrations

Abstract While citrus rootstocks differ in capacity for sodium and chloride ion exclusion, citrus scion species also vary in foliar sensitivity to NaCl salinisation. Of two common scions, ‘Lisbon’ lemon appears more sensitive, whereas ‘Valencia’ orange in less sensitive to leaf salt. In an attempt to explain this difference. ‘Valencia’ orange (Citrus sinensis [L.] Osbeck) and ‘Prior Lisbon’ lemon (Citrus limon [L.] Burm. F.) were budded to rootstocks known to differ in their ability to exclude sodium ions viz, the strong excluder Trifoliata (Poncirus trifoliata [L.] Raf.), and the weaker excluder Troyer citrange (C. sinensis×P. trifoliata); neither rootstock shows strong exclusion of chloride ions. Budded trees were held under a photosynthetic photon flux density of 450 μmol m ² S ¹ and watered with nutrient solution containing either 0 or 50 mol m ³ NaCl. Growth and photosynthetic responses were measured over 58 d following onset of salinization: salinity effects on leaf gas exchange were studied in relation to changes in leaf water status, compatible solutes and foliar content of sodium and chloride ions, over that same period. Once root-zone salinization began to influence leaf solutes (day 30 onwards), lemon showed a steeper increase in leaf chloride than occurred for orange. Although rootstock differences were without effect on this ingress of chloride ions for either scion, sodium ions were excluded from both scions to a larger extent by Trifoliata than by Troyer citrange. Carbon dioxide assimilation of scion foliage was reduced earlier and to a much larger extent by rootzone salinization in lemon than in orange. Furthermore, comparisons of CO₂ assimilation in relation to leaf tissue solutes between scions (on either rootstock) showed stronger responses for both sodium and chloride ions in lemon than in orange. Faster ingress of chloride into lemon leaves was identified as the crucial factor which predisposed towards expression of that contrast between scions. Although contrasts between scions in photosynthetic responses to salinization matched a faster ingress of chloride into lemon than into orange leaves, the sharper photosynthetic response of ‘Prior Lisbon’ lemon to salinity was not solely attributable to higher concentrations of chloride ions (cell sap basis). A difference between species in subcellular compartmentation of the chloride ion under saline conditions was invoked.     

Relationships of leaf Fatty acids to cold hardening of citrus seedlings

Three cultivars of citrus with different sensitivities to freezing temperatures (citron, Citrus medica L.; rough lemon, C. limon Burm. F; sour orange, C. aurantium L.) were cold hardened for 4 weeks. Lipids from leaves of hardened and control seedlings were fractionated and analyzed for fatty acids. The absolute amount of triglycerides and phospholipids increased in the leaves upon hardening. With hardening, total linoleic acid also increased 141% in citron, 210% in rough lemon, and 233% in sour orange. Specific increases in linoleic acid were found in triglycerides, in the four phospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylglycerol), and in neutral lipids more polar than triglycerides. Trans-3-hexadecenoic acid was found only in phosphatidylglycerol.     

Increased Arginine Biosynthesis during Phosphorus Deficiency : A Response to the Increased Ammonia Content of Leaves

The accumulation of arginine in leaves of four citrus rootstock cultivars during P deficiency has been demonstrated to be due to increased de novo synthesis rather than decreased catabolism or increased protein degradation (E Rabe, CJ Lovatt, 1984, Plant Physiol 76: 747-752). In this report, we provide evidence (a) that the increased activity of the arginine biosynthetic pathway observed for citrus rootstocks grown under P-deficient conditions for 7 months is due to an increase in the concentration of ammonia in leaves of P-deficient plants and (b) that ammonia accumulation and removal through arginine systhesis are early responses to phosphorus deficiency for both a woody perennial, rough lemon (Citrus limon), and an herbaceous annual, summer squash (Cucurbita pepo). Transferring 5-day-old squash plants to a phosphorus-deficient nutrient solution for only 10 days resulted in a 2-fold increase in the concentration of nitrate in the youngest fully expanded leaves (YFE). Concomitantly, the specific activity of nitrate reductase doubled and the ammonia content of P-deficient YFE leaves increased to a concentration significantly greater that of leaves from healthy control plants (P < 0.05). Consistent with increased availability of ammonia, the incorporation of NaH¹⁴CO₃ into arginine plus urea doubled during phosphorus deficiency and arginine accumulated. Despite the accumulation of nitrate and ammonia in YFE leaves during phosphorus deficiency, the total nitrogen content of these leaves was less than that of the healthy control plants. Similar results were obtained for rough lemon. Nitrate content of the YFE leaves increased 1.5- and 3.0-fold in plants deprived of phosphorus for 6 and 12 weeks, respectively. Ammonia content of the leaves increased as P deficiency progressed to 1.4 ± 0.08 mg (± se, n = 4) per gram dry weight, a level 1.8-fold greater than that of the P-sufficient control plants. During P deficiency de novo arginine biosynthesis in rough lemon increased 10-fold. Immersing the petiole of YFE leaves from P-sufficient squash and rough lemon plants in 50 millimolar NH₄⁺ for 3 hours resulted in the accumulation of ammonia in the leaves, and a 4-fold increase in the incorporation of NaH¹⁴CO₃ into arginine plus urea. Taken together, these results provide strong evidence that the accumulation of nitrate and ammonia in leaves is an early response of both woody and herbaceous plants to P deprivation. The data are consistent with the hypothesis that increased de novo arginine biosynthesis in leaves during P deficiency is in response to ammonia content of the leaves.     

Distribution ofGeotrichum candidum citrus race in citrus groves and non-citrus fields in Japan

The presence ofGeotrichum candidum citrus race, the citrus sour rot pathogen, was examined in soils of citrus groves and non-citrus fields of Japan. Soil samples were collected from 223 sites (118 sites in citrus groves, and 105 sites in fields cultivated with 33 species of non-citrus plants and in evergreen broad-leaved forest) in 11 main citrus-growing prefectures, and Hokkaido, a non-citrus-growing area. Of 236 soil samples from citrus groves, 95.76% containedG. candidum citrus race and 0.42% contained the non-citrus race; while of 210 samples from non-citrus fields, 62.85% and 4.76% contained the citrus race and the non-citrus race respectively. All of the citrus race isolates obtained either from citrus groves or non-citrus fields were pathogenic on lemon (Citrus limon) and satsuma mandarin (Citrus unshiu), but some of these isolates failed to infect orange (Citrus sinensis). The non-citrus races were pathogenic on ripe tomato fruit (Lycopersicon esculentum) and ripe muskmelon fruit (Cucumis melo var.reticulatus). Results indicated that citrus sour rot pathogen is widely distributed in citrus groves and non-citrus fields of diverse plant species in Japan.     

Growth responses and vascular plugging of citrus inoculated with rhizobacteria and xylem-resident bacteria

Summary Bacteria, isolated from roots (xylem tissue) of healthy and Young Tree Decline (YTD, Blight)-affected citrus trees, and also from nursery seedlings, were screened for potential pathogenicity by the tobacco hypersensitive reaction (HR). A majority (>75%) of the HR positive strains were classified as nonfluorescent pseudomonads. These HR positive strains were subsequently inoculated into rough lemon (Citrus jambhiri Lush.) and sweet orange (C. sinsensis Osbeck) seedlings or into Valencia sweet orange budded on rough lemon root-stock. Many of the HR positive pseudomonads reduced fresh weights (up to 94%) of roots and shoots and some reduced xylem water conductance and caused scion dieback. There was no evidence of necrosis or root rot in inoculated roots. A few HR negative Pseudomonas and Enterobacter strains significantly, but less severely, inhibited (to 43%) root growth of sweet orange seedlings. HR negative mutants derived from HR positive strains were considerably less inhibitory. Postinoculation stresses (dark and cold) markedly decreased susceptibility of seedlings to bacterial-induced inhibition. Evidence of cultivar-specific effects was obtained in comparable inoculations of rough lemon and sweet orange seedlings. Soil application of a fluorescent pseudomonad, which alone was growth stimulatory, intensified inhibitory effects of nonfluorescent, growth inhibitory, psuedomonads. This study demonstrates that many rhizobacteria isolated from xylem tissue of roots have detrimental effects on citrus.     

Study on Citrus Response to Huanglongbing Highlights a Down-Regulation of Defense-Related Proteins in Lemon Plants Upon ‘Ca. Liberibacter asiaticus’ Infection

Citrus huanglongbing (HLB) is a highly destructive disease of citrus presumably caused by ‘ Candidatus Liberibacter asiaticus ’ (Las), a gram-negative, insect-transmitted, phloem-limited α-proteobacterium. Although almost all citrus plants are susceptible to HLB, reports have shown reduced susceptibility to Las infection in lemon ( Citrus limon ) plants. The aim of this study is to identify intra-species specific molecular mechanisms associated with Las-induced responses in lemon plants. To achieve this, comparative 2-DE and mass spectrometry, in addition to Inductively Coupled Plasma Spectroscopy (ICPS) analyses, were applied to investigate differences in protein accumulation and the concentrations of cationic elements in leaves of healthy and Las-infected lemon plants. Results showed a differential accumulation of 27 proteins, including an increase in accumulation of starch synthase but decrease in the production of photosynthesis-related proteins in Las-infected lemon plants compared to healthy plants. Furthermore, there was a 6% increase (P > 0.05) in K concentration in leaves of lemon plants upon Las infection, which support results from previous studies and might represent a common response pattern of citrus plants to Las infection. Interestingly, contrary to reports from prior studies, this study showed a general reduction in the production of defense-related pathogen-response proteins but a 128% increase in Zn concentration in lemon plants in response to Las infection. Taken together, this study sheds light on general and intra-species specific responses associated with the response of citrus plants to Las.     

Prediction of citrus canker epidemics generated through different inoculation methods

Citrus canker epidemics were generated with 10^8?cfu/ml of Xanthomonas axonopodis pv. citri (ex Hasse) on Citrus limonia cv. China lemon, Citrus reticulate cv. kinnow, Citrus jambhiri, Citrus reticulate cv. Feutral’s early and Citrus limettioides using four inoculation techniques. Natural inoculum was also relied upon for infection. Overall, the injection infiltration method led to maximum disease generation followed by spray, pinprick and smear inoculation methods. Citrus canker incidence along with environmental data were recorded and subjected to stepwise regression analysis. Except relative humidity, the relationship of weekly air temperature (maximum and minimum), rainfall and wind speed with citrus canker disease development in all citrus cultivars was positively correlated and best explained by linear regression. Overall, two environmental variable model containing maximum and minimum air temperature fit the data well explaining 93% variability in disease development. The observed citrus canker incidence values and those predicted by the model were close in most of citrus cultivars. This two environmental variable model can be used to issue advance warning forecasts for the timely management of the citrus canker in Pakistan.     

Molecular systematics of citrus-associated Alternaria species

The causal agents of Alternaria brown spot of tangerines and tangerine hybrids, Alternaria leaf spot of rough lemon and Alternaria black rot of citrus historically have been referred to as Alternaria citri or A. alternata. Ten species of Alternaria recently were described among a set of isolates from leaf lesions on rough lemon (Citrus jambhiri) and tangelo (C. paradisi × C. reticulata), and none of these isolates was considered representative of A. alternata or A. citri. To test the hypothesis that these newly described morphological species are congruent with phylogenetic species, selected Alternaria brown spot and leaf spot isolates, citrus black rot isolates (post-harvest pathogens), isolates associated with healthy citrus tissue and reference species of Alternaria from noncitrus hosts were scored for sequence variation at five genomic regions and used to estimate phylogenies. These data included 432 bp from the 5' end of the mitochondrial ribosomal large subunit (mtLSU), 365 bp from the 5' end of the beta-tubulin gene, 464 bp of an endopolygalacturonase gene (endoPG) and 559 and 571 bp, respectively, of two anonymous genomic regions (OPA1-3 and OPA2-1). The mtLSU and beta-tubulin phylogenies clearly differentiated A. limicola, a large-spored species causing leaf spot of Mexican lime, from the small-spored isolates associated with citrus but were insufficiently variable to resolve evolutionary relationships among the small-spored isolates from citrus and other hosts. Sequence analysis of translation elongation factor alpha, calmodulin, actin, chitin synthase and 1, 3, 8-trihydroxynaphthalene reductase genes similarly failed to uncover significant variation among the small-spored isolates. Phylogenies estimated independently from endoPG, OPA1-3 and OPA2-1 data were congruent, and analysis of the combined data from these regions revealed nine clades, eight of which contained small-spored, citrus-associated isolates. Lineages inferred from analysis of the combined dataset were in general agreement with described morphospecies, however, three clades contained more than one morphological species and one morphospecies (A. citrimacularis) was polyphyletic. Citrus black rot isolates also were found to be members of more than a single lineage. The number of morphospecies associated with citrus exceeded that which could be supported under a phylogenetic species concept, and isolates in only five of nine phylogenetic lineages consistently were correlated with a specific host, disease or ecological niche on citrus. We advocate collapsing all small-spored, citrus-associated isolates of Alternaria into a single phylogenetic species, A. alternata.     

Growth and root hydraulic conductivity of several citrus rootstocks under salt and polyethylene glycol stresses

The water relations responses to salt of several important citrus rootstocks such as Swingle citrumelo, sour orange, and Milam lemon have not been studied in detail before. Studies were set up to compare growth and root hydraulic properties of these rootstocks to other citrus rootstocks by exposing them to NaCl and polyethylene glycol (PEG) stresses. Seedlings of 7 citrus rootstocks were irrigated for 5 months with nutrient solutions containing NaCl or PEG that had been adjusted to osmotic potentials of -0.10, -0.20 or -0.35 MPa. The 7 rootstocks studied were sour orange (Citrus aurantium), Cleopatra mandarin (Citrus reticulata Blanco), Swingle citrumelo (C. paradisi x P. trifoliata), Carrizo citrange (C. sinensis x P. trifoliata), rough lemon (Citrus jambhiri Lush), Milam lemon (C. jambhiri hybrid), and trifoliate orange (Poncirus trifoliata [L.] Raf.). In both shoot and root growth, Cleopatra mandarin and sour orange were the least sensitive to salt, Milam and trifoliate orange were the most sensitive, and rough lemon, Swingle, and Carrizo were intermediate in sensitivity. Even though the roots were exposed to solutions of equal osmotic potentials, plant growth and root conductivity were reduced more by the PEG treatments than the corresponding NaCl treatments. At -0.10 and -0.20 MPa, shoot and root dry weights were reduced 16 to 55% by NaCl and 24 to 68% by PEG. Shoot root ratio was lowered at the higher concentrations, particularly by PEG. There was a major decrease in root conductivity caused by NaCl at -0.10 MPa (19 to 30% in sour orange and Cleopatra mandarin and 78 to 85% in trifoliate orange and Milam). Conductivity decreased more at -0.20 and -0.35 MPa, but not proportionally as much as at -0.10 MPa. Root weight per unit length increased at the higher salt levels, particularly in trifoliate orange. Water flow rate through root systems followed the same trend as root conductivity; salt affected sour orange and Cleopatra mandarin the least and trifoliate orange and Milam the most. However, reductions in fibrous root length by salt treatment differed. Root lengths of Swingle and Carrizo were least affected by salt while sour orange. Milam, and rough lemon were the most affected. Hence, even though sour orange and Cleopatra mandarin were more tolerant than the other rootstocks in terms of water flow rate or root conductivity, these 2 rootstocks showed a proportionally greater decrease in root length than Carrizo, Swingle, or trifoliate orange.     

Citrus Tree Decline Caused by Pratylenchus coffeae

The pathogenic effects of Pratylenchus coffeae on growth and yield of tangelo (Citrus paradisi × C. reticulata) scions grafted on rough lemon (C jambhiri), sour orange (C. aurantium) and ''Cleopatra'' mandarin (C. reticulata) rootstocks were evaluated under field conditions for 4 years. Pratylenchus coffeae on inoculated trees increased to significantly damaging population densities on rough lemon rootstock the second year, on sour orange the third and on Cleopatra mandarin the fourth year after planting. Mean growth reduction of P. coffeae-infected trees after 4 years was 80, 77 and 49%, respectively, for the three rootstocks. Noninoculated trees on rough lemon and sour orange rootstocks yielded significantly more fruit than comparable inoculated trees. Natural migration of P. coffeae occurred horizontally on roots for a distance of 4.5 m.     

Exploring non-host plant-based management strategy with lemongrass,garlic and guava volatiles for the African citrus triozid

The African citrus triozid (ACT), Trioza erytreae, is an important pest of citrus. Both nymphs and adults damage the plant by feeding on the sap causing young shoots to die. Trioza erytreae also vectors Candidatus Liberibacter africanus, the bacteria that cause citrus greening disease. Since certain non-host plants are known to repel insect pests, it is important to investigate how such plants can be exploited to manage T. erytreae. Here, we screened effects of odours of three non-host plants namely guava (Psidium guajava), garlic (Allium sativum) and lemongrass (Cymbopogon citratus) against T. erytreae's location of a common host plant, rough lemon (Citrus jambhiri) and showed that repellence varied interspecifically with the plants. Using cage assays, we found that guava and garlic decreased the attraction of females but not males of T. erytreae to rough lemon volatiles. Chemical analysis by coupled gas chromatography/mass spectrometry (GC/MS) showed that volatiles of three of the plants were dominated by terpenoids; guava (69% comprised of limonene, 34%, (E)-β-ocimene, 29% and (Z)-β-Ocimene, 6%), lemongrass (56% comprised of geranial, 26%, neral, 19% and myrcene, 11%) and rough lemon (74% comprised of limonene, 53%, sabinene, 11% and (E)-β-ocimene, 10%). On the other hand, the volatile profile of garlic was dominated by benzenoids and saturated compounds (85% comprised of benzaldehyde, 12%, benzyl alcohol, 17%, nonanal, 31%, decanal, 13% and hexadecane, 12%). Our results suggest that non-host plant volatile composition and richness in specific compounds may contribute to influencing T. erytreae response to its host, with garlic and guava as potential non-host plants that can be exploited in the management of the pest.