Field effectiveness of fosetyl-AI against citrus foot rot and brown rot1

The effectiveness of fosetyl-A1 against citrus foot rot caused by Phytophthora citrophthora has been evaluated in a 25-year-old orchard of sweet orange cv. Tarocco, showing severe symptoms of the disease and in a 10-year-old orchard of the clementine-type mandarin cv. Monreal, apparently healthy. All the trees were grafted on sour orange. In both orchards, three sprays at 200 g a.i. per 100 1 were applied in May, July and September for 3 years. The results were evaluated on the basis of yield and fruit quality. Trees of cv. Tarocco had yield increased by 25–44% whereas cv. Monreal yielded 3–16% higher than the unsprayed trees. Fruits of orange cv. Tarocco were collected from trees sprayed with fosetyl-A1 and plunged in a water suspension of P. citrophthora. Fruits treated 11 days before inoculation showed an infection rate of 8.7%) whereas those unsprayed were 77% infected. Fruits inoculated 21 days after the treatment with fosetyl-A1 were 32% infected whereas those unsprayed were 91%, infected. In other trials, trees of volkamer lemon were sprayed with a conidial and mycelial suspension of P. citrophthora at different times after application of fosetyl-A1. The number of infected fruits and leaves in the unsprayed trees was very high (up to total leaf drop) and decreased sharply with the number of sprays (1 to 3).     

Histological Comparison of Fibrous Root Infection of Disease-Tolerant and Susceptible Citrus Hosts by Phytophthora nicotianae and P. palmivora

ABSTRACT Phytophthora nicotianae and P. palmivora infect and cause rot of fibrous roots of susceptible and tolerant citrus rootstocks in Florida orchards. The infection and colonization by the two Phytophthora spp. of a susceptible citrus host, sour orange (Citrus aurantium), and a tolerant host, trifoliate orange (Poncirus trifoliata), were compared using light and electron microscopy. Penetration by both Phytophthora spp. occurred within 1 h after inoculation, regardless of the host species. No differences were observed in mode of penetration of the hypodermis or the hosts' response to infection. After 24 h, P. palmivora had a significantly higher colonization of cortical cells in susceptible sour orange than in tolerant trifoliate orange. Intracellular hyphae of both Phytophthora spp. were observed in the cortex of sour orange, and cortical cells adjacent to intercellular hyphae of P. palmivora were disrupted. In contrast, the cortical cells of sour orange and trifoliate orange adjacent to P. nicotianae hyphae and the cortical cells of trifoliate orange adjacent to P. palmivora were still intact. After 48 h, the cortical cells of both hosts adjacent to either Phytophthora spp. were disrupted. After 48 and 72 h, P. palmivora hyphae colonized the cortex of sour orange more extensively than the cortex of trifoliate orange; P. palmivora also colonized both hosts more extensively than P. nicotianae. A higher rate of electrolyte leakage among host-pathogen combinations reflected the combined effects of greater cell disruption by P. palmivora than by P. nicotianae, and the higher concentration of electrolytes in healthy roots of trifoliate orange than of sour orange. Although cellular responses unique to the tolerant host were not observed, reduced hyphal colonization by both pathogens in the cortex of trifoliate orange compared with sour orange is evidence for a putative resistance factor(s) in the trifoliate orange roots that inhibits the growth of Phytophthora spp.     

Relationships between agronomic factors and epidemics of Phytophthora branch canker of citrus in southwestern Spain

Phytophthora branch canker, caused by Phytophthora citrophthora, has been an increasing problem in clementine (Citrus reticulata) production in Spain during last years. The disease was particularly severe in the new citrus-growing areas of the southwestern coastal areas in Huelva Province. Recent studies revealed that disease emergence was not related to either genetic drift or host specificity changes in P. citrophthora population. Therefore, the possible association of agronomic factors with the disease was investigated. A total of 110 orchards were selected arbitrarily from the main citrus-growing areas in Huelva Province. The presence of branch cankers together with agronomic factors including soils, cultivars, rootstocks, irrigation, pruning, techniques to improve fruit production, fungicide treatments, presence of brown rot of fruit and frost damage were recorded. Logistic regression analysis was used to detect correlations between the agronomic factors studied and disease prevalence. Phytophthora branch canker was significantly associated with mature clementine orchards. Sweet orange and hybrid cultivars as well as young clementine orchards were less affected by the disease. Although disease was less frequent in Salorthid soils, alternative high resolution procedures are required to draw conclusions about the effect of soil properties on disease prevalence. As in other Phytophthora-induced diseases, soil flooding during the rainy season was correlated positively with the prevalence of branch cankers. Improving fruit production by branch scoring showed a strong positive correlation with Phytophthora branch canker. This is the first time that girdling has been associated with Phytophthora disease epidemics on a fruit tree crop, but further research is needed to determine the cause of this relationship. Cultural practices including pruning, regulated deficit irrigation, additional phosphonate sprays, and abiotic and disease factors such as frost damage and presence of brown rot of fruit were not significantly correlated with disease prevalence.     

Role of the Helix aspersa snail as a vector of Phytophthora citrophthora causing branch cankers on clementine trees in Spain

This study investigated the suspected role of invertebrate vectors in the transmission of phytophthora branch canker, a severe disease of clementine cultivars in Spain, caused by Phytophthora citrophthora . Ants ( Lasius grandis ) and snails ( Helix aspersa and Rumina decollata ) were collected in spring and autumn 2005 from 15 commercial citrus fields which were severely affected by the disease. Isolations made from L. grandis and R. decollata bodies did not yield positive results. However, P. citrophthora was isolated from 5·0% of bodies of H. aspersa and 4·8% of samples of their faeces. In one assay, after snails were allowed to feed for 5 h on citrus branches which had been artificially infected with P. citrophthora , the pathogen was isolated from 79% of their faeces. In another experiment, snails were infested by placing them in contact with a substrate colonized by P. citrophthora and then transferred to the base of potted 4-year-old trees of cvs Clemenules, Fortune and Nova in the glasshouse. One day after their release, infested snails were widely distributed throughout the tree canopies and 10 days later bark discoloration and gum exudations were observed on the trees. Phytophthora citrophthora was readily isolated from tissues showing symptoms.     

Phytophthora crown rot of almonds in Australia 1

Phytophthora crown rot, caused primarily by Phytophthora cambivora, has caused considerable losses in almond orchards in the Willunga and northern plains areas approximately 50 km north and south of Adelaide (AU), respectively. Other Phytophthora species including P. citrophthora, P. cryptogea, P. megasperma and P. syringae have also been associated with diseased trees. Tree losses have been associated with P. cactorum isolated from trunk cankers on scaffold limbs on trees in the Riverland almond-growing area approximately 350 km north-east of Adelaide. The Al mating type of P. cambivora was the most pathogenic to almond seedlings. Cvs Mission and Chellaston, which are commonly used as rootstocks, were highly susceptible to crown rot whereas peach cv. Nemaguard was resistant. An excised twig assay has been developed to screen micropropagated shoots for resistance to P. cambivora. Metalaxyl applied at 5 g active substance per tree in spring, winter and autumn in a shallow trench close to the trunk prevented the development of trunk cankers. Foliar sprays of phosphonate (H₃PO₃) at 2 g active substance per litre in autumn and spring also inhibited canker development.     

Characterization of Phytophthora spp. Causing Outbreaks of Citrus Brown Rot in Florida

ABSTRACT Epidemics of citrus brown rot from 1994 to 1997 in the south-central and east-coast citrus areas of Florida were characterized and the causal Phytophthora spp. identified. Two species of Phytophthora, P. palmivora and P. nicotianae, were consistently associated with brown rot. Epidemics caused by P. palmivora appeared to be initiated on immature fruit dropped on the orchard floor. The soilborne fungus infected and sporulated on these fruit and was then disseminated to fruit above 1 m in the canopy. In contrast, infection by P. nicotianae, the common cause of root rot, was confined to the lowest 1 m of the canopy. Fruit infected by P. palmivora produced large amounts of ellipsoidal sporangia available for splash dispersal, whereas those infected by P. nicotianae produced far fewer spherical sporangia. Isolates from brown rot epidemics were compared with P. nicotianae from citrus in Florida and Texas, P. citrophthora in California, P. palmivora, and selected Phytophthora spp. from other hosts. Brown rot symptoms produced by the different pathogenic citrus isolates on inoculated fruit were indistinguishable. Morphology, mating behavior, and isozyme patterns of brown rot isolates from 1988 to 1997 matched P. palmivora from citrus roots, other host plants, and other locations, but were different from characterized isolates of P. citrophthora in California and P. nicotianae in Florida and Texas. Cellulose acetate electrophoresis of the isozyme glucose-6-phosphate isomerase rapidly identified the causal citrus pathogen from infected fruit and soil isolation plates. Although P. palmivora is an aggressive pathogen of citrus roots, bark, and fruit, populations in orchard soils were low compared with P. nicotianae.     

新疆主要农作物疫霉菌种类鉴定

 1993~1995年,对侵染新疆主要农作物的疫霉菌进行了较全面的调查研究,共从9个地区的28种作物上采集表现根腐、根颈腐、果腐症状(包括病土)的病样1531个,结果从其中17种作物上分离到452个疫霉菌株。根据形态特征、生理生化特性、致病性和菌体可溶性蛋白电泳测定,鉴定为7个种:辣椒疫霉(Phytophthora capsici Leon.),恶疫霉[P.cactorum(Leb.et Cohn) Schroeter],掘氏疫霉(P.drechsleri Tucker),菸草疫霉(P.nicotianae van Breda de Haan),苎麻疫霉(P.boehm eriaeSaw.),柑桔褐腐疫霉[P.citrophthora (Sm.et Sm.) Leonian],隐地疫霉(P.cryptogea Pethyb.etLaff.)。这些疫菌是造成新疆茄果类、瓜类、棉花、苹果、梨、枸杞、草霉、红花、白术等幼苗及成株大量死亡及烂果的主要病原,在农业生产中具有十分重要的意义。     

Protection of Citrus Rootstocks Against Phytophthora spp. with a Hypovirulent Isolate of Phytophthora nicotianae

ABSTRACT Phytophthora root rot of citrus in Florida is caused by Phytophthora nicotianae and P. palmivora. A naturally occurring isolate of P. nicotianae (Pn117) was characterized as hypovirulent on citrus roots. Pn117 infected and colonized fibrous roots, but caused significantly less disease than the virulent isolates P. nicotianae Pn198 and P. palmivora Pp99. Coincident inoculation of rootstock seedlings of Cleopatra mandarin (Citrus reticulata) or Swingle citrumelo (C. paradisi x Poncirus trifoliata) with the hypovirulent Pn117 and the virulent isolates Pn198 and Pp99 did not reduce the severity of disease caused by the virulent Phytophthora spp. When either rootstock was inoculated with the hypovirulent Pn117 for 3 days prior to inoculation with virulent isolates, preinoculated seedlings had significantly less disease and greater root weight compared with seedlings inoculated with the virulent isolates alone. Recovery of the different colony types of Phytophthora spp. from roots of sweet orange (C. sinensis) or Swingle citrumelo was evaluated on semiselective medium after sequential inoculations with the hypovirulent Pn117 and virulent Pp99. Pn117 was isolated from roots at the same level as the Pp99 at 3 days post inoculation. Preinoculation of Pn117 for 3 days followed by inoculation with Pp99 resulted in greater recovery of the hypovirulent isolate and lower recovery of the virulent compared with coincident inoculation.     

Seasonal susceptibility of citrus scions to <Emphasis Type="Italic">Phytophthora citrophthora</Emphasis> and <Emphasis Type="Italic">P. nicotianae</Emphasis> and the influence of environmental and host-linked factors on infection development

Three citrus scions were evaluated to determine seasonal changes in susceptibility to infections by Phytophthora citrophthora and Phytophthora nicotianae. In a period of 24 months, the Clementine mandarin cv. Hernandina, the hybrid Fortune mandarin and the sweet orange cv. Lane-Late were branch-inoculated under field and laboratory conditions. Field studies showed that the cultivars inoculated with P. citrophthora developed the highest lesion areas during March–June (spring) and September–October (autumn) and with P. nicotianae from June to August (summer). However, lesion areas on detached citrus branches did not show a definite pattern of infection because lesion sizes fluctuated irregularly during the study. The lesion area caused by P. nicotianae in different citrus scions correlated significantly with the monthly mean maximum values of temperature, relative humidity, and the percentage of the relative water content in the 24-month period of inoculations. In contrast, there was no correlation between these variables and the extent of colonisation by P. citrophthora. Nevertheless, a significant relationship was observed between lesion areas caused by P. citrophthora from October to May of each year and the same variables that were significant in inoculations with P. nicotianae. Seasonal changes in the susceptibility of citrus cultivars to P. citrophthora and P. nicotianae may facilitate timing of disease control measures to coincide with periods when disease development is greatest.     

Phyllocnistis citrella and its parasitoids in three citrus species in Greece

The relative abundance as well as the percentage of parasitism of Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae) and its parasitoids were studied in a citrus orchard with orange, mandarin and lemon trees in Greece. Infestation of P. citrella on each citrus host, as expressed by the number of mines per leaf, was significantly higher on lemon than on mandarin. The P. citrella parasitoid complex included the native species Neochrysocharis formosa and Pnigalio pectinicornis, as well as the introduced Citrostichus phyllocnistoides. The most abundant of those was N. formosa in all of the citrus species. The average percentage of parasitism was 13.1%, 13.8% and 11.7% on orange, mandarin and lemon, respectively. No significant differences in parasitism rate by each of the three parasitoid species separately were recorded among the three citrus species.     

Phytophthora species in oak ecosystems in Turkey and their association with declining oak trees

From 1999 to 2001, a survey on the occurrence of Phytophthora spp. in the rhizosphere soil of healthy and declining oak trees was conducted in 51 oak stands in Turkey. Seven Phytophthora spp. were recovered from six out of the nine oak species sampled: P .  cinnamomi , P .  citricola , P .  cryptogea , P .  gonapodyides , P .  quercina , Phytophthora sp. 1 and Phytophthora sp. 2. The most frequently isolated species, P .  quercina , was very common on slopes susceptible to drought. It occurred in four different climatic zones and on six Quercus spp., suggesting that it is native to oaks. The second most common species, P .  citricola , was separated into three subgroups: type C was recovered only in Anatolia, whereas A and B occurred only in the European part of Turkey. Phytophthora cinnamomi was recovered at one site only, and may not be involved in oak decline in Turkey. The other four species were recovered sporadically. On affected sites there was a significant association between deteriorating crown status and the presence of Phytophthora spp., particularly P .  quercina . The occurrence of Phytophthora species was significantly influenced by soil pH. Stem inoculation tests on oak seedlings revealed that Q .  petraea was the most susceptible species.     

Variability among Spanish citrus tristeza virus isolates revealed by double-stranded RNA analysis

A survey for citrus tristeza virus (CTV) strains, based on double-stranded RNA (dsRNA) analysis, was carried out in five locations on the eastern citrus-growing area of Spain. CTV was recovered from 137 trees of different ages, citrus species and varieties, sampled in 53 orchards. The best months for dsRNA recovery were April, May, September, October, and November, and the highest dsRNA yield was obtained from sweet orange cultivars. Sixteen dsRNA profiles differing by the number and/or position of subgenomic bands were detected. One of these profiles was detected in more than half the trees analysed. Maximum diversity of dsRNA patterns was found in the location with the oldest citrus orchards and the highest CTV incidence (Alzira-Carcaixent). In many instances, several dsRNA profiles were detected in neighbouring trees of the same orchard, notably in Alzira-Carcaixent, where 70% of the plots sampled contained more than one profile. The possible causes for the diversity of CTV isolates found in this specific area are discussed.     

Molecular analysis of the major Phytophthora species on cocoa

The internally transcribed spacer (ITS) regions of the ribosomal RNA (rRNA) gene cluster of 161 isolates of Phytophthora species involved in pod rot, stem canker and leaf blight of cocoa were analysed to determine inter- and intraspecific variation in this disease complex. The species P. palmivora , P. megakarya , P. capsici , P. citrophthora and P. nicotianae could all be clearly distinguished by PCR amplification of the ITS region followed by restriction analysis with Hae III, Hinf I, Pvu II and Alu I. This method provided a relatively rapid identification procedure for these species, and was able to distinguish isolates that had previously been misidentified by morphological methods. Sequence analysis showed that the four main cocoa-associated species formed two distinct groups, one comprising P. capsici and P. citrophthora , and the other P. palmivora and P. megakarya . Detailed sequence analysis and comparison with published literature suggested that P. capsici isolates from cocoa may be closely related to P. tropicalis , a species recently described from Cyclamen and Dianthus .     

Relative significance of nursery infections and orchard inoculum in the development and spread of apple canker (Nectria galligena) in young orchards

Three nurseries produced apple rootstocks (M9) and budwood (cv. Royal Gala), which they exchanged at the end of the first year. Each nursery then budded its own budwood onto the rootstocks it had produced and that from the other two nurseries. Budded trees were grown on for a further year before being planted at HRI, East Malling in southern England; NIHPBS, Loughgall in Northern Ireland; and ADAS, Rosemaund in the West Midlands of England. Canker development was monitored twice a year. The position of the infected trees within the orchard was recorded, as was the position of the canker on each tree (main-stem or peripheral). Nectria galligena was isolated from representative cankers and analysed using molecular techniques. At the sites in Northern Ireland and HRI there was a strong positional effect, especially of peripheral cankers, indicating that most of the inoculum was external and had been spread from neighbouring orchards. There was little or no positional effect on main-stem cankers at any of the three sites. The proportions of different isolates taken from peripheral cankers was different in Northern Ireland from that in England, suggesting different populations associated with the geographic areas. In contrast, the populations of N. galligena obtained from main-stem cankers were very similar in England and Northern Ireland. It was concluded that a small proportion of trees developing canker were infected during propagation, with no symptom development until after planting. In a second trial it was demonstrated that trees infected during the propagation phase, and particularly at budding and heading back, could develop canker up to 3 years later. While it is clear that some canker developing in the orchard can be associated with the nursery of production, in climatic conditions conducive to the formation and dissemination of conidia, inoculum from surrounding infected orchards is the primary source of the pathogen. Aerial spread is therefore an essential element of the epidemiology of N. galligena, and its control is a crucial part of any canker-control programme.     

四川柑桔根腐病病原真菌种类研究

 本文报道四川柑桔根腐病的病原真菌12种,其中腐皮镰孢(Fusarium solani)、寄生疫霉(Phytophthora parasitica)、柑桔褐腐疫霉(P. citrophthora)、德氏腐霉(Pythium debaryanum)和腐霉-1(Pythium sp.-1)等5种为主要种群,其余7种为次要种群。     

Effect on Resistance to Tylenchulus semipenetrans of Hybrid Citrus Rootstocks Subjected to Continuous Exposure to High Population Densities of the Nematode

Experiments simulating interplanting of resistant rootstocks with susceptible rootstocks that maintain high population densities of Tylenchulus semipenetrans in field soil were carried out in microplots at two locations, and in an naturally infested orchard. Selections of Cleopatra mandarin (03) × Poncirus trifoliata (01) 03.01.5 and 03.01.13, Citrus volkameriana (23) × P. trifoliata 23.01.17, Troyer citrange (02) × Cleopatra mandarin 02.03.24, Troyer citrange × Common mandarin (04) 02.04.18, King mandarin (05) × P. trifoliata 05.01.7, and Carrizo citrange were exposed to continuous high population densities of a population of the Mediterranean biotype of T. semipenetrans. The selection 23.01.17 retained its resistance in the microplots and in the field (< 1.2% females and eggs per gram fibrous root of those on Carrizo citrange). The selection 03.01.5 also retained its resistance in the microplots at Moncada (< 0.5% females and eggs per gram fibrous root of those on Carrizo citrange) but numbers of females and eggs per gram fibrous root were 27% and 22% at Amposta, and 139% and 18% in the orchard of those on Carrizo citrange, respectively. The selection 05.01.7 supported equal number of females and 43% eggs per gram fibrous root of those on Carrizo citrange in the nematode-infested orchard. The remaining selections supported high populations of T. semipenetrans.     

Alternaria Brown Spot of Minneola in Greece; Evaluation of Citrus Species Susceptibility

During the last three years, a new disease was observed in northwestern Greece on Minneola trees, hybrid of mandarin and grapefruit. On May small brown necrotic leaf spots surrounded by yellow halo areas of various sizes appeared and covered a major portion of the leaves with extension of necrosis into the veins. On young fruits small, slightly depressed black spots were the first symptoms, which later became 2–7 mm in diameter. Brown spots were observed on the leaves and fruits in several orchards in the same area, causing leaves and fruits to drop. In some orchards over 50% of the fruits were affected. From the fruit and leaf spots the typical small-spore species Alternaria alternata was isolated. Pathogenicity tests were performed by artificially inoculating fruits of Minneola, common mandarin and Clementine. The symptoms of the disease were reproduced only on fruits of Minneola hybrids by the specific strain of the fungus Alternaria alternata pv. citri. Different citrus susceptibility tests indicated that mandarins Minneola, Nova and Page were very susceptible to tested isolates while Clementine SRA and Poros Clementine were not. All lemons and lime Seedless were not susceptible. Grapefruit New Hall was not susceptible, while the Star Ruby was. Orange Lane Late, Navel Late, Oval Poros, Olinda, Navel Athos were not susceptible and only Moro showed reaction being slightly susceptible only to one isolate.     

Monitoring of <Emphasis Type="Italic">Phytophthora</Emphasis> species on fruit trees in Bulgaria

Disease on fruit trees in Bulgaria caused by Phytopthora cactorum and P. citrophthora was found in the period 1998–1999. Leaves of some trees become reddish during July, and later in the season fall off. Infected trees die during the same season, or the next season. Observations on symptom development and spread of Phytophthora root and crown rot of fruit trees was undertaken from 1999 to 2009. Disease incidence is between 2% and 14% in some gardens and nurseries. The disease was registered in the regions of Plovdiv, Kjustendil, Sliven, Yambol, Karnobat, Bourgas and Svishtov. Samples from infected plant tissues were taken and isolations were done on selective PARP media, or by applying a baiting bioassay. Based on morphological and cultural characteristics and temperature requirements the following Phytophthora species have been identified: Phytophthora cactorum, P. citrophthora, P. drechsleri, P. cryptogea, hybrid and Pythium. Pathogenicity of the isolates was tested on green apple fruits or one-year-old apple rootstocks. Laboratory studies of the effect of temperature on mycelia growth showed that most isolates can grow from 5° up to 30°C, with an optimum from 18° to 25°C. Only three strains grew at 35–36°C, two developed slowly, one grew well. The optimal pH for mycelia development was tested. Aiming at control of disease, in vivo pot trials have been carried out for studying resistance of rootstocks to P. cactorum. At the end of the growing season a good level of resistance has been shown in the rootstocks M29C, Gizela 6, and MAXMA 14.