Urediniospore Production Research Articles

Evaluating the Utility of Simplicillium lanosoniveum, a Hyperparasitic Fungus of Puccinia graminis f. sp. tritici, as a Biological Control Agent against Wheat Stem Rust

Wheat stem rust is one of the wheat diseases caused by Puccinia graminis Pers. f. sp. tritici (Pgt). This disease has been responsible for major losses to wheat production worldwide. Currently used methods for controlling this disease include fungicides, the breeding of stem rust-resistant cultivars, and preventive agricultural measures. However, the excessive use of fungicides can have various deleterious effects on the environment. A hyperparasitic fungus with white mycelia and oval conidia, Simplicillium lanosoniveum, was isolated from the urediniospores of Pgt. When Pgt-infected wheat leaves were inoculation with isolates of S. lanosoniveum, it was found that S. lanosoniveum inoculation inhibited the production and germination of urediniospores, suggesting that S. lanosoniveum could inhibit the growth and spread of Pgt. Scanning electron microscopy revealed that S. lanosoniveum could inactivate the urediniospores by inducing structural damage. Overall, findings indicate that S. lanosoniveum might provide an effective biological agent for the control of Pgt.

SSR genotypes of Puccinia triticina in 10 provinces of China reveal a possible source of the wheat leaf rust fungus

AbstractWheat leaf rust is a fungal disease caused by Puccinia triticina and spread by wind. In recent years, with changes in the climate and farming systems, damage caused by wheat leaf rust has become increasingly severe. To study the genetic structure of P. triticina populations in China and provide a basis for control of wheat leaf rust, collections of P. triticina were obtained from Xinjiang, Inner Mongolia, Hebei, Henan, Jiangsu, Anhui, Hubei, Sichuan, Guizhou and Yunnan provinces in 2019. A total of 177 single uredinial isolates were tested with 13 simple‐sequence repeat (SSR) primers. The results showed that there was frequent gene exchange within P. triticina populations in China, but a distinct population divergence was also revealed between those in Xinjiang and Yunnan provinces. Results also suggested that Inner Mongolia might be a source of P. triticina for other regions. In addition, the linkage disequilibrium test indicated that the P. triticina populations reproduced by clonal production of urediniospores.

Monocyclic Components and Photosynthetic Damage Caused by Myrtle Rust in Guava Leaves.

Austropuccinia psidii, the causal agent of myrtle rust, was, for many years, restricted to the Americas, but since reaching Hawaii in 2005, thepathogen has expanded its global range exponentially. In Brazil, myrtle rust is the main fungal disease in guava plants. Despite this, there are few studies on guava rust epidemiology. The objectives of this study were to quantify the monocyclic components of rust and to evaluate the photosynthetic damage caused by A. psidii in young and old leaves of 'Paluma' guava. The monocyclic components of guava rust and gas exchange in healthy or inoculated (105 ml-1 urediniospores of A. psidii) leaves were quantified over time. Additionally, young leaves were inoculated with varying concentrations of A. psidii inoculum, and leaf gas exchange and chlorophyll fluorescence were measured at 25 days postinoculation. The relationship between the relative CO2 assimilation of a diseased leaf (Px) and a healthy leaf (Po) is related to disease severity (x) by Px/Po = (1 - x)β. The density of lesions, disease severity, and urediniospore production were high in young leaves, averaging 58lesions cm-2, 50% leaf area diseased, and 2.5 × 104 urediniospores per lesion, respectively. Rust symptoms were not observed in old leaves, and resistance to infection did not cause any photosynthetic cost to these leaves. On young leaves, β was 2.13, indicating a reduction on CO2 assimilation at green tissues from symptomatic leaves. Our data revealed that photosynthesis reduction in diseased guava leaves was caused by biochemical and photochemical damage rather than by stomatal limitation.

Resistance in Maize (Zea mays) to Isolates of Puccinia sorghi from Eastern Australia

The causal agent of maize common rust (CR), Puccinia sorghi Schwein., has increased in incidence and severity in Australia in recent years, prompting the assessment of sources of resistance and a preliminary survey of the diversity of P. sorghi populations. The maize commercial hybrids tested carried no resistance to 14 isolates of P. sorghi and had infection types (IT) comparable to that of a susceptible check. The resistance gene Rp1_D that remained effective in the US for 35 years, was ineffective against seven of the 14 isolates. Maize lines carrying known Rp genes were inoculated with the five isolates considered most diverse based on year of collection (2018/2019), location (Queensland/Victoria), and host from which they were isolated (maize/sweet corn). Lines carrying the resistance genes RpG, Rp5, Rp1_E, Rp1_I, Rp1_L, RpGDJ, RpGJF and Rp5GCJ were resistant to all five isolates and to isolates collected in many agro-ecological regions. These lines were recommended as donors of effective resistance for maize breeding programs in Australia. Lines carrying no known resistance or resistance genes Rp8_A, Rp8_B, Rp1_J, Rp1_M, Rp7 and Rpp9 (conferring resistance to P. polysora Underw.) were susceptible to all five isolates. Differential lines carrying the resistance genes Rp1_B, Rp1_C, Rp1_D, Rp1_F, Rp1_K, Rp3_D, or Rp4_A were either resistant or susceptible depending upon the isolate used, showing that the isolates varied in virulence for these genes. Urediniospore production was reduced on adult compared to juvenile plants, presumably due to changes in plant physiology associated with age or the presence of adult plant resistance.

Identification of a Hyperparasitic Simplicillium obclavatum Strain Affecting the Infection Dynamics of Puccinia striiformis f. sp. tritici on Wheat.

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most serious threats to wheat production worldwide. Changes of Pst virulence may circumvent resistance in wheat varieties, and application of fungicides may cause environmental problems. Parasites of Pst can be used to develop biological agents for environmentally friendly control of this fungal disease. Here, we report a hyperparasitic fungus isolated from Pst and identified it as Simplicillium obclavatum through molecular and morphological characterizations. We demonstrated that inoculation of Pst-infected wheat leaves with S. obclavatum reduced the production and germination rate of Pst urediniospores. Therefore, S. obclavatum has the potential to be developed into a biological control agent for managing wheat stripe rust.

Comparative study on the monocycle of Phakopsora meliosmae-myrianthae and Phakopsora pachyrhizi

Asian grapevine leaf rust, caused by Phakopsora meliosmae-myrianthae, presents symptoms similar to Asian soybean rust, caused by P. pachyrhizi, such as the presence of satellite uredinia and early leaf fall. To shed light on the epidemiological behaviour of these rusts, we compared the monocyclic components of these Phakopsora species and the effects of these pathogens on host photosynthesis over time. Both hosts were inoculated with a 104 urediniospores mL−1 suspension and incubated at 23–25 °C. Incubation and latent periods were simultaneous at 8 days in Asian grapevine leaf rust and 13 days in Asian soybean rust. The minimum infectious periods were 21 days for P. meliosmae-myrianthae and 13 days for P. pachyrhizi. The continuous production of urediniospores through a long infectious period was observed for both pathogens. Both Phakopsora rusts showed an increase in rust severity with similar progress rates to those estimated with the monomolecular model as 0.07 day−1 and 0.02 day−1 for grapevine and soybean rusts, respectively. No increase in the number of lesions was observed from the first assessment, although the number of uredinia increased over time. P. meliosmae-myrianthae and P. pachyrhizi infection reduced the net photosynthetic rates by 22% and 5%, respectively, before the onset of symptoms. Both pathogens withstand non-ideal environmental conditions for secondary infections. We concluded that, although P. meliosmae-myrianthae has a restricted host range on Vitis sp., unlike to P. pachyrhizi with a broad host range, the epidemiological behaviour of P. meliosmae-myrianthae is in some ecological aspects similar to that of P. pachyrhizi.

Carfentrazone‐ethyl and glyphosate drift inhibits uredinial formation of Austropuccinia psidii on Eucalyptus grandis leaves

The response to infection of Austropuccinia psidii in resistant (CLR-383) and susceptible (CLR-384) Eucalyptus grandis clones, exposed to herbicide drift of carfentrazone-ethyl, glyphosate and a mixture of these two herbicides, was evaluated at microscopic and physiological levels. Plants of the two clones showed symptoms of phytotoxicity caused by herbicide drift. However, net CO2 assimilation rate, height and shoot dry matter were lower in CLR-384 than in CLR-383. At the ultrastructure level, the leaves of both clones exposed to the herbicides showed thylakoid disorganization and accumulation of starch grains in the chloroplasts. Only plants of CLR-384 were infected by A. psidii, but when exposed to herbicide drift, rust severity was lower than in control plants. Six days after inoculation (dai), plants of this clone exposed to the herbicides had smaller uredinia than control plants. At 12 dai, non-herbicide treated plants showed normal uredinia, containing abundant urediniospores. By contrast, plants exposed to the herbicides were less colonized by the fungus, and the uredinia were smaller with reduced production of urediniospores, which were sometimes not even detected. Glyphosate and carfentrazone-ethyl herbicide drift reduce infection and uredinial formation of A. psidii and to some extent induce basal resistance in a susceptible clone of E. grandis. © 2018 Society of Chemical Industry.

Identification of a Novel Alternaria alternata Strain Able to Hyperparasitize Puccinia striiformis f. sp. tritici, the Causal Agent of Wheat Stripe Rust.

The obligate bitrophic fungus Puccinia striiformis f. sp. tritici (Pst) causes stripe (yellow) rust on wheat worldwide. Here, we report a novel fungal strain able to hyperparasitize Pst. The strain was isolated from gray-colored rust pustules, and was identified as Alternaria alternata (Fr.: Fr.) keissler based on a combination of morphological characteristics and multi-locus (ITS, GAPDH, and RPB2) phylogeny. Upon artificial inoculation, the hyperparasite reduced the production and viability of urediniospores, and produced a typical gray-colored rust pustule symptom. Scanning electron microscopy demonstrated that the strain could efficiently penetrate and colonize Pst urediniospores. This study first demonstrates that A. alternata could parasitize Pst and indicates its potential application in the biological control of wheat stripe rust disease.

A novel fungal hyperparasite of Puccinia striiformis f. sp. tritici, the causal agent of wheat stripe rust.

Puccinia striiformis f. sp. tritici (Pst), the causal fungus of wheat stripe rust, was previously reported to be infected by Lecanicillium lecanii, Microdochium nivale and Typhula idahoensis. Here, we report a novel hyperparasite on Pst. This hyperparasitic fungus was identified as Cladosporium cladosporioides (Fresen.) GA de Vries based on morphological characteristics observed by light and scanning electron microscopy together with molecular data. The hyperparasite reduced the production and viability of urediniospores and, therefore, could potentially be used for biological control of wheat stripe rust.

Wheat stripe (yellow) rust caused by Puccinia striiformis f. sp. tritici.

Stripe (yellow) rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a serious disease of wheat occurring in most wheat areas with cool and moist weather conditions during the growing season. The basidiomycete fungus is an obligate biotrophic parasite that is difficult to culture on artificial media. Pst is a macrocyclic, heteroecious fungus that requires both primary (wheat or grasses) and alternate (Berberis or Mahonia spp.) host plants to complete its life cycle. Urediniospores have the capacity for wind dispersal over long distances, which may, under high inoculum pressure, extend to thousands of kilometres from the initial infection sites. Stripe rust, which is considered to be the current major rust disease affecting winter cereal production across the world, has been studied intensively for over a century. This review summarizes the current knowledge of the Pst-wheat pathosystem, with emphasis on the life cycle, uredinial infection process, population biology of the pathogen, genes for stripe rust resistance in wheat and molecular perspectives of wheat-Pst interactions. The stripe rust pathogen, Puccinia striiformis Westend. (Ps), is classified in kingdom Fungi, phylum Basidiomycota, class Urediniomycetes, order Uredinales, family Pucciniaceae, genus Puccinia. Ps is separated below the species level by host specialization on various grass genera, comprising up to nine formae speciales, of which P. striiformis f. sp. tritici Erikss. (Pst) causes stripe (or yellow) rust on wheat. Uredinial/telial hosts: Pst mainly infects common wheat (Triticum aestivum L.), durum wheat (T. turgidum var. durum L.), cultivated emmer wheat (T. dicoccum Schrank), wild emmer wheat (T. dicoccoides Korn) and triticale (Triticosecale). Pst can infect certain cultivated barleys (Hordeum vulgare L.) and rye (Secale cereale L.), but generally does not cause severe epidemics. In addition, Pst may infect naturalized and improved pasture grass species, such as Elymus canadensis L., Leymus secalinus Hochst, Agropyron spp. Garetn, Hordeum spp. L., Phalaris spp. L and Bromus unioloides Kunth. Pycnial/aecial (alternative) hosts: Barberry (Berberis chinensis, B. koreana, B. holstii, B. vulgaris, B. shensiana, B. potaninii, B. dolichobotrys, B. heteropoda, etc.) and Oregon grape (Mahonia aquifolium). Stripe rust appears as a mass of yellow to orange urediniospores erupting from pustules arranged in long, narrow stripes on leaves (usually between veins), leaf sheaths, glumes and awns on susceptible plants. Resistant wheat cultivars are characterized by various infection types from no visual symptoms to small hypersensitive flecks to uredinia surrounded by chlorosis or necrosis with restricted urediniospore production. On seedlings, uredinia produced by the infection of a single urediniospore are not confined by leaf veins, but progressively emerge from the infection site in all directions, potentially covering the entire leaf surface. Individual uredinial pustules are oblong, 0.4-0.7 mm in length and 0.1 mm in width. Urediniospores are broadly ellipsoidal to broadly obovoid, (16-)18-30(-32) × (15-)17-27(-28) μm, with a mean of 24.5 × 21.6 μm, yellow to orange in colour, echinulate, and with 6-18 scattered germ pores. Urediniospores can germinate rapidly when free moisture (rain or dew) occurs on leaf surfaces and when the temperatures range is between 7 and 12 °C. At higher temperatures or during the later growing stages of the host, black telia are often produced, which are pulvinate to oblong, 0.2-0.7 mm in length and 0.1 mm in width. The teliospores are predominantly two-celled, dark brown with thick walls, mostly oblong-clavate, (24-)31-56(-65) × (11-)14-25(-29) μm in length and width, and rounded or flattened at the apex.

Effects of Frequency of “Extreme” Temperature Highs on Development of Soybean Rust

Previously, we hypothesized that summer "extreme" diurnal temperature highs in the southeastern United States were responsible for the yearly absence or delay of soybean rust development until fall. Utilizing temperature-controlled growth chambers, a diurnal temperature pattern of 33°C high and 20°C low reduced urediniospore production by 81%. However, that study did not consider the influence of frequency of extreme temperatures on soybean rust. We now report that a temperature high of 35°C for 1 h on three consecutive days, initiated 15 days after inoculation, when lesions had formed, reduced urediniospore production by 50% and required 9 to 12 days for sporulation to resume once the extreme temperature highs ceased. Furthermore, three consecutive days in which the temperature high was 37°C, beginning immediately after inoculation and subsequent dew period, reduced lesion numbers by 60%. The combined effects of reduced numbers of lesions and urediniospores per lesion caused by extreme temperature highs can account for observed absence or delay of soybean rust development in the southeastern United States until fall. A comparison of frequency of extreme temperature highs with numbers of counties reporting presence of soybean rust from 2005 to 2012 verified that extreme temperature highs may be largely responsible for absence or delay of soybean rust development. This is the first report showing the effect of frequency of extreme temperature highs on development of soybean rust. Because the south-to-north progression of soybean rust is required for the disease to occur in the major soybean-production regions of the United States, temperatures in the southeastern United States have a major effect on the entire U.S. soybean industry.

Effects of Daily Temperature Highs on Development of Phakopsora pachyrhizi on Soybean

Although considerable information exists regarding the importance of moisture in the development of soybean rust, little is known about the influence of temperature. The purpose of our study was to determine whether temperature might be a significant limiting factor in the development of soybean rust in the southeastern United States. Soybean plants infected with Phakopsora pachyrhizi were incubated in temperature-controlled growth chambers simulating day and night diurnal temperature patterns representative of the southeastern United States during the growing season. At 3-day intervals beginning 12 days after inoculation, urediniospores were collected from each plant and counted. The highest numbers of urediniospores were produced when day temperatures peaked at 21 or 25°C and night temperatures dipped to 8 or 12°C. When day temperatures peaked at 29, 33, or 37°C for a minimum of 1 h/day, urediniospore production was reduced to 36, 19, and 0%, respectively, compared with urediniospore production at the optimum diurnal temperature conditions. Essentially, no lesions developed when the daily temperature high was 37°C or above. Temperature data obtained from the National Climatic Data Center showed that temperature highs during July and August in several southeastern states were too high for significant urediniospore production on 55 to 77% of days. The inhibition of temperature highs on soybean rust development in southeastern states not only limits disease locally but also has implications pertaining to spread of soybean rust into and development of disease in the major soybean-producing regions of the Midwestern and northern states. We concluded from our results that temperature highs common to southeastern states are a factor in the delay or absence of soybean rust in much of the United States.

First Report of White Pine Blister Rust Caused by Cronartium ribicola on Immune Black Currant Ribes nigrum Cv. Titania in Preston, Connecticut.

Since the relaxation of federal sanctions on the planting of Ribes crops because of the development of white pine blister rust (WPBR) immune Ribes cultivars (3), a small industry for the production of Ribes fruit for fresh and processing markets was established in New York and surrounding New England states. The majority of Ribes acreage in the region is planted to a WPBR immune black currant R. nigrum cv. Titania. From 2008 to June 2011, symptoms resembling those caused by WPBR pathogen Cronartium ribicola were observed at a large (>12 ha) R. nigrum cv. Titania planting in Preston, CT. In 2008, infection was restricted to a single field (100% incidence), but in 2009, all fields suffered from premature defoliation by late July. In 2010 and 2011, there was considerable incidence (>85%), but premature defoliation was kept in check by chemical management. Symptoms began as chlorotic lesions (0.5 to 4.3 mm in diameter) on both sides of the leaf. These chlorotic lesions had margins delineated by leaf veins and many developed necrotic (0.3 to 0.9 mm in diameter) centers on the upper surface of leaves within 2 to 3 weeks. The undersides of lesions developed blisters containing orange uredinia (0.1 to 0.33 mm in diameter) with smooth peridia that broke with the production of yellow-orange urediniospores (30 × 15 to 25 μm). Symptoms and signs were consistent with published descriptions of C. ribicola (1) and with WPBR infections on highly susceptible R. nigrum cv. Ben Alder planted at the New York State Agricultural Experiment Station in Geneva. Additional confirmation was provided by sequencing the two internal transcribed spacer (ITS) regions and the 5.8S gene (GenBank Accession No. JN587805; 98% identity with No. DQ533975) in the nuclear ribosomal repeat using primers ITS1-F and ITS4 as described previously (2,4). Furthermore, an attempt was made to confirm pathogenicity in the greenhouse by inoculating shoots of potted nursery stock of R. nigrum cv. Titania. Shoots were inoculated by rubbing leaves with either an uninfected currant leaf or a currant leaf from Preston, CT sporulating with urediniospores. Plants were subsequently misted with dH20 and covered with plastic bags for 24 h. Plants were watered biweekly and kept in a greenhouse with 39 to 65% relative humidity at 21 to 26°C. Shoots were monitored for symptom development on a weekly basis. After 3 weeks, 2 of the 10 plants inoculated with infected leaves developed chlorotic lesions and uredinia identical to those on leaves from Preston, CT, while all others remained healthy. Although not easily reproducible in a greenhouse, the breakdown of immunity in R. nigrum cv. Titania was observed for the last 4 years in Connecticut. Given that WPBR immunity was one of the conditions for legalized planting of Ribes, the breakdown of immunity has potentially deleterious implications particularly for nearby states like Massachusetts and New York, in which the Ribes industries are expanding. Moreover, Ribes growers may need to rely on chemical management programs to manage WPBR in the future.

Effect of Postsymptom Application of Fungicides on Urediniospore Production by Puccinia triticina on Wheat and P. hemerocallidis on Daylily.

Greenhouse experiments were conducted to evaluate the effect of post-symptomatic fungicide applications on urediniospore production of leaf rust of wheat, Puccinia triticina, and daylily rust, P. hemerocallidis. Fungicides from five chemical classes were evaluated for their effect on urediniospore production at 24-h intervals post application for 96 h. The quinone outside inhibitor (QoI) fungicide (azoxystrobin) significantly reduced cumulative urediniospore production by P. triticina (40.1 and 11.3% of control) and P. hemerocallidis (37.9 and 35.6% of control) in both trials. The demethylation inhibitor, myclobutanil, significantly reduced urediniospore production by P. hemerocallidis 24 h post treatment in both trials and cumulative production in one trial. Myclobutanil did not affect urediniospore production by P. triticina. Four QoI fungicides significantly reduced urediniospore production by P. triticina at all collection times (24, 48, and 72 h) and cumulative production in both trials. No differences were observed between the QoI fungicides with P. triticina. The four QoI fungicides significantly reduced urediniospore production by P. hemerocallidis compared with the nonfungicide control, with significantly fewer urediniospores produced in the pyraclostrobin-treated lesions compared with azoxystrobin, fluoxastrobin, and trifloxystrobin. Differences in leaf morphology between daylily and wheat such as leaf thickness and waxiness may contribute to the differences observed between the two pathosystems. The significant reductions in urediniospore production by postsymptom applications of QoI fungicides, combined with their known fungistatic properties, suggest that these chemistries would be useful for reducing the spread of viable inoculum to noninfested plants.

Culturing Phakopsora pachyrhizi on Detached Leaves and Urediniospore Survival at Different Temperatures and Relative Humidities.

Soybean rust, caused by Phakopsora pachyrhizi, is one of the most important foliar diseases of soybean worldwide. In a series of experiments, multiple objectives were addressed to determine the (i) longevity of detached soybean leaves, (ii) reproductive capacity of uredinia on leaves inoculated and/or incubated on the abaxial versus adaxial side of the leaf, (iii) reproductive capacity of uredinia and urediniospore germination when spores were harvested at regular intervals or all at once, and (iv) effect of temperature and relative humidity (RH) on urediniospore germination. A detached-leaf assay using agar medium amended with 6-benzylaminopurine performed better in retarding leaf chlorosis than filter paper alone among five soybean genotypes. Among the three susceptible genotypes tested, detached leaves of cv. Williams 82 had the lowest level of leaf chlorosis and often allowed for the greatest urediniospore production and germination rate. Temperature and RH played significant roles in survival of urediniospore as measured by germination rates. Viable urediniospores were harvested from infected soybean leaves maintained at room temperature (23 to 24°C at 55 to 60% RH) for up to 18 days, whereas freshly harvested urediniospores that were desiccated for 12 h before being placed in vials and maintained at room temperature remained viable for up to 30 days. Urediniospore hydration was the major factor for the dormancy reversion; thermal shock with hydration and no thermal shock with hydration treatments had consistently similar urediniospore germination rates. In the RH experiment, urediniospores harvested from inoculated leaf pieces maintained at 85% RH had the highest germination rates compared with higher and lower RH. Improvement in P. pachyrhizi cultural techniques and understanding of urediniospore survival will enhance our knowledge of the pathogen biology, host-plant relationship, and conditions that favor the infection, reproduction, and survival of the pathogen.

Comparative Susceptibilities of Legume Species to Infection by Phakopsora pachyrhizi.

Knowledge of the host range of Phakopsora pachyrhizi is important to agriculture in the United States because of the distinct possibility that economic losses could occur to crops other than soybean. Furthermore, it is possible that alternative hosts could provide a means of overwintering of the pathogen, providing inoculum to initiate epidemics in future years. To clarify the potential importance of soybean rust on nonsoybean legumes and their role in overwintering of the disease, multiple accessions of clover, cowpea, pea, kudzu, lima bean, snap bean, and single accessions of coffee senna, Florida beggarweed, hemp sesbania, hyacinth bean, partridge pea, and showy crotalaria were inoculated under greenhouse conditions with urediniospores of P. pachyrhizi; infected soybean plants served as a control. The four criteria used to assess susceptibility were lesion density, proportion of lesions with sporulating uredinia, average number of uredinia per lesion, and average uredinia diameter, each determined 2 weeks following inoculation. Based on lesion densities, percentage of lesions with sporulation, and average numbers of uredinia per lesion, soybean, kudzu, and pea were the most susceptible species, followed by snap bean. However, because infected pea plants defoliated rapidly, urediniospore production presumably was limited, lessening the potential for epidemics on pea. Cultivars of snap bean produced numerous brown to reddish-brown lesions, many of which sporulated, but numbers of uredinia per lesion were lower than on soybean, kudzu, or pea. The presence of both tan (susceptible) and reddish-brown (resistant) lesions on kudzu demonstrated physiological differentiation on that host. Some kudzu plants appeared to be potentially excellent hosts for overwintering of the disease. The average number of uredinia per lesion appeared to be a valid measurement with which to compare host susceptibilities, and may have epidemiological significance. High susceptibility of a host was characterized by numerous uredinia with a wide range of sizes within individual lesions. In contrast, low susceptibility to rust was characterized by no or a few small uredinia.

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.

Interaction of Fungicide Physical Modes of Action and Plant Phenology in Control of Stem Rust of Perennial Ryegrass Grown for Seed.

Azoxystrobin provided protective and curative effects against stem rust (caused by Puccinia graminis subsp. graminicola) of inoculated perennial ryegrass under field conditions, significantly reducing disease severity compared with the nontreated check when applied as much as 15 days before infection or 14 days after infection. Propiconazole had a significant effect when applied 13, but not 15, days before infection or 7, but not 9, days after infection. Either fungicide was very effective when applied near the time of infection, and the effectiveness of each fungicide was well described by a second- or third-order polynomial with time (days or stem rust latent periods) as the independent variable. When symptomatic plants were sprayed with a fungicide, subsequent urediniospore production per pustule was reduced by 73% in propiconazole-treated plants and by 95% in azoxystrobin-treated plants. Azoxystrobin modestly but significantly reduced germinability of urediniospores from sprayed pustules, unlike propiconazole. These differences between the two fungicides in physical modes of action resulted in a marked difference in their effects on secondary, within-plant spread of the disease. In normal disease development, sporulation from the inner (adaxial) face of pustules on the flag-leaf sheath produces inoculum that leads to numerous contiguous secondary infections along the length of the emerging inflorescence, but only 7% of azoxystrobin-treated sheath pustules had sporulation from the adaxial surface compared with 72 and 90% of propiconazole-treated and nontreated pustules, respectively. Tillers treated with propiconazole early in the process of within-plant spread had significantly greater final stem rust severity than those treated with azoxystrobin at this time. Results of these experiments will allow effects of fungicide application to be incorporated into quantitative epidemic models that describe disease development as a function of environment and plant phenology.

(44) Microscopic and Macroscopic Studies on the Development of Puccinia hemerocallidis in Resistant and Susceptible Daylily Cultivars

Infection process of Puccinia hemerocallidis, the causal agent of daylily rust, and resistance responses in eight daylily cultivars, were studied macroscopically and microscopically. After germination of urediniospores, appressoria were formed at the tip of germ tubes and penetrated through stomatal openings. Intercellular hyphae aggregated and formed uredia under the infection sites, and released urediniospores after rupturing the epidermis. In highly resistant cultivars `Prairie Blue Eyes' and `Bertie Ferris', intercellular hyphal growth was restricted and uredia were not formed. No macroscopic symptoms of the disease were present on the leaf surface, although a few collapsed cells were observed microscopically. Both resistant and moderately resistant reactions were characterized by necrotic lesions with many collapsed cells under infection sites. The difference between these two reactions was that uredia and urediniospores were observed on the moderately resistant cultivar `Chicago Apache', but not on resistant cultivars, `Buttered Popcorn' and `Stella De Oro'. Sporulation was observed on both moderately susceptible and susceptible cultivars, but latent periods were delayed and the amount of urediniospore production was reduced on moderately susceptible cultivars, `Mary Todd' and `Chorus Line', compared to the susceptible cultivar `Pardon Me'. The results indicate that the hypersensitive cell death is one of the resistance responses to daylily rust, but necrotic lesions on leaf surfaces are associated with the amount of collapsed host cells. The delayed latent periods and reduced sporulation that resulted from restricted intercellular hyphal growth could represent another resistance mechanism in the daylily rust pathosystem.

Effect of host genotype on leaf rust (Puccinia triticina) lesion development and urediniospore production in wheat seedlings

Experiments were performed in controlled conditions to compare several aggressiveness components of an isolate of Puccinia triticina on different wheat (Triticum aestivum) genotypes. Latency period, urediniospore production per lesion and per unit of sporulating surface, lesion size, and nitrogen and carbon content of the spores were measured on four host genotypes. Three of the host cultivars (Soissons, Altria and Isengrain) were susceptible and the fourth (Trémie) was resistant to the isolate used in the experiments, producing a mesothetic infection type. In all the analyses, lesion density was used as a covariable. The latent period was identical on the susceptible host cultivars, and there were no marked differences in the urediniospore production capacity when related to the sporulating surface area. In cv. Isengrain, differences in lesion size relative to the other susceptible cultivars resulted in lower urediniospore production per lesion. Urediniospore production per lesion and lesion size were density dependent and decreased exponentially with increasing lesion density. Urediniospore production per unit of sporulating surface was either independent of lesion density or marginally dependent (decreased linearly with a limited rate). On the resistant cultivar, Trémie, the latent period was longer and of a much greater variance compared with the susceptible cultivars. Urediniospore production per lesion was considerably reduced, but this reduction was fully accounted for by a reduction in lesion size, the urediniospore production capacity of the diseased tissue remaining equivalent to that observed in the susceptible cultivars. In all cultivars the C and N contents of the urediniospores were considered unchanged, even if minor differences were detected.