Rust Pustules Research Articles

Registration of Three Soybean Germplasm Lines Resistant to Phakopsora pachyrhizi (Soybean Rust)

Soybean rust, caused by Phakopsora pachyrhizi Sydow, is one of the most important foliar diseases of soybean [Glycine max (L.) Merr.]. Development of rust‐resistant lines is one objective of many soybean breeding programs. Three soybean germplasm lines designated as TGx 1987–76F (Reg. No. GP‐369, PI 657702), TGx 1987–118F (Reg. No. GP‐370, PI 657703), and TGx 1987–129F (Reg. No. GP‐368, PI 657701) with resistance to P. pachyrhizi and to Xanthomonas axonopodis pv. glycines (Nakano) Vauterin, Hoste, Kersters & Swings (bacterial pustule) were selected from lines developed by the International Institute of Tropical Agriculture (IITA) in collaboration with the USDA‐ARS at the University of Illinois. These lines were derived from a tropical soybean rust‐resistant cultivar UG5, and a rust‐susceptible, high‐yielding elite breeding line, TGx 1805–31F. A total of 297 F7 lines were evaluated for rust resistance under greenhouse conditions using whole inoculated plants, in the laboratory using detached leaves, and under field conditions in Nigeria and Alabama (287 lines). Based on assessments from these various tests, these three lines have combinations of high levels of rust and bacterial pustule resistance, good agronomic traits, and adaptable maturity to the southern U.S. soybean breeding programs. Breeders will find the three lines useful as rust‐resistant parental material in rust‐prone areas.

Control of Bean Rust using Antibiotics Produced by Bacillus and Streptomyces species - Translocation and Persistence in Snap Beans

Antibiotic culture filtrates produced by Bacillus (CA5) and Streptomyces spp. were tested for translocation and persistence when applied on snap beans inoculated with rust (Uromyces appendiculatus) in greenhouse pot experiments. The antibiotics were applied on the first trifoliate leaves and translocation was assessed as the number of rust pustules on non-treated leaflets or trifoliates while persistence was assessed as the number of rust pustules on rust infected plants at different times after antibiotic treatment. The treatments were replicated three times, each replicate consisting of a pot containing three plants. Antibiotics from both Bacillus and Streptomyceswere found to have up to 100% trans-lamina and leaflet-to-leaflet translocation but no significant trifoliate-totrifoliatetranslocation. The antibiotic culture filtrates also retained significant rust control for up to 10 days after application on the bean plant. However, no significant rust control was found on the plants after 16 days of treatment. The study indicated that the antibiotics produced by antagonistic Bacillus and Streptomyces species possess systemicactivity that can persist within the plant for over one week. These metabolites are potential bean rust control products that could be incorporated in integrated disease management programs

Leaf rust induced volatile organic compounds signalling in willow during the infection

Plants are known to emit volatile organic compounds (VOC) in response to various biotic or abiotic stresses. Although the VOC emission in the case of insect attacks is well described, there is only little known about the impact of pathogens on plant emission. In the present study, we used a willow-leaf rust system to describe the effects of a biotrophic fungal infection on the VOC emission pattern of willow leaves. We detected that isoprene emissions from rust-infected leaves decreased threefold compared to control. The total monoterpene emissions did not change although a stress-signalling compound (Z)-beta-ocimene showed an increase in infected plants on several days. The infection also increased the emission of sesquiterpenes and lipoxygenase products (LOX) by factors of 175-fold and 10-fold, respectively. The volatile emission signals showed two clear peaks during the experiment. At 6, 7 and 12 days post-infection (dpi), the relative volatile emission signal increased to about sixfold compared to uninfected plants. These time points are directly connected to rust infection since at 6 dpi the first rust pustules appeared on the leaves and at 12 dpi necrosis had developed around several pustules. We present correlations between LOX and sesquiterpene emission signals, which suggest at least two different steps in eliciting the volatile emission.

The influence of canopy density on willow leaf rust (Melampsora epitea) severity in willow short rotation coppice

Willow short rotation coppice is used as a renewable energy source and also as a vegetation filter for purifying wastewater. Wastewater irrigation might change microclimatic conditions and increase the canopy density in plantations, which might decrease production due to leaf rust ( Melampsora epitea ). The aim of this study was to estimate the impact of the canopy density on rust abundance on willows. For that, we counted rust pustules on leaves of five different willow clones from dense and sparse areas in both the wastewater irrigated and control part of the plantation. The results demonstrated clear differences between clones; clone ‘81090’ was very susceptible, ‘78183’ susceptible, ‘78021’ fairly tolerant and ‘Tora’ rust resistant. Clone ‘Gudrun’, which was previously reported resistant, had severe rust damages in Estonia. In the case of clones ‘78183’ and ‘78021’ there were significantly more rust pustules per leaf unit area at areas with denser canopy, which confirmed that higher plant density could result in biomass losses caused by leaf rust. No differences, however, were detected between dense and sparse areas of hybrid clone ‘Gudrun’, most probably because in this particular case leaves from upper canopy layer were used. There was a tendency detected that clones with a higher number of shoots per plant had more rust damages on their leaves, however, the correlation was not statistically confirmed. In conclusion, the impact of canopy density on rust abundance is clone-specific and significant in the case of clones on which infection starts from the lower part of the canopy.

Interactions between Melamspora larici‐epitea pathotypes and the mycoparasite Sphaerellopsis filum from willow rusts

SummaryFive pathotypes of the willow rust Melamspora larici‐epitea were inoculated with 12 isolates of Sphaerellopsis filum derived from Melampsora species/forms occurring on willows. On average, 20.5% uredinial pustules produced S. filum pycnidia and rust spore production was reduced by 38.4% on leaf discs inoculated with S. filum. Some rust isolates were more readily infected by S. filum than others while some S. filum isolates caused higher levels of infections than other S. filum isolates. In general, the suppressive effects of these S. filum isolates on rust spore production were similar on the majority of rust pathotypes tested. There appeared to be a positive link between the rust pustule area and the rate of infection by S. filum. Sphaerellopsis filum inoculum densities were positively correlated with the reduction in rust spore production but not with the number of rust pustules. Implications from the results were discussed in relation to the deployment of S. filum in biological control of willow rust in willow mixture plantations which harbour more diverse rust pathotypes compared with monocultures.

Phakopsora pachyrhizi, the causal agent of Asian soybean rust

The plant pathogenic basidiomycete fungi Phakopsora pachyrhizi and Phakopsora meibomiae cause rust disease in soybean plants. Phakopsora pachyrhizi originated in Asia-Australia, whereas the less aggressive P. meibomiae originated in Latin America. In the New World, P. pachyrhizi was first reported in the 1990s to have spread to Hawaii and, since 2001, it has been found in South America. In 2004, the pathogen entered continental USA. This review provides detailed information on the taxonomy and molecular biology of the pathogen, and summarizes strategies to combat the threat of this devastating disease. Phakopsora pachyrhizi Syd. & P. Syd; uredial anamorph: Malupa sojae (syn. Uredo sojae); Domain Eukaryota; Kingdom Fungi; Phylum Basidiomycota; Order Uredinales; Class Urediniomycetes; Family Phakopsoraceae; Genus Phakopsora (http://www.indexfungorum.org). The nomenclature of rust spores and spore-producing structures used within this review follows Agrios GN (2005) Plant Pathology, 5th edn. London: Elsevier/Academic Press. In the field, P. pachyrhizi infects leaf tissue from a broad range (at least 31 species in 17 genera) of leguminous plants. Infection of an additional 60 species in other genera has been achieved under laboratory conditions. At the beginning of the disease, small, tan-coloured lesions, restricted by leaf veins, can be observed on infected soybean leaves. Lesions enlarge and, 5-8 days after initial infection, rust pustules (uredia, syn. uredinia) become visible. Uredia develop more frequently in lesions on the lower surface of the leaf than on the upper surface. The uredia open with a round ostiole through which uredospores are released.

Relation between leaf rust (Melampsora epitea) severity and the specific leaf area in short rotation coppice willows

Willow leaf rust (Melampsora epitea) is an important leaf pathogen in short rotation coppice plantations. Although ecological factors influencing rust severity are rather well known, to date, the connections between leaf morphological traits and rust infections are poorly studied. The aim of this study was to examine the relation between rust severity and specific leaf area (SLA). Willow leaves of three different clones from waste-water-irrigated and non-irrigated plantation areas were used. The results revealed a significant positive correlation between SLA and rust severity. There were clone specific differences whereas more rust pustules were always accompanied with higher SLA. Waste-water-irrigated plants had higher SLA and always more rust damages. These results suggest that leaves with low SLA are less susceptible to leaf rust in various conditions.

Field Assessment of a Model for Fungicide Effects on Intraplant Spread of Stem Rust in Perennial Ryegrass Seed Crops

Intraplant spread of stem rust (Puccinia graminis subsp. graminicola) in perennial ryegrass during tiller extension is a major determinant of epidemic severity and is dominated by stem extension dynamics. Simple equations for extension of inflorescence and internodes are presented and parameterized. These equations are combined with previously published equations for pathogen latent period and for postinfection efficacy of fungicides to produce a model for effects of fungicide type and timing on intraplant spread. The model is driven by thermal units, calculated from air temperature measurements. Three field experiments, conducted independently from the field experiments that provided data for plant growth model parameterization, were conducted to assess performance of the disease spread model. Either propiconazole or azoxystrobin, the two most commonly used fungicides for stem rust control, was applied to tillers that had stem rust pustules on the flag sheath and in which the inflorescence was partially extended. Intraplant spread of disease to the extending inflorescence (stem and flowerhead) was observed at several dates following treatment and compared with modeled severities. The model estimated accurately the severities of inflorescence infection for most treatments and observation times, with a correlation coefficient of 0.93 for modeled versus observed disease severities across the three experiments. The model correctly estimated the rank order of final severities among the treatments (fungicide type and timing). The model can be extended to intraplant spread of stem rust at all internodes and incorporated into decision support tools for fungicide type and timing in management of this disease.

First Report of Soybean Rust Caused by Phakopsora pachyrhizi on Erythrina herbacea (Coral Bean).

Soybean rust (SBR), caused by the obligate fungus Phakopsora pachyrhizi Syd. & P. Syd., was initially reported on soybean (Glycine max L.) in Louisiana in 2004 and has since been reported on soybean and/or kudzu (Pueraria lobata (Willd.) Ohwi) in 9 states in 2005, 15 states in 2006, and 19 states in 2007 (1). The host range of P. pachyrhizi includes plants that are all in the Fabaceae or legume family. Six plant species in the United States have been reported as hosts of P. pachyrhizi: soybean, kudzu, Florida beggarweed (Desmodium tortuosum (Sw) DC.), dry bean (Phaseolus vulgaris L.), lima bean (P. lunatus L.), and scarlet runner bean (P. coccineus L.) (4). On 17 April 2008, a rust disease was observed on a weedy legume host with red showy flowers that was growing with kudzu in an overgrown vacant lot in the understory of live oak trees (Quercus virginiana Mill.) in Citra, FL. The discovery was made during routine scouting of this Integrated Pest Management Pest Information Platform for Extension and Education (IPM PIPE) mobile sentinel plot (3). The plant was confirmed by University of Florida botanists to be Erythrina herbaceae L., commonly known as coral bean. Coral bean is native to the southeastern United States and also is planted as a perennial ornamental. A sample of leaves exhibiting rust pustules characteristic of P. pachyrhizi uredinia was collected and examined with a microscope. Brown-to-brick red, angular lesions that were 3 to 11 mm in diameter (average 6.75 mm) were observed on the undersides of the leaves of two trifoliates. Within these lesions, there were several uredinia, some exuding hyaline, echinulate urediniospores (20 × 25 μm). The visual diagnosis and the species of the rust fungus were confirmed to be P. pachyrizi by a real-time PCR protocol (2). The diagnosis on this new host was verified by a USDA, APHIS National Mycologist in Beltsville, MD. Coral bean may serve as an additional overwintering host for P. pachyrhizi in the southeast. To our knowledge, this is the first report of soybean rust caused by P. pachyrhizi on E. herbaceae.

Concentration and intervals of hydrogen dioxide applications to control Puccinia hemerocallidis on daylily

Hydrogen dioxide (H 2O 2) is a disinfestant used to kill fungal spores, such as urediniospores of Puccinia hemerocallidis, on plant and production surfaces. Excised sections of daylily leaves with sporulating rust pustules were sprayed with concentrations between 19 and 270 g active ingredient (a.i.) H 2O 2 l −1. Treated spores were rubbed onto the surface of PDA, and germination of >100 spores per replication per treatment was determined 24 h later. An exponential decay model was fitted to the data and a lethal dose (LD) of 57 and 114 g a.i. H 2O 2 l −1 solution was predicted to cause 90 and 99% urediniospore mortality, respectively. In an irrigation pad study, one label concentration (2.7 g a.i. H 2O 2 l −1) and approximate LD 90 and LD 99 concentrations (54 and 108 g a.i. H 2O 2 l −1) were sprayed 1 or 2 times per week on healthy daylily plants exposed to naturally dispersing inoculum. Disease incidence and severity decreased with increasing concentration and number of applications of H 2O 2 per week. H 2O 2, at 108 g a.i. l −1 applied once per week, provided control equal to a fungicide treatment (azoxystrobin and chlorothalonil plus thiophanate-methyl rotation), but was also significantly phytotoxic. In a greenhouse study, two concentrations (2.0 and 2.7 g a.i. H 2O 2 l −1) registered for application on plants and one concentration (3.4 g a.i. H 2O 2 l −1) registered for application on production surfaces were sprayed 2, 3, and 5 times per week on healthy daylily plants exposed to naturally dispersed inoculum. In greenhouse experiment I where the maximum mean incidence was 59%, 2.7 and 3.4 g a.i. H 2O 2 l −1 applied 2–5 times per week provided control equal to the fungicide treatment. In greenhouse experiment II where the maximum mean incidence was 89%, 5 applications of H 2O 2 at 2.0–3.4 g a.i. H 2O 2 l −1 provided control that was better than the water treatment but not as good as the fungicide treatment. Higher label concentrations (2.7 g a.i. H 2O 2 l −1) and multiple applications per week (≥5) can provide disease control equal to a fungicide when disease pressure is low, but may not be effective when disease pressure is high.

Superação da resistência qualitativa da cultivar de trigo "BRS 194" por uma nova raça de Puccinia triticina

A população de Puccinia triticina, agente causal da ferrugem da folha do trigo, é extremamente dinâmica na região do Cone Sul da América do Sul, onde o surgimento de novas raças é freqüente. A cultivar de trigo "BRS 194" foi a segunda variedade com maior disponibilidade de sementes para plantio no Rio Grande do Sul e em Santa Catarina em 2005, por apresentar características como rusticidade, elevado rendimento de grãos e resistência qualitativa a todas as raças de P. triticina ocorrentes no Brasil e em outros países da América do Sul. Em 2005, pústulas de ferrugem da folha foram observadas sobre plantas desta cultivar, tanto em lavouras quanto em campos experimentais, indicando a possível superação de sua resistência. O objetivo deste trabalho foi identificar a raça de P. triticina presente em amostras oriundas de "BRS 194" e verificar se esta se tratava de uma nova virulência do patógeno, a qual teria superado sua resistência. Oito amostras de ferrugem provenientes da cultivar "BRS 194" foram enviadas em 2005 à Embrapa Trigo, Passo Fundo, Rio Grande do Sul, para isolamento e identificação por meio da Série Internacional de Hospedeiros Diferenciais específica para este patógeno. Todas as amostras apresentaram a mesma combinação de virulência, a qual correspondeu ao código MFP-CT, conforme o Sistema Norte-Americano de nomenclatura do patógeno. Esta foi a primeira vez que este código foi detectado no Brasil, caracterizando o surgimento de uma nova raça de P. triticina virulenta à "BRS 194". Outras cultivares de trigo, também amplamente semeadas, foram inoculadas com a nova raça, sendo que algumas foram suscetíveis e outras resistentes. Entre as cultivares resistentes estão "Fundacep 30" e "IPR 84", as quais permanecem também resistentes a todas as raças do patógeno ocorrentes no Brasil.

First Report of Soybean Rust Caused by Phakopsora pachyrhizi in Iowa and its Statewide Occurrence.

On 25 September 2007, soybean leaves with lesions typical of soybean rust were found in two commercial fields located in Dallas County, Iowa. Growth stage of the infected soybean plants was near physiological maturity. Diagnosis of Phakopsora pachyrhizi on leaves was confirmed by morphological observation of uredinia and urediniospores and conventional PCR conducted by the Iowa State University Plant and Insect Diagnostic Clinic using P. pachyrhizi-specific primers Ppm1 and Ppa2 as described (1). Water blanks were used as negative controls in PCR testing. Leaves were collected from additional counties throughout Iowa and examined microscopically. Soybean rust was identified on leaves from 14 counties, almost all the counties sampled, ranging from far western to far eastern Iowa. The northernmost detection was at 42.9°N in Hancock County, which also is the northernmost detection of soybean rust in the continental United States so far. In a commercial field in Fremont County, in the southwestern corner of Iowa along the Missouri River, disease incidence was approximately 20% and disease severity was 5%. Observed disease incidence was 1 to 2% and severity was less than 1% from all other samples. Most uredinia were scattered on the leaves as single pustules or clustered in groups of three to five pustules. Pustules on some leaf samples were sporulating, depending on weather conditions at the time when samples were collected. Fresh urediniospores collected from the leaf samples were placed on glass slides with free water on the surface and incubated in a dew chamber under darkness for 8 h to test for germination. Germination rates ranged from 80 to 90%. Rust spores also were used to inoculate detached soybean trifoliate leaves, which were kept in a dew chamber under darkness for 12 h with water-soaked cotton wrapped around the petioles. Typical soybean rust pustules developed within 10 to 14 days after incubation. The detections of soybean rust in Iowa were consistent with the predictions of an aerobiological spore dispersal model, which predicted spore showers in central to western Iowa in August. Above normal wet weather in the Great Plains and Iowa may have favored the statewide disease occurrence. To our knowledge, this is the first report of soybean rust in Iowa. Reference: (1) R. D. Frederick et al. Phytopathology 92:217, 2002.

Occurrence of Rust Caused by Puccinia carthami on Snow Lotus in China.

Snow lotus (Saussurea involucrata (Kar. & Kir.) Sch. Bip.) is an economically important medicinal herb increasingly grown in China in recent years. In August 2005, a rust disease on snow lotus plants commercially grown was found in the Tianshan mountain area of Xinjiang at 2,100 m above sea level. Disease incidence was approximately 15% of the plants observed in a commercial field in 2006. At the initial stage, tiny (1 to 2 mm long), orange brown pustules are formed on leaves. Later in the season, pustules turned chestnut brown to form bigger rust patches. Severely attacked leaves may die prematurely. During the growing season, rust pustules broke open to release reddish brown spores that cause secondary infection. The infected snow lotus plants were sampled, and the urediniospores and teliospores were observed for identification with a light microscope (4). Urediniospores were globose or broadly ellipsoid, 22 to 28 × 22 to 25 μm; teliospores were slightly or not constricted at the septum, 31.5 to 41.5 × 21.5 to 26.5 μm, wall was sienna to fulvous, occasionally chestnut colored, and pedicels were basal or oblique, verrucose, hyaline, and fragile. The pathogenicity test of the fungus was done by burying five leaves bearing telia around the roots of healthy safflower seedlings grown in a greenhouse under controlled conditions (25°C and 70% relative humidity) and healthy snow lotus seedlings grown under natural conditions with five replications. Symptoms were evaluated 60 days after inoculation. Similar rust symptoms were observed on both the snow lotus and safflower seedlings (1). The pycnidial and aecial stages of this autoecious rust were not observed in nature or the pathogenicity tests. The teliospores were reisolated and deposited at the Key Oasis Eco-Agriculture Laboratory of Xinjiang Production and Construction Group, Xinjiang and the Institute of Biotechnology, Zhejiang University. The causal organism of rust of snow lotus was identified as Puccinia carthami Corda on the basis of the morphology and pathogenicity test. To our knowledge, this is the first report of this rust on snow lotus (S. involucrata) (1-4).

Expansão da pústula da ferrugem em três cultivares do cafeeiro

A expansão da lesão é considerada importante componente epidemiológico em vários patossistemas. Com o propósito de avaliar a expansão da pústula da ferrugem (Hemileia vastatrix) em três cultivares de cafeeiro e o valor das respectivas taxas de expansão da lesão (TEL) sob condições controladas delineou-se este estudo. O experimento foi instalado em câmaras de crescimento de plantas com temperatura de 20±2 ºC e umidade relativa de 70%. Foram inoculadas vinte plantas de cada uma das cultivares Acaiá, Catuaí e Rubi, de três meses de idade, com 0,5mg/mL da suspensão de urediniósporos. Após 42 dias da inoculação, sete pústulas foram marcadas aleatoriamente, em 7 plantas de cada cultivar e, em seguida, fotografadas na face abaxial da folha, com câmara digital, durante 15 dias, para posterior mensuração da área, utilizando o programa Image Tool da UTHSCA. Todas as imagens digitalizadas foram padronizadas em 4,0 x 4,0 cm. Ao final do experimento, foi avaliada a produção de urediniósporos/cm² de pústula, em hemacitômetro. As curvas de progresso da expansão da pústula nas três cultivares foram ajustadas ao modelo exponencial. A TEL foi de 0,14, 0,16 e 0,16 mm²/dia e o número de urediniósporos produzidos foi de 918,6, 929,3 e 934,8/cm² de pústula, para as cultivares Acaiá, Catuaí e Rubi, respectivamente. Não houve diferenças, entre as três cultivares, consideradas suscetíveis, em relação à TEL e à produção final de urediniósporos. Estes resultados podem contribuir para maior compreensão dos possíveis mecanismos que atuam no aumento da severidade da ferrugem do cafeeiro, sem que ocorram novas infecções.

A Comparative Analysis of Diagnostic Protocols for Detection of the Asian Soybean Rust Pathogen, Phakopsora pachyrhizi

Plant diagnosticians routinely process soybean rust samples with one or more of the following diagnostic protocols: visual assessment using a dissecting microscope, an enzyme-linked immunosorbant assay (ELISA), and pathogen-specific PCR (conventional and real-time). Results of these three diagnostic protocols and time requirements for diagnosis were collected for samples with signs (sporulation) of soybean rust (presumed positive) and samples without sporulating rust pustules (presumed negative). Data were collected for samples processed in triplicate at six university plant disease clinics. The effects of sample treatment and storage on results of two diagnostic protocols also were examined. Visual diagnosis required the least amount of time (4 min), ELISA and PCR took approximately six-and-a-half times longer per sample (multiple samples being processed at one time) than a visual diagnosis. ELISA could not detect the pathogen in positive samples after desiccation followed by long-term storage at room temperature and following one or more autoclave treatments for 30 min. However, conventional PCR was capable of detecting P. pachyrhizi-infected plant material following all sample storage regimes. Accepted for publication 9 February 2007. Published 31 May 2007.

First Report of Soybean Rust Caused by Phakopsora pachyrhizi on Phaseolus spp. in Argentina.

Asian soybean rust (ASR), caused by Phakopsora pachyrhizi Syd. & P. Syd., has been reported in Argentina on soybean (Glycine max) and kudzu (Pueraria lobata and Pueraria javanica) since the 2002 growing season (1-4). On 29 May 2006, plants of Phaseolus spp. were found to have tan ASR-like rust lesions on leaves at eight different field plots located in the northwestern province of Salta, Argentina. Growth stages of infected bean plants within plots were between pod setting and physiological maturity. Diagnosis of ASR on bean leaves was performed with a stereoscopic microscope to view rust pustules, and suspected uredinia of P. pachyrhizi were observed, furthermore, typical ASR urediniospores also were also observed at ×400. ELISA and PCR methods gave positive results for ASR. Rust spores from these plants were used to inoculate soybean plants at the V3 growth stage with rust spores from field bean plants produced under greenhouse conditions. Typical ASR tan pustules developed within 21 days of inoculation. Bean rust caused by Uromyces phaseoli also was seen in some of the bean plots but was easily differentiated from ASR because the uredinia were much darker and affected the upper leaves, while the ASR uredinia were lighter and spread from the lower leaves to the upper leaves. This finding is of significance in Argentina because bean is an important crop grown in the northwestern region of the country and is planted approximately 2 months after soybean planting. Given this planting time difference, bean may provide an overwintering host for the survival of ASR spores, thereby providing a green bridge for infection of soybean plants during the following growing season.

Relações entre intensidade da doença, refletância da radiação solar e rendimento de grãos no patossistema ferrugem da folha de trigo Embrapa 16

Em experimentos com o cultivar de trigo Embrapa 16 conduzidos em Passo Fundo e Não-Me-Toque nas safras de 1999 e 2000 houve uma relação inversa entre a incidência foliar da ferrugem da folha do trigo, a severidade, o número de pústulas por centímetro quadrado e a área abaixo da curva de progresso da doença com o rendimento de grãos, e uma relação direta entre a refletância da radiação solar, na faixa de 810 nm, com o rendimento de grãos. A refletância da radiação solar na faixa permitiu estimar o rendimento de grãos do trigo, a severidade da doença e o número de pústulas cm-2. As equações da função de dano com maior R² obtidas pela análise de regressão permitem o cálculo do limiar de dano econômico (LDE) em diferentes estádios fenológicos da cultura. O LDE pode servir de critério indicador do momento para o controle da ferrugem da folha do trigo. Os danos causados pela doença atingiram um máximo de 43%, com uma média de 40 %.

First Report of Soybean Rust Caused by Phakopsora pachyrhizi on Phaseolus spp. in the United States.

Phakopsora pachyrhizi Syd. & P. Syd., the cause of soybean rust, was first observed in the continental United States in November 2004 (2). During the growing season of 2005, P. pachyrhizi was confirmed on soybean (Glycine max) and/or kudzu (Pueraria montana) in nine states in the southern United States. It is known that P. pachyrhizi has a much larger host range within the Fabaceae family. On 29 September 2005 in Quincy, FL, 45 entries of mostly large-seeded legumes were planted next to soybeans that were infected with P. pachyrhizi. Several seeds of each entry were planted on one hill. Soybean plants growing adjacent to these potential hosts had 15 to 25% of the leaf area affected, 95% incidence, and 73% defoliation on 16 November. On 7 December 2005, all the plants of Phaseolus coccineus L. (scarlet runner bean, PI311827), Phaseolus lunatus L. (lima bean, PI583558), and two Phaseolus vulgaris L. (kidney bean) cvs. Red Hawk and California Early Light Red Kidney (CELRK) were found to have leaves with suspected rust lesions. These plants were at physiological maturity but had not senesced. None of the hosts had been inoculated other than from spores produced by the adjacent rust-infected soybean plants or from unknown locations. On the basis of microscopic examination, suspected infected leaves from plants of the Phaseolus spp. had rust pustules characteristic of P. pachyrhizi uredinia. Uredinia were counted within a randomly selected 2-cm2 area of one leaf of each sample. The mean and range of uredinia per lesion for Phaseolus coccineus was 29 uredinia with a range of 0 to 3 uredinia per lesion, Phaseolus lunatus had 2 uredinia with 0 to 1 uredinium per lesion, Phaseolus vulgaris cv. Red Hawk had 22 uredinia with 0 to 5 uredinia per lesion, and Phaseolus vulgaris cv. CELRK had 43 uredinia with 0 to 4 uredinia per lesion. Polymerase chain reactions using two sets of primers (Ppa1/Ppa2 and Pme1/Pme2) were performed on DNA extracted from leaves of the three species with sporulating rust pustules (1). The results of these tests and further tests conducted by the USDA/APHIS confirmed that P. pachyrhizi was the causal organism for the observed rust.

First Report of a Rust Disease on Ohia Caused by Puccinia psidii in Hawaii.

Several species of Metrosideros (Myrtaceae), referred to as ohia in Hawaii, are endemic trees that comprise as much as 80% of the native Hawaiian forests. For centuries, these trees have provided niches for many indigenous and endangered plants and animals and are treasured by Hawaiians for their beauty and role in folklore and legends. During April 2005, a cultivated ohia plant was diagnosed by the Agricultural Diagnostic Service Center at the University of Hawaii at Manoa as infected by a rust fungus. Rust pustules containing abundant urediniospores were observed on leaves, stems, and sepals, causing discolored spots and severe deformity of young leaves and growing tips. By July 2005, a similar rust disease was observed on other plants in the family Myrtaceae; namely Syzygium jambos (L.) Alston, Eugenia koolauensis Degener, E. reinwardtiana (Blume) DC, and Psidium guajava L. Microscopic examination of the uredinia and urediniospores showed that the rust was morphologically similar to Puccinia psidii, which is reported as the guava or eucalyptus rust in Florida and Central and South America (1,2). To confirm the identity of this fungus, DNA was extracted from urediniospores of two isolates collected from ohia plants, and their nuclear ribosomal internal transcribed spacer (ITS) was amplified with two universal primers, ITS4 and ITS5 (3). Sequences of the ITS region of these isolates from ohia were identical to the P. psidii isolates provided by A. Alfenas in Brazil and M. Rayachhetry in Florida. Koch's postulate of the isolates, obtained from ohia, was performed using 1 × 108 spores/ml of urediniospores suspension in distilled water. The suspension was sprayed onto 6-month-old ohia seedlings. These inoculated seedlings were placed in clear plastic chambers maintained at 100% relative humidity and 22°C with a combination of 10-h fluorescent light period and a 14-h dark period. After 48 h of incubation, the seedlings were removed from the chambers and transferred to a greenhouse where the ambient temperature ranged from 20 to 24°C. Rust pustules appeared after 1 to 2 weeks of incubation. Symptoms first appeared as tiny, bright yellow, powdery eruptions that developed into circular, uredinial pustules on the stem and foliage. These pustules later expanded, coalesced, and became necrotic, spreading over the entire leaf and stem surfaces, and then leaves and stems were deformed and tip dieback ensued. These symptoms were the same as those observed on the naturally infected cultivated ohia plant mentioned above. P. psidii is reported to be native to South and Central America that later spread to some Myrtaceous plants in the Caribbean countries (1). It has a very wide host range within the family Myrtaceae (2). To our knowledge, this is the first report of P. psidii in Hawaii. This rust disease may pose a formidable threat to Myrtaceous species that make up the native Hawaiian forests and are grown as ornamental plants or for the production of wood chips.

Host Range of Rust Isolates on Oregano and Mint in Florida

Urediniospores were collected separately from rust pustules on oregano (Origanum vulgare) and spearmint (Mentha spicata) plants purchased from retail stores in Gainesville, Alachua Co., FL, in April 2004 and March 2005. Morphological characteristics of the urediniospores from both hosts were consistent with descriptions of Puccinia menthae. Spearmint, peppermint (M. × piperita), oregano (O. vulgare), sweet marjoram (O. majorana), and Greek oregano (Oregano heracleoticum) were inoculated with urediniospore suspensions of each rust isolate in a series of cross-inoculation experiments. Rust developed on oregano, sweet marjoram, and Greek oregano only when inoculated with urediniospores from the oregano plant, and this rust isolate did not infect any of the Mentha spp. tested. Rust developed on spearmint when inoculated with urediniospores from the mint plant, but this isolate did not infect peppermint, oregano, sweet marjoram, or Greek oregano. The two isolates of Puccinia menthae appear to represent different populations, “spearmint rust” and “oregano rust.” This is the first report of P. menthae on oregano in Florida. Accepted for publication 2 March 2006. Published 17 April 2006.