Showing papers in "Plant Disease in 2007"

First Report of Alternaria dianthicola Causing Leaf Blight on Withania somnifera from India.

Abstract: Withania somnifera (L.) Dunal, a potential medicinal plant used for the treatment of nervous disorders, intestinal infection, leprosy, and cancer, is a perennial herb belonging to Solanaceae and distributed throughout the drier parts of India. Leaf blight disease of this plant generally occurs during March in various districts of South Bengal, India. At the initial stage of infection, symptoms appear as small, light brown spots, gradually becoming irregular, dark brown, concentrically zonate with a diffuse margin, frequently surrounded by light yellow haloes, conspicuous brownish concentric rings in the advance stage of infection. A species of Alternaria was isolated from the lesions. The pathogen was isolated on potato dextrose agar (PDA) media. On PDA, the fungus grew slowly with colonies reaching approximately 35 to 40 mm in diameter in 7 days when incubated at 30°C. Conidiophores arose singly or in groups, straight or flexous, cylindrical, septate, pale to olivaceous brown, as much as 155 μm long, 4 t...

Characterization of seedling infection types and adult plant infection responses of monogenic Sr gene lines to race TTKS of Puccinia graminis f. sp. tritici.

Abstract: Stem rust, caused by Puccinia graminis f. sp. tritici, historically was one of the most destructive diseases of wheat and barley. The disease has been under effective control worldwide thr...

Re-emergence of Cassava Brown Streak Disease in Uganda.

Abstract: During November 2004, veinal chlorosis on mature cassava leaves, typical of cassava brown streak disease (CBSD), was observed at Mukono in central Uganda. Five out of 11 cultivars at the site showed CBSD symptoms (incidence range 4 to 64%). In a survey of farmers' fields, CBSD was observed in Wakiso and Mukono districts. Incidence of cassava mosaic disease was also recorded and averaged 60% for landraces (range 16.7 to 100%) and 20% for resistant varieties (range 0 to 65%). Leaf samples of plants with CBSD symptoms produced an amplicon of 222 bp using reverse transcription-polymerase chain reaction with primers that amplify a fragment of the coat protein (CP) gene of Cassava brown streak virus. Sequence comparisons based on the amplified CP gene fragment indicated that the isolates have 77 to 82.9% nucleotide and 43.9 to 56.8% amino acid identity with those from Mozambique and Tanzania. There was 95.9 to 99.5% nucleotide and 85.1 to 90.5% amino acid identity among the Ugandan isolates. These results confirm the re-emergence of CBSD in Uganda after it was first observed in the 1930s in cassava introduced from Tanzania and controlled by eradication. Prior to this report, CBSD was known to be restricted to the coastal lowlands of East Africa.

Resistance of Botrytis cinerea Isolates from Vegetable Crops to Anilinopyrimidine, Phenylpyrrole, Hydroxyanilide, Benzimidazole, and Dicarboximide Fungicides.

Abstract: During February 2005, 55 single-spore isolates of Botrytis cinerea were collected at the end of the season from vegetable crops grown in 18 greenhouses on the island of Crete, Greece. They were tested for sensitivity to the anilinopyrimidine fungicides pyrimethanil and cyprodinil, the hydroxyanilide fungicide fenhexamid, the phenylpyrrole fungicide fludioxonil, the dicarboximide fungicide iprodione, and the benzimidazole fungicide carbendazim. Results of the study showed the existence of benzimidazole- and dicarboximide-resistant strains at frequencies of 61.8 and 18%, respectively. Moreover, for first time, the development of resistance to anilinopyrimidine fungicides by B. cinerea was detected in greenhouse vegetable crops on the island of Crete. High resistance frequencies of 49.1 and 57.4% were observed for pyrimethanil and cyprodinil, respectively. In addition, one isolate was found to be resistant to the hydroxyanilide fungicide fenhexamid, while no strains resistant to the phenylpyrrole fu...

Resistance to Boscalid Fungicide in Alternaria alternata Isolates from Pistachio in California

Abstract: Boscalid is a new carboxamide fungicide recently introduced in a mixture with pyraclostrobin in the product Pristine for the control of Alternaria late blight of pistachio. In all, 108 isolates of Alternaria alternata were collected from pistachio orchards with (59 isolates) and without (49 isolates) prior exposure to boscalid. The sensitivity to boscalid was determined in conidial germination assays. The majority of isolates from two orchards without a prior history of boscalid usage had effective fungicide concentration to inhibit 50% of spore germination (EC50) values ranging from 0.089 to 3.435 μg/ml, and the mean EC50 was 1.515 μg/ml. Out of 59 isolates collected from an orchard with a history of boscalid usage, 52 isolates had EC50 values ranging from 0.055 to 4.222 μg/ml, and the mean EC50 was 1.214 μg/ml. However, in vitro tests for conidial germination and mycelial growth also revealed that seven A. alternata isolates, originating from the orchard exposed to boscalid were highly resistant (EC50 > 100 μg/ml) to this fungicide. Furthermore, in vitro tests showed no significant differences between wild-type and boscalid-resistant mutants in some fitness parameters such as spore germination, hyphal growth, sporulation, or virulence on pistachio leaves. Experiments on the stability of the boscalid-resistant phenotype showed no reduction of the resistance after the mutants were grown on fungicide-free medium. Preventative applications of a commercial formulation of boscalid (Endura) at a concentration which is effective against naturally sensitive isolates failed to control disease caused by the boscalid-resistant isolates in laboratory tests. To our knowledge, this is first report of field isolates of fungi resistant to boscalid.

Characterization of Pythium spp. Associated with Corn and Soybean Seed and Seedling Disease in Ohio.

Abstract: Cool, moist conditions in combination with minimum tillage, earlier planting, and recent shifts in commercial fungicide seed-treatment active ingredients have led to an increase in corn (Zea mays) and soybean (Glycine max) seedling establishment problems. This situation resulted in an investigation of Pythium spp. associated with seed and seedling diseases. Samples of diseased corn and soybean seedlings were collected from 42 production fields in Ohio. All isolates of Pythium recovered were identified to species using morphological and molecular techniques and evaluated in an in vitro pathogenicity assay on both corn and soybean seed, and a subset of the isolates was tested for sensitivity to fungicides currently used as seed treatments. Eleven species and two distinct morphological groups of Pythium were identified, of which six species were moderately to highly pathogenic on corn seed and nine species were highly pathogenic on soybean seed. There was significant variation (P < 0.05) in sensitivity to mefenoxam, azoxystrobin, trifloxystrobin, and captan both across and within species. Multiple species of Pythium had the capacity to reduce germination of both corn and soybean seed. Results indicated that mefenoxam, azoxystrobin, trifloxystrobin, or captan, when used individually, may not inhibit all pathogenic species of Pythium found in Ohio soils.

Synergistic Interaction of Sweet potato chlorotic stunt virus (Crinivirus) with Carla-, Cucumo-, Ipomo-, and Potyviruses Infecting Sweet Potato.

Abstract: Untiveros, M, Fuentes, S, and Salazar, L F 2007 Synergistic interaction of Sweet potato chlorotic stunt virus (Crinivirus) with carla-, cucumo-, ipomo-, and potyviruses infecting sweet potato Plant Dis 91:669-676 Co-infection of Sweet potato chlorotic stunt virus (SPCSV, genus Crinivirus) with Sweet potato feathery mottle virus (SPFMV, genus Potyvirus) results in sweet potato virus disease (SPVD), a synergistic disease that is widely distributed in the sweet potato (Ipomoea batatas) growing regions of the world Since both SPCSV and SPFMV are common and often detected as part of multiple co-infections of severely diseased plants, the occurrence of synergistic interactions with other viruses was investigated Data from this study show that SPCSV, but not SPFMV, can cause synergistic diseases in sweet potato with all viruses tested, including members of the genus Potyvirus (Sweet potato latent virus, Sweet potato mild speckling virus), Ipomovirus (Sweet potato mild mottle virus), Cucumovirus (Cucumber mosaic virus), and putative members of the genus Carlavirus (Sweet potato chlorotic fleck virus and C-6 virus) The synergism was expressed as an increase in the severity of symptoms, virus accumulation, viral movement in plants, and as an effect on yield of storage roots The presence of a third different virus in plants affected with SPVD increased the severity of symptoms even further compared with SPVD alone There was a positive correlation between increase in virus accumulation and symptom expression in double and triple SPCSV-associated co-infections The epidemiological implications of the results are discussed

NAPPFAST: An Internet System for the Weather-Based Mapping of Plant Pathogens.

Abstract: In recent years, the number of exotic pest introductions has increased rapidly as a result of increased volume of trade (22). The serious and sometimes irreparable ecological and economic damage of exotic pathogens, such as Cryphonectria parasitica, Ophiostoma novo-ulmi, and Phytophthora ramorum, the causal agents of chestnut blight, Dutch elm disease, and Sudden Oak Death, respectively, are amply documented (1,6,42). An estimate of annual losses for exotic plant pathogens is $21 billion dollars (32). The Plant Protection and Quarantine (PPQ) (Sidebar 1) division within the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (USDA-APHIS) has the goal of safeguarding agriculture and natural resources from the risks associated with the entry, establishment, and spread of exotic pathogens. Two important components of the APHIS-PPQ mission are risk analysis and pest detection. A key goal of the risk analysis program is to identify exotic pest pathways and to assess the risks these exotic pests pose to plants and plant products as well as to the environment. Three types of risk assessments that evaluate the probability of the introduction and establishment of exotic plant pests are pathway analysis, organism pest risk assessment, and commodity risk assessment. The PPQ pest detection program and its state cooperators provide a continuum of pest surveillance, from offshore preclearance programs through port inspections, to surveys in rural and urban sites across the United States. The Center for Plant Health Science and Technology (CPHST) and the Cooperative Agricultural Pest Survey (CAPS) programs are instrumental in APHIS-PPQ’s pest detection programs. CAPS is responsible for supplying a means of detection, documentation, and rapid dissemination of information regarding the survey of regulated significant plant pests and weeds in the United States. The survey information gathered by CAPS is entered into a central database known as National Agricultural Pest Information System (NAPIS). CPHST, headquartered in Raleigh, NC, is a multi-program scientific support organization for PPQ. One way CPHST scientists help facilitate the APHIS-PPQ activities of risk analysis and pest detection is by mapping the potential introduction and establishment of exotic pathogens in the United States. These maps are the result of pathogen-specific information analyses, including climate, pathogen distribution, host distribution, and trade. Given its influence on pest phenology, reproduction, dispersion, and overwintering survival, climate is a critical component for the geographic assessment of potential pathogen distribution. A large number of climate-based risk mapping systems, such as CLIMEX, BIOCLIM, and GARP, have been used for pest risk analysis (3,10,38,44). Literature typically focuses on the development and/or evaluation of the best modeling techniques (10); however, often the quality of the inputs, including biological parameters, weather

Evaluation of Fusarium graminearum Associated with Corn and Soybean Seed and Seedling Disease in Ohio.

Abstract: Fusarium graminearum is an important pathogen of cereal crops in Ohio causing primarily head blight in wheat and stalk and ear rot of corn. During the springs of 2004 and 2005, 112 isolates of F. graminearum were recovered from diseased corn and soybean seedlings from 30 locations in 13 Ohio counties. These isolates were evaluated in an in vitro pathogenicity assay on both corn and soybean seed, and 28 isolates were tested for sensitivity to the seed treatment fungicides azoxystrobin, trifloxystrobin, fludioxonil, and captan. All of the isolates were highly pathogenic on corn seed and moderately to highly pathogenic on soybean seed. Fludioxonil was the only fungicide that provided sufficient inhibition of mycelial growth; however, several fludioxonil-resistant mutants were identified during the sensitivity experiments. These results indicate that F. graminearum is an important pathogen of both corn and soybean seed and seedlings in Ohio, and that continued use of fludioxonil potentially may select for less sensitive isolates of F. graminearum.

Comparative Efficacy of the New Postharvest Fungicides Azoxystrobin, Fludioxonil, and Pyrimethanil for Managing Citrus Green Mold

Abstract: Three new fungicides (i.e., azoxystrobin, fludioxonil, and pyrimethanil) are currently being introduced for postharvest management of citrus green mold in the United States. The effectiveness of each fungicide was evaluated when applied alone (at 1,000 to 1,200 mg/liter) or in mixtures (at 500 mg/liter each component) to lemon fruit that were wound-inoculated with imazalil/thiabendazole (TBZ)-sensitive or -resistant isolates of Penicillium digitatum. In laboratory studies when aqueous fungicide solutions were applied 9 to 21 h after inoculation, pyrimethanil showed the highest level of green mold control. The efficacy of fludioxonil and azoxystrobin was very high at the early timings, but decreased as time after inoculation increased. Differences in fungicide performance were not due to multiple fungicide resistance, but more likely due to differences in fungicide mobility in fruit tissue. Azoxystrobin-fludioxonil mixtures were significantly more effective when compared to single-fungicide treatments. Mixtures of imazalil with pyrimethanil were the most effective in controlling decay. The efficacy of all fungicides was significantly lower when mixed into a packing fruit coating as compared to aqueous or storage fruit coating applications. In laboratory and packingline studies, the lowest incidence of green mold decay was obtained when azoxystrobin-fludioxonil and imazalil-pyrimethanil were applied as aqueous solutions that were followed by a fruit coating. Among the new fungicides, azoxystrobin and fludioxonil applied in water or storage fruit coating, respectively, provided the best anti-sporulation activity. Storage fruit coating improved the activity of both fungicides. Pyrimethanil was the least effective fungicide in suppressing sporulation of the pathogen on decaying fruit. Overall, among the mixtures, azoxystrobin-fludioxonil and TBZ-fludioxonil had high anti-sporulation activity in aqueous and storage fruit coating applications. New integrated management programs should be based on monitoring of fungicide sensitivities in pathogen populations, rotating mixtures of products with different modes of action, and using appropriate fungicide application strategies.

Phytophthora spp. associated with forest soils in eastern and north-central U.S. oak ecosystems

Abstract: A survey of soils associated with oak species was conducted in 2003 and 2004 in Indiana, Illinois, Maryland, Michigan, Minnesota, Pennsylvania, Ohio, West Virginia, and Wisconsin to investigate the occurrence of Phytophthora spp. Soils taken from around the base of healthy and declining oak trees were flooded with H2O and Quercus robur leaflets were used as bait for Phytophthora spp. From 829 soil samples collected near trees, 21% were positive for Phytophthora spp., with 55% of the 125 sites surveyed yielding a Phytophthora sp. Phytophthora cinnamomi was the most frequently isolated species, representing 69.4% of the Phytophthora-infested sites surveyed. Other species, in decreasing order of isolation frequency were Phytophthora sp. 2, P. citricola, P. europaea, P. cambivora, P. quercina-like isolates, and Phytophthora sp. 1. No significant association was found between the presence of Phytophthora organisms and site characteristics such as latitude, elevation, soil pH, or the crown condition of the trees. However, in P. cinnamomi-infested sites, a significant association was found with the deteriorating crown status of Q. alba and the presence of P. cinnamomi. The absence of P. cinnamomi above the 40°N latitude range also was noteworthy.

International Fungicide Efficacy Trials for the Management of Soybean Rust

Abstract: The efficacy of fungicides in managing soybean rust was evaluated in 12 environments in South America and southern Africa over three growing seasons from 2002 to 2005. There were differenc...

Evaluation of Essential Oils and Their Components for Broad-Spectrum Antifungal Activity and Control of Late Leaf Spot and Crown Rot Diseases in Peanut

Abstract: Clove oil, cinnamon oil, and five essential oil components (citral, eugenol, geraniol, limonene, and linalool) were tested for growth inhibition of 14 phytopathogenic fungi. Citral completely inhibited the growth of Alternaria alternata, Aspergillus flavus, Curvularia lunata, Fusarium moniliforme, F. pallidoroseum, and Phoma sorghina in paper disc agar diffusion assays. Cinnamon oil, citral, and clove oil as low as 0.01% (vol/vol) inhibited the spore germination of Cercospora arachidicola, Phaeoisariopsis personata, and Puccinia arachidis by >90% in vitro. Limonene and linalool were observed to be the least antifungal against the test fungi and were not used in further studies. Clove oil (1% vol/vol) applied as a foliar spray 10 min before Phaeoisariopsis personata inoculation reduced the severity of late leaf spot of peanut up to 58% when challenge inoculated with 104 conidia ml-1. This treatment was more effective (P = 0.01) than 0.5% (vol/vol) citral, cinnamon oil, or clove oil and 1% (vol/vol...

Rapid Determination of Rice Cultivar Responses to the Sheath Blight Pathogen Rhizoctonia solani Using a Micro-Chamber Screening Method

Abstract: An accurate greenhouse screening method has not been developed previously to identify host response to sheath blight disease caused by Rhizoctonia solani Kuhn that causes significant economic losses in rice yield worldwide. The unavailability of a robust screening system in the greenhouse has made it difficult to quantify disease reactions to R. solani, and has hampered studies on the genetics of resistance and plant breeding efforts to improve resistance. In an effort to develop a standardized laboratory micro-chamber screening method to quantify resistance to R. solani in rice, five rice cultivars, representing a wide range of observed disease reactions under field conditions, were examined in a blind inoculation test at three locations (Arkansas, Texas, and Colombia). Rice seedlings were inoculated at the three- to four-leaf stage with potato dextrose agar plugs containing mycelium and then covered with a 2- or 3-liter transparent plastic bottle for maintaining high humidity after inoculation. Two cultivars, Jasmine 85 and Lemont, that consistently have shown the highest and lowest levels of resistance, respectively, in previous field and greenhouse studies, were used as standards. Concurrent field experiments in Arkansas and Texas also were performed to compare the greenhouse disease ratings with those observed under field conditions. Overall, the relative disease ratings of the seven test cultivars were consistent between test locations and with field evaluations. Thus, the micro-chamber screening method can be used as an effective approach to accurately quantify resistance to the sheath blight pathogen under controlled greenhouse conditions and should help expedite the selection process to improve resistance to this important pathogen.

Susceptibility of grapevine pruning wounds and symptom development in response to infection by Phaeoacremonium aleophilum and Phaeomoniella chlamydospora

Abstract: Fresh to 10-month-old pruning wounds on grapevine cvs. Thompson Seedless and Cabernet Sauvignon were inoculated with Phaeoacremonium aleophilum or Phaeomoniella chlamydospora at approximately 1 × 106 spores/ml. Successful infection was determined by isolation of the pathogen from necrotic margins of cankers or from vascular discoloration assessed 4 months after each inoculation date. Disease incidence decreased as the length of time between pruning and inoculation increased; however, wounds remained susceptible for up to 4 months. Vascular discoloration was significantly less extensive in the noninoculated control than in inoculated spurs. Reduction of shoot length in both cultivars varied depending on pruning wound age at the time of inoculation. In a separate study, inoculation of fresh pruning wounds of Cabernet Sauvignon resulted in successful infection regardless of the time of pruning from February to December.

Fungicidal Activity of Plant Volatile Compounds for Controlling Monilinia laxa in Stone Fruit

Abstract: Nine plant-volatile compounds were tested for their activity against Monilinia laxa, the cause of brown rot in stone fruit. In vitro trials on conidial germination and mycelial growth showed a consistent fungicidal activity of trans-2-hexenal, carvacrol, and citral, whereas trans-cinnamaldehyde, hexanal, (-)-carvone, eugenol, 2-nonanone, and p-anisaldehyde exhibited a progressively lower inhibition. The best inhibitor of conidial germination was trans-2-hexenal (effective dose for 50 and 90% inhibition [ED50 and ED95] = 7.53 and 9.4 μl/liter, respectively; minimal inhibitory concentration [MIC] = 12.3 μl/liter], whereas carvacrol was the best inhibitor of mycelial growth (ED50 and ED95 = 2 and 3.4 μl/liter, respectively; MIC = 6.1 μl/liter). The three most active compounds in in vitro studies also were tested in vivo as postharvest biofumigants. The best control of brown rot was with trans-2-hexenal (efficacy ranging from 46.2 to 80.3%, depending on cultivar), whereas citral and carvacrol resulte...

Molecular detection of Plasmodiophora brassicae, causal agent of clubroot of crucifers, in plant and soil

Abstract: Cao, T., Tewari, J., and Strelkov, S. E. 2007. Molecular detection of Plasmodiophora brassicae, causal agent of clubroot of crucifers, in plant and soil. Plant Dis. 91:80-87. Clubroot of crucifers, caused by Plasmodiophora brassicae, recently has been identified in canola (Brassica napus) fields in Alberta, Canada. An effective strategy for managing the disease is to avoid planting cruciferous crops in P. brassicae-infested soil, because the pathogen produces resting spores that can remain infectious for many years. A simple, one-step polymerase chain reaction (PCR) protocol was developed to detect the pathogen in plant and soil samples. The primers TC1F and TC1R, based on a P. brassicae partial 18S ribosomal RNA (rRNA) gene sequence from GenBank, yielded a 548-bp product in the optimized PCR. A second pair of primers, TC2F and TC2R, which amplified a fragment of the 18S and internal transcribed spacer (ITS) 1 regions of the rDNA repeat, also was tested and produced a 519-bp product. Neither set of primers amplified any DNA fragment from noninfected plant hosts, noninfested soil, or common soil fungi and bacteria tested in this study. Quantities of 100 fg or less of total P. brassicae DNA, or 1 × 10 3 resting spores per gram of soil, could be detected consistently using these primers and PCR protocol, corresponding to an index of disease of 11% or lower when the soil was bioassayed. The protocol also enabled detection of P. brassicae in symptomless root tissue 3 days after inoculation with the pathogen. Therefore, the PCR assay described in this study could provide a reliable diagnosis for routine detection of P. brassicae in plant and soil materials in a specific and rapid manner.

Diseases of Tropical Perennial Crops: Challenging Problems in Diverse Environments.

Abstract: The world’s oldest ecosystems are found in the tropics. They are diverse, highly evolved, but barely understood. Diseases that impact crops in these regions can be significant contraints to production, especially when they occur in lowland environments with high rainfall and uniform, warm temperatures; respites from disease pressure there are often infrequent. Difficulties in managing diseases in the humid tropics are multiplied when the affected crops are perennial. The favorable conditions for disease development and the presence of susceptible host tissue over long periods make diseases of tropical perennial crops serious management challenges. This topic is introduced with a few concepts on the occurrence and development of these pathosystems. Peculiar aspects and scenarios that influence the types of and extent to which different diseases develop are summarized. Measures that are useful on annual or short-term crops may be ineffective against these diseases. They are scientifically interesting problems. New vectors, as for mango malformation, or pathogens, as for bunchy top of papaya, are associated with some of the diseases. And some of the diseases are caused by two or more distinct taxa; for example, citrus greening, mango malformation, Panama disease, and tracheomycosis of coffee. Some of the most important diseases are host-specific and are caused by either coevolved or new-encounter pathogens. Resistance, the most effective tool with which many of these diseases are managed, is usually available in coevolved pathosystems but may be uncommon in new-encounter situations. Inadequate host resistance can be a significant barrier in the management of both coevolved and new encounter diseases. General tactics are described that are useful against diseases of tropical perennials. The successful management of plant disease utilizes several principles and practices, regardless of the host and environment in which it is grown. These include the avoidance, exclusion, and eradication of the causal agents. Host protection is of great importance, as is the identification and incorporation of resistance in the host plant. All of these approaches are discussed with tropical perennial examples.

Impact of Sclerotinia stem rot on yield of canola

Abstract: Sclerotinia sclerotiorum is the causal agent of Sclerotinia stem rot (SSR) of canola (Brassica napus). In North Dakota, the leading canola producer in the United States, SSR is an endemic disease. In order to estimate the impact of this disease on canola yield, field experiments were conducted from 2000 to 2004 at several locations in North Dakota and Minnesota. Experimental plots were either inoculated with laboratory-produced ascospores or infected by naturally occurring inoculum in commercial fields. Applying fungicides at different concentrations and timings during the flowering period created epiphytotics of diverse intensities. Disease incidence was measured once prior to harvesting the crop on 50 to 100 plants per plot. Results of the study indicated that 0.5% of the potential yield (equivalent to 12.75 kg/ha) was lost for every unit percentage of SSR incidence (range of 0.18 to 0.96%). Considering the current cost of fungicide applications and the market value of this commodity, a 17% SSR incidence could cause losses similar to the cost of a fungicide application. Additional efforts are required to improve current levels of tolerance of canola plants to this pathogen.

Characterization of Cylindrocarpon Species Associated with Black Foot Disease of Grapevine in Spain.

Abstract: In this work, 82 Cylindrocarpon isolates associated with black foot disease of grapevines in Spain were studied by means of phenotypical characterization, DNA analyses, and pathogenicity tests Partial sequences of the beta-tubulin (BT) gene, BT1, were amplified using primers BT1a and BT1b A unique and conserved 52-bp insertion in the BT1 sequence, which is a specific marker for C macrodidymum, was found in 56 of the isolates The rest of the isolates (26) were identified as C liriodendri The BT phylogeny grouped all the isolates of each species into two well-supported clades Phenotypical data were subjected to multivariate factorial analysis According to this study, the isolates were clearly separated into two groups which were in agreement with BT species identification C macrodidymum isolates were differentiated from C liriodendri by producing fewer conidia, presenting longer and wider macroconidia, and lower growth rate at 5 and 10°C Selected isolates of each species inoculated onto rooted cuttings of grapevine rootstock cv 110 R caused typical black foot disease symptoms This is the first report of C liriodendri and C macrodidymum causing black foot disease of grapevines in Spain

Evaluation of DNA Amplification Methods for Improved Detection of "Candidatus Liberibacter Species" Associated with Citrus Huanglongbing.

Abstract: Li, W., Hartung, J. S., and Levy, L. 2007. Evaluation of DNA amplification methods for improved detection of “Candidatus Liberibacter species” associated with citrus huanglongbing. Plant Dis. 91:51-58. Citrus huanglongbing (HLB), also known as citrus greening or citrus yellow shoot, is considered the most serious disease of citrus worldwide. The disease has Asian, African, and American forms caused by “Candidatus Liberibacter asiaticus”, “Ca. L. africanus”, and “Ca. L. americanus”, respectively, which can be spread efficiently by the psyllid vectors Diaphorina citri and Trioza erytreae and through contaminated plant materials. Infected citrus groves are usually destroyed or become unproductive in 5 to 8 years. The presumed low concentration and uneven distribution of the pathogens in citrus plants and vector insects make the phloem-limited bacterium difficult to detect consistently. In this study, we compared and validated four conventional polymerase chain reaction (PCR)-based protocols, one loop-mediated isothermal amplification (LAMP) protocol, and three TaqMan real-time PCR protocols. The detection sensitivity of the validated conventional PCR assays reported are improved compared with the original protocols. All of the validated conventional and the newly developed real-time methods were reliable for confirmatory tests for the presence of “Ca. Liberibacter spp.” in symptomatic samples. There were no differences in assay specificity among the standard format PCR-based methods evaluated. The TaqMan real-time PCR was 10- to 100-fold more sensitive than conventional PCR and LAMP, showing the potential to become a valuable tool for early detection and identification of “Ca. Liberibacter spp.” prior to the appearance of disease symptoms. The methods validated in this study will be very useful for regulatory response, effective management of infected trees, and development of a “Ca. Liberibacter spp.”–free nursery system.

Genetic Diversity and Distribution of Korean Isolates of Ralstonia solanacearum

Abstract: Genetic diversity among 478 isolates of Ralstonia solanacearum collected from various plants in Korea between 1997 and 2005 was determined based on biovar, pathogenicity, amplified fragment length polymorphism (AFLP), 16S rRNA, endoglucanase, hrpB, and mutS gene sequence analyses. Of the isolates, 440 belonged to biovars 1, 3, or 4, and 38 belonged to biovar 2. Biovar N2 isolates were not found. The biovar 1 and 2 isolates were found mainly in southern Korea, whereas the biovar 3 and 4 isolates were widely distributed throughout all nine provinces. AFLP analysis divided the 109 representative Korean isolates into six clusters that were distinct from most of the foreign isolates. Grouping of 8 representative isolates based on their 16S rRNA gene sequences indicated that biovars 1, 3, and 4 belonged to division 1, while biovar 2 belonged to subdivision 2b. Sequence analysis of the endoglucanase, hrpB, and mutS genes from the same isolates indicated that the biovar 1, 3, and 4 isolates belonged to phylotype I, while the biovar 2 isolate belonged to phylotype IV. This study is the first comprehensive analysis of genetic diversity among Korean isolates of R. solanacearum.

First Report of Pitch Canker Caused by Fusarium circinatum on Pinus halepensis and P. pinea in Apulia (Southern Italy).

Abstract: Since 2005, pitch canker symptoms have been observed in Apulia (southern Italy, 41°27'42.84″N, 15°33'0.36″E) on numerous trees of Pinus halepensis and P. pinea in urban parks and gardens. Trees showed crown decline as a consequence of dieback of twigs and branches and withering of needles. Bleeding cankers with abundant resin were visible on twigs and branches. The needles of affected twigs and branches wilted, faded, turned yellow, then red, and were discarded. Isolations from symptomatic needles, twigs, and branches were performed on water agar, potato dextrose agar (PDA), and pentachloronitrobenzene medium. A species of Fusarium was consistently isolated from all infected tissues, and pure cultures were obtained by single hyphal tip transfers on PDA and synthetic nutrient agar medium (2). Colonies were incubated at 22 ± 3°C for 7 to 10 days. They produced white aerial mycelia, violet pigment, typically 3-septate macroconidia with slightly curved walls, single-celled microconidia, and characteristic sterile hyphal coils. Microconidia were ovoid or allantoid and born in false heads on aerial polyphialides. The species was identified as Fusarium circinatum Nirenberg & O'Donnell (= F. subglutinans Wollenweb & Reinking) on the basis of morphological and cultural characteristics (3). The identification was confirmed by PCR with specific primers CIRC1A/CIRC4A. The specific primer pair amplified a 360-bp DNA fragment of the two nuclear ribosomal IGS region (4). The pathogenicity of three Italian isolates of F. circinatum from Pinus spp. (Fc1640, Fc1642, and Fc1643 stored in the collection of Dipartimento Scienze Agroambientali, Chimica and Difesa Vegetale, University of Foggia) was evaluated by artificial inoculations on 2-year-old potted seedlings of P. halepensis, P. pinea, P. nigra, P. sylvestris, P. domestica, P. pinaster, P. excelsa, P. radiate, and Pseudotsuga menziesii (10 seedlings for each species and fungal isolate). Small PDA plugs from actively growing colonies of F. circinatum were introduced into a U-shaped cut on the stem of the seedlings and wrapped with moist sterile cottonwool. An equal number of control plants of each Pinus spp. was inoculated with sterile agar. All plants were grown in a nursery at ambient temperature (20 to 28°C). Within 30 days after inoculation, resinous cankers appeared on the stem of the seedlings of P. halepensis, P. pinea, P. domestica, P. pinaster, and P. radiata. Basal needles began to wilt, turn yellow, then red, and were discarded. F. circinatum was reisolated from stems of symptomatic seedlings. No symptoms were observed on seedlings of Pseudotsuga menziesii, P. sylvestris, P. excelsa, and P. nigra or on control seedlings. In Europe, pitch canker caused by F. circinatum previously has been reported only in Spain on P. radiata and P. pinaster (1). There was an unconfirmed report of this disease in Italy ( http://www.eppo.org ), but to our knowledge, this is the first definite conclusive evidence of the presence of pitch canker of pine in Italy. References: (1) E. Landeras et al. Plant Dis. 89:1015, 2005. (2) H. I. Niremberg. Mitt. Biol. Bundesanst. Land-Forstwirtsch. Berl.-Dahl, 169:1, 1976. (3) H. I. Niremberg and K. O'Donnell. Mycologia 90:434, 1998. (4) W. Schweigkofler et al. Appl. Environ. Microbiol. 70:3512, 2004.

Postharvest Decay Management for Stone Fruit Crops in California Using the "Reduced-Risk" Fungicides Fludioxonil and Fenhexamid.

Abstract: In laboratory studies and experimental packingline trials that closely simulated fungicide treatments under commercial conditions, the "reduced-risk" fungicides fenhexamid and fludioxonil were highly effective in managing postharvest brown rot and gray mold decays of peach, nectarine, and plum caused by Monilinia fructicola and Botrytis cinerea, respectively. Tebuconazole was more effective against brown rot than against gray mold. Additionally, fludioxonil and tebuconazole significantly reduced the incidence of Rhizopus rot caused by Rhizopus stolonifer. In laboratory studies, the QoI fungicide azoxystrobin was not consistent in its decay control activity. The high efficacy of fenhexamid and fludioxonil against brown rot and gray mold was substantiated by low effective concentrations necessary (≤0.063 mg/liter)for 50% inhibition of mycelial growth in vitro. In general, fungicides applied 14 to 16 h after wound inoculation were significantly more effective than those applied before inoculation. These results indicate that the fungicides act mainly as protectants that do not penetrate deeply enough into the fruit to prevent decay from wounds that extend below the fruit epidermis. Laboratory treatments of plum fruit generally were less effective than those of peach or nectarine fruit. Comparative studies using either low-volume spray or high-volume, in-line drench applications over a roller bed demonstrated that drench applications were significantly more effective in reducing postharvest decaysof inoculated plum fruit. Decay incidence using fenhexamid or fludioxonil was ≤1.1%, whereas incidence after the spray applications was between 25.2 and 40.4% for brown rot, between 12.0 and 24.3% for gray mold, or 62.6% for Rhizopus rot (fludioxonil only). This research identified effective replacements for iprodione, a fungicide voluntarily canceled by the manufacturer in 1996.

Phytophthora species associated with diseased woody ornamentals in Minnesota nurseries.

Abstract: Schwingle, B. W., Smith, J. A., and Blanchette, R. A. 2007. Phytophthora species associated with diseased woody ornamentals in Minnesota nurseries. Plant Dis. 91:97-102. Phytophthora species are responsible for causing extensive losses of ornamental plants worldwide. Recent international and national surveys for the detection of P. ramorum have led to the finding of previously undescribed Phytophthora species. Since no previous Phytophthora surveys have been carried out in Minnesota, surveys of ornamental nurseries were performed over 4 years to isolate and identify the Phytophthora species causing diseases of woody plants in Minnesota. Species were identified by direct sequencing of internal transcribed spacer (ITS) rDNA, β-tub, and mitochondrial coxI genes. Species associated with diseased ornamental plants include P. cactorum, P. cambivora, P. citricola, P. citrophthora, P. hedraiandra, P. megasperma, P. nicotianae, and the previously identified but undescribed taxon P. Pgchlamydo. The most common species encountered were P. cactorum, P. citricola, and P. citrophthora. Two additional isolates obtained did not match known species. One was similar to P. alni subsp. alni, and the other appeared to be a new species and is referred to as P. sp. MN1. In addition, species are reported for the first time from several hosts. Results indicated that several Phytophthora species were more widespread in the nursery industry than previously thought, and undescribed species were causing disease in Minnesota ornamental nurseries.

Seed Transmission of Pepino mosaic virus and Efficacy of Tomato Seed Disinfection Treatments

Abstract: Rates of seed transmission for Pepino mosaic virus (PepMV) were estimated in seedlings grown from seeds obtained from symptomatic tomato (Lycopersicon esculentum) fruits of plants naturally infected with the virus. The proportion of seeds infected with PepMV was at least 25% as estimated from enzyme-linked immunosorbent assay (ELISA) analysis of grouped seeds. The seeds from symptomatic fruits were planted, and seedlings at the cotyledon and transplant stage were assayed for PepMV by ELISA. Three of 168 seedlings grown from infected seeds were PepMV-positive, corresponding to a seed-to-seedling transmission rate of 1.84%. Various tomato seed treatments were evaluated for their ability to prevent seed transmission of PepMV. This virus was largely eradicated by immersing the seeds in 10% trisodium phosphate for 3 h. Although heat treatments of 24 h at 80°C and 48 h at 74°C eliminated PepMV in seedlings, these treatments did not eradicate the virus in whole seeds. The three treatments did not adversely affect seed germination. The results suggest that trisodium phosphate can be used to eradicate PepMV in tomato seed without hindering germination.

Spread of Tomato apical stunt viroid (TASVd) in Greenhouse Tomato Crops Is Associated with Seed Transmission and Bumble Bee Activity

Abstract: Tomato apical stunt viroid (TASVd) has been reported as a devastating pathogen of greenhouse tomato in Israel. This isolate shares 92 and 99% identity with the Ivory Coast type strain and an Indonesian strain, respectively. No information is available regarding the epidemiology of this viroid complex. The present study indicates that TASVd is not transmitted by the aphid Myzus persicae or the whitefly Bemisia tabaci, nor through root infection in infested soil. However, the results indicate that the viroid may be able to invade the embryonic tissues of the seed and transmission rates through seed may reach 80%. Moreover, it was confirmed that bumble bees (Bombus terrastris) can transmit the viroid from infected tomato source plants to healthy plants. Based on these findings, it is suggested that the primary spread of the viroid in greenhouse tomato plants is by seed transmission, and secondary distribution occurs by the pollination activity of bumble bees.

Management of soilborne diseases in strawberry using vegetable rotations

Abstract: Subbarao, K. V., Kabir, Z., Martin, F. N., and Koike, S. T. 2007. Management of soilborne diseases in strawberry using vegetable rotations. Plant Dis. 91:964-972. The influence of crop rotation on soilborne diseases and yield of strawberry (Fragaria × ananassa) was determined at a site infested with Verticillium dahliae microsclerotia and at another with no known history of V. dahliae infestation during 1997 to 2000. The rotations studied at the V. dahliae-infested site were (i) broccoli-broccoli-strawberry, (ii) Brussels sproutsstrawberry, and (iii) lettuce-lettuce-strawberry; the treatments at the site with no history of V. dahliae were (i) broccoli-broccoli-strawberry, (ii) cauliflower-cauliflower-strawberry, and (iii) lettuce-lettuce-strawberry. The effects of rotation on V. dahliae and Pythium populations, strawberry vigor, Verticillium wilt severity, and strawberry fruit yield were compared with a standard methyl bromide + chloropicrin fumigated control treatment at both sites. Rotations did not alter total population levels of Pythium spp. at either study site. However, V. dahliae microsclerotia were significantly reduced with broccoli and Brussels sprouts rotations compared with lettuce rotations at the V. dahliae-infested site. Reduced propagules led to lower Verticillium wilt severity on strawberry plants in the broccoli and Brussels sprouts rotations than in lettuce-rotated plots. Strawberry vigor and fruit yield were significantly lower in lettuce-rotated plots than in broccoli- and Brussels sprouts-rotated plots. Despite no detectable microsclerotia at the other site, strawberry vigor and fruit yield were greatest in plots rotated with broccoli, intermediate with cauliflower, and lowest with lettuce. None of the rotation treatments were better than the fumigated control for all variables measured. In the absence of fumigation, rotation with broccoli and Brussels sprouts is an effective cultural practice for managing Verticillium wilt in strawberry production; whereas, in fields with no detectable V. dahliae, broccoli is also a feasible rotational crop that enhances strawberry growth and yield. According to a cost-benefit analysis, the broccoli-strawberry rotation system could be an economically viable option provided growers are able to alternate years for strawberry cultivation.