Neofusicoccum parvum and Diaporthe foeniculina associated with twig and shoot blight and branch canker of citrus in Greece

In a survey performed in Chania and Aetoloacarnania, Greece in years 2013–2014, fungal isolates causing twig and shoot blight and branch canker of citrus trees were morphologically characterized and identified by multiple gene sequence analysis. By sequencing the ITS‐5.8S rRNA, the elongation factor 1‐α (EF1‐α), the β‐tubulin and the RNA polymerase II subunit (Rpb2) genes, the isolates examined were associated with Diaporthe foeniculina (six isolates) and Neofusicoccum parvum (one isolate). All six D. foeniculina isolates showed slow colony growth rates (7.4 ± 3.2 mm/day), while the N. parvum isolate exhibited fast growth (41.6 mm/day). Koch's criteria were met after re‐isolation of D. foeniculina isolates from all inoculated Citrus spp. and N. parvum from inoculated C. reticulata “Ortanique” and after having developed symptoms similar to those detected on shoots and branches collected from citrus fields. Based on lesion length on detached C. medica “Lia Kritis” shoots, N. parvum caused long necrotic lesions (58 mm in length) in comparison with a length of 12–21 mm lesions caused by D. foeniculina isolates. Pathogenicity trials on nine Citrus spp., which had been inoculated with D. foeniculina and N. parvum, revealed different levels of susceptibility, indicating a host‐dependent infection effect, with Poncirus trifoliate × C. paradisi (“Citrumelo Swingle”) being the most resistant citrus genotype. Lack of host specificity suggests that their pathogen–host association could be attributed to ecological rather to co‐evolutionary factors. This work represents the first report, accompanied with pathogenicity tests, on botryosphaeriaceous and diaporthaceous pathogens associated with twig and shoot blight and branch canker of citrus in Greece.     

Assessment of glucosinolate-derived isothiocyanates as potential natural antifungal compounds against citrus sour rot disease agent Geotrichum citri-aurantii

In this study, the antifungal effects of six different isothiocyanate (ITCs) compounds (methyl, allyl, butyl, ethyl, benzyl and 2-phenylethyl ITCs) were investigated to be use against the citrus sour rot disease caused by Geotrichum citri-aurantii in vitro and semi-commercial (in vivo) conditions. Antifungal activities of the vapour phases of different ITC compounds were examined on the arthroconidia germination and mycelial growth of G. citri-aurantii. Mycelial growth of G. citri-aurantii was inhibited in a concentration-dependent way. The minimum inhibitory concentrations of benzyl, methyl, allyl and ethyl ITCs on mycelial growth were 0.06, 0.08, 0.10 and 0.10 µl/L, respectively. Arthroconidia germination of G. citri-aurantii was completely inhibited by benzyl, methyl, allyl and ethyl ITCs at concentrations of 0.05, 0.07, 0.07 and 0.07 µl/L, respectively. Light microscopy observations revealed that the ITC compounds, at completely inhibiting concentrations, caused considerable morphological changes in the fungal hyphae. Under in vivo conditions, the average rotting area caused by G. citri-arantii was inhibited 100% by ethyl, methyl and allyl ITC compounds at concentrations of 8.0, 12.0 and 12.0 µl/L, respectively. Results suggest that ITC’s may be useful and effective natural antifungal compounds to control the citrus sour rot disease agent.     

The relationship among host plant species,egg clutch size,and level of parasitism for the sharpshooter Tapajosa rubromarginata

The sharpshooter Tapajosa rubromarginata (Signoret) (Hemiptera: Cicadellidae, Proconiini), a vector of the bacterium Xylella fastidiosa Wells et al. (Xanthomonadaceae) that causes citrus variegated chlorosis, has more than 30 reported host plant species. The fitness of a phytophagous insect is determined by the host plant suitability, plant resistance, and the natural enemies. The aim of this study was to: (1) identify plant species utilized as oviposition substrate by T. rubromarginata in the field; (2) establish the relationship between plants and clutch size; (3) establish the relationship among host plants, clutch size, and level of parasitism; and (4) establish variations in parasitoid composition and abundance in the various host plants. Egg masses of the sharpshooter were surveyed on plants reported as hosts, or those that were abundant in the study site. The number of eggs of the sharpshooter and emerged parasitoids were recorded for all the collected masses. We found egg masses of T. rubromarginata on 12 out of 21 plant species sampled. The size of the egg masses was greatly influenced by the type of leaf venation and to a lesser extent by the plant species. Parasitism rates were influenced by both leaf venation and host plant. Trichogrammatidae species were mostly associated with egg masses in plants with parallel-veined leaves, whereas Mymaridae attacked masses laid in reticular-veined leaves. The choice between a good host plant, but heavily attacked by parasitoids, and the host plants that are less suitable for nymphs but less frequently attacked by natural enemies, was a trade-off for T. rubromarginata females to increase their fitness. We conclude that the host plant utilization by T. rubromarginata females in the field could be influenced by leaf structure and the strategy to avoid parasitism by selecting plants that were less attractive for parasitoids.     

Temporal response and attraction of Diaphorina citri to visual stimuli

As the vector of the global disease of citrus greening or huanglongbing, Asian citrus pysllids, Diaphorina citri Kuwayama (Hemiptera: Liviidae), are the greatest threat to the worldwide citrus industry. Critical to management of D. citri and huanglongbing is optimization of surveillance methodologies. Although phytophagous insects may find host plants by multimodal cues, some appear to primarily use visual cues. In this study, we examined the behavior of Asian citrus psyllids toward light from light‐emitting diodes (LEDs) in the insect visible spectrum. The periodicity of attraction of psyllids to visual cues was evaluated in the field (yellow sticky traps) and laboratory (multi‐colored LEDs) with a strong peak of activity during the afternoon in both the field and the laboratory (both 14:00 to 18:00 hours). In laboratory evaluations of psyllids to differently colored LEDs, strongest attraction was to LEDs emitting ultraviolet (390 nm), green (525 nm), and yellow (590 nm) light. Male and female psyllids did not differ significantly in their responses to visual cues. These findings provide the basis for formulating better traps that reflect UV and yellow light and potentially incorporate UV LEDs for monitoring psyllids and a better understanding of Asian citrus psyllid visual behavior.     

Detection and Inoculation of Peanut Witches' Broom Phytoplasma (16SrII‐A) and Periwinkle Leaf Yellowing Phytoplasma (16SrI‐B) in Citrus Cultivars in Taiwan

To clarify the phytoplasma associated with Huanglongbing (HLB), a detection survey of phytoplasma in field citrus trees was performed using the standardized nested PCR assay with primer set P1/16S‐Sr and R16F2n/R16R2. The HLB‐diseased citrus trees with typical HLB symptoms showed a high detection of 89.7% (322/359) of HLB‐Las, while a low detection of phytoplasma at 1.1% (4/359) was examined in an HLB‐affected Wentan pummelo (Citrus grandis) tree (1/63) and Tahiti lime (C. latifolia) trees (3/53) that were co‐infected with HLB‐Las. The phytoplasma alone was also detected in a healthy Wentan pummelo tree (1/60) at a low incidence total of 0.3% (1/347). Healthy citrus plants were inoculated with the citrus phytoplasma (WP‐DL) by graft inoculation with phytoplasma‐infected pummelo scions. Positive detections of phytoplasma were monitored only in the Wentan pummelo plant 4 months and 3.5 years after inoculation, and no symptoms developed. The citrus phytoplasma infected and persistently survived in a low titre and at a very uneven distribution in citrus plants. Peanut witches' broom (PnWB) phytoplasma (16SrII‐A) and periwinkle leaf yellowing (PLY) phytoplasma belonging to the aster yellows group (16SrI‐B) maintained in periwinkle plants were inoculated into healthy citrus plants by dodder transmission. The PnWB phytoplasma showed infection through positive detection of the nested PCR assay in citrus plants and persistently survived without symptom expression up to 4 years after inoculation. Positive detections of the phytoplasma were found in a low titre and several incidences in the other inoculated citrus plants including Ponkan mandarin, Liucheng sweet orange, Eureka lemon and Hirami lemon. None of the phytoplasma‐infected citrus plants developed symptoms. Furthermore, artificial inoculation of PLY phytoplasma (16SrI‐B) into the healthy citrus plants demonstrated no infection. The citrus symptomless phytoplasma was identified to belong to the PnWB phytoplasma group (16SrII‐A).     

Evaluating auxin distribution in tomato (Solanum lycopersicum) through an analysis of the PIN and AUX/LAX gene families

The temporal and spatial control of auxin distribution has a key role in the regulation of plant growth and development, and much has been learnt about the mechanisms that influence auxin pools and gradients in vegetative tissues, particularly in Arabidopsis. For example polar auxin transport, mediated by PIN and AUX/LAX proteins, is central to the control of auxin distribution. In contrast, very little information is known about the dynamics of auxin distribution and the molecular basis of its transport within and between fruit tissues, despite the fact that auxin regulates many aspects of fruit development, which include fruit formation, expansion, ripening and abscission. In addition, functional information regarding the key regulators of auxin fluxes during both vegetative and reproductive development in species other than Arabidopsis is scarce. To address these issues, we have investigated the spatiotemporal distribution of auxin during tomato (Solanum lycopersicum) fruit development and the function of the PIN and AUX/LAX gene families. Differential concentrations of auxin become apparent during early fruit growth, with auxin levels being higher in internal tissues than in the fruit pericarp and the pattern of auxin accumulation depended on polar transport. Ten tomato PIN (SlPIN1 to 10) and five AUX/LAX (SlLAX1 to 5) genes were identified and found to display heterogeneous expression patterns, with tissue and developmental-stage specificity. RNAi-mediated co-silencing of SlPIN4 and SlPIN3 did not affect fruit development, which suggested functional redundancy of PIN proteins, but did lead to a vegetative phenotype, and revealed a role for these genes in the regulation of tomato shoot architecture.     

10种农业化学品对柑橘内生菌枯草芽孢杆菌L1-21的影响

[目的]明确农业化学品对柑橘内生菌的影响,指导柑橘合理用药.[方法]采用平板菌落计数法,测定了6种杀虫剂、3种杀菌剂和微肥EDTA-Cu对生防菌株枯草芽孢杆菌L1-21的相容性及对柑橘植株内生菌的抑制作用.[结果]56.00 mg·L-1吡虫啉、121.80 mg·L-1虫螨腈、4.00 mg·L-1甲氨基阿维菌素、5...     

Spatial and temporal spread of Citrus tristeza virus and its aphid vectors in the North western area of Morocco

First report of Citrus tristeza virus (CTV,Closterovirus) in Morocco datesback to 1961 in collections of citrus varieties. An exhaustive survey of citrus in the north of the country in 2009 revealed that CTV was spread all over the citrus production area. We attempted to evaluate the relative contribution of different aphid species in the spread of CTV disease in a Citrus reticulata orchard at the Loukkous region during 2 years (2012 and 2013). The overall CTV incidence estimated in the experimental site increased from 17.8% in 2012 to 31.15% in 2013. The most abundant aphid species colonising clementine trees was Aphis spiraecola and A. gossypii. Both aphid species reached their maximum peaks during the spring season. The rate of viruliferous aphids, estimated by real‐time RT‐PCR of single aphid, revealed that 35.4% of winged A. gossypii and 28.8% of winged A. spiraecola were viruliferous, confirming a high inoculum pressure in the area surrounding the experimental site. The aphid species Toxoptera citricida, which is able to transmit the aggressive isolates of CTV, was not found in the Loukkous region. The study of the spatial distribution of the CTV showed that in general, the disease was randomly distributed in the field. Overall, the results seem to indicate that A. spiraecola may be considered as the major aphid species contributing to CTV spread in our experimental conditions. The prevalence of mild strains in the region and the high level of aphid flight activity could explain the rapid evolution of CTV incidence in the experimental area.