Web Blight Research Articles

Mejoramiento genético del frijol rojo y negro mesoamericano para Centroamérica y El Caribe.

Bean production in Central America is mainly a small farm operation on hillside, marginal areas, limited by several biotic and abiotic constraints. More than 350,000 t of beans are produced in nearly 0.5 millions of hectáreas, with a rather low yield average of 678 kg/ha. The majority of bean producers utilize low inputs and few farms are mechanized. Autoconsumption is rather high; however, a great portion of the beans is sold by intermediaries. In Honduras, beans are the 7th most important crop in economic value, and has the highest economic return among corn, rice and sorghum. The major market classes in Central America are small red and black (race Mesoamerican) beans. Improvement of these bean types is focused in developing resistance to diseases (common mosaic, golden mosaic, anthracnose, angular leaf spot, rust, web blight, and common bacterial blight) and pests (mainly pod weevil); and tolerance to low fertility, drought and heat. Appropriate hybridization and selection programs are utilized for developing multiple resistant, high yielding, well adapted and commercially accepted cultivars. A broad genetic base is accomplished by using Andean and Mesoamerican sources of germplasm in the hybridization stage. Simultaneous selection for various traits and multilocation testing of advanced lines are practiced. Yield and adaptation nurseries and trials of improved lines are distributed to Central America, Mexico, Panama and Caribbean countries. Testing of advanced lines and on-farm validation and varietal release, are carried out by National Bean Programs and institutions, members of the PROFRIJOL regional program, in collaboration with Zamorano, CIAT and the Bean/Cowpea CRSP.

Diagnóstico de la actividad productiva del frijol (Phaseolus vulgaris L.) en la Región Brunca de Costa Rica: Cosecha 1995-1996.

Nine localities were selected by their tradition in the bean cultivation in order to carry out a survey and diagnose of the situation of this productive activity. Based on surveys and visits to properties of farmers, it was that in the majority of the localities prevailed the use of Creole varieties. Creole varieties with a major commercial use were important for their precocity. A longer vegetative cycle of the improved varieties was a disadvantage which caused grain deterioration during the rain season. Quality and seed availability were the main problems. Inoculants of Antracnosis was found in the field and in the seeds. Web blight was an important pathogen affecting bean crops followed by leaf spot anthracnose. It was found that weeds were an important problem in the Brunca region.

Evaluación de la resistencia a la mustia hilachosa (Thanatephorus cucumeris (Frank), Donk). en líneas y variedades de frijol. Caisán. Panamá, 1997.

This study was carried out with the purpose of identifying genotypes of common bean, with a greater level of resistance to the web blight, in Caisán, Panama. 4335 lines and varieties of common bean were evaluated from seven experiments. A completely randomized design and a simple lattice design were utilized according to the experiments. Experiment repetitions varied from one, two or three depending on the amount of seed available. 62 materials showed an inferior reaction to the web blight in relation to that of the resistant Talamanca control (5 grade). The Barriles susceptible control showed grade reaction of 8. In several lines it was observed an apparent reaction of hyper-sensitivity to the web blight.

Diversidade genética de isolados de Rhizoctonia solani coletados em feijão-caupi no Estado de Roraima

O feijão-caupi (Vigna unguiculata) é uma das principais fontes de proteína para a população de baixa renda, principalmente nas Regiões Norte e Nordeste do Brasil. Esta leguminosa é suscetível a várias doenças incluindo a mela ou murcha-da-teia-micélica, cujo agente causal é o fungo Rhizoctonia solani (teleomorfo: Thanatephorus cucumeris). Embora Rhizoctonia solani seja um agente causal de doença muito importante, no Brasil inexiste qualquer informação sobre as características de seus isolados associados ao feijão-caupi. O objetivo do presente estudo foi avaliar, utilizando-se das técnicas Random Amplified Polymorphic DNA (RAPD) e RFLP-ITS (Internal Transcribed Spacer), a diversidade genética de isolados de R. solani coletados de plantas de feijão-caupi oriundas da região de cerrado e de mata do Estado de Roraima. Pelos resultados obtidos pode-se concluir que existe diversidade genética em R.. solani coletada de feijão-caupi e que os dois métodos moleculares utilizados foram eficientes em avaliar a divergência genética deste patógeno.

Mejoramiento genético para tolerancia a altas temperaturas y resistencia a mosaico dorado en frijol común.

In 1994-95, sources of heat tolerance were identified in common beans after screened several germplasm and advanced lines nurseries in Choluteca and Nacaome (≤ 50 masl), in the southern region of Honduras. The best heat tolerant genotypes were validated under greenhouse controlled conditions (35/27 °C day/night) in Geneva, New York. Afterward, crosses were made to develop small-red bean lines with heat tolerance and resistance to BGMV and other limiting factors. The F2- F5 segregant populations were evaluated for multiple traits including BGMV, common bacterial blight, angular leaf spot, rust, low fertility conditions, and selected for agronomic performance and commercial value. In 1998, 217 F6 and F7 advanced lines were evaluated in the irrigation district Lempa-Acahuapa (20 masl), in the Pacific region of El Salvador for heat tolerance and BGMV. The same nurseries were evaluated in Liberia, in the Northwest region of Costa Rica, for tolerance to high temperatures; and at Zamorano, Honduras for resistance to common bacterial blight, angular leaf spot and rust. A group of lines were identified having superior yield and adaptation than commercial checks, under high temperature conditions in El Salvador and Costa Rica. Some of these lines were also resistant to BGMV in El Salvador, and moderate resistant to web blight in Costa Rica, and they had excellent agronomic performance and good commercial value.

Occurrence of Web Blight in Soybean Caused by Rhizoctonia sol ani AG-l(IA) in Korea

Web blight symptoms were frequently observed on soybean plants grown in a farmer's fields located in Jincheon in Korea during a disease survey in August, 2005. Incidence of the disease was <TEX>$5-20\%$</TEX> infected plants in two of four soybean fields investigated. A total of 31 isolates of Rhizoctonia sp. were obtained from leaves, leaf petioles, and pods of diseased soybean plants. The isolates were identified as Rhizoctonia solani AG-l(IA) by anastomosis test and based on the morphological and cultural characteristics. Three isolates of R. solani AG-l(IA) were tested for pathogenicity to five cultivars of soybean by artificial inoculation. All the isolates induced blight symptoms on the leaves of soybean and formed sclerotia on the lesions, which were similar to those observed in the field. The pathogenicity tests revealed that all the soybean cultivars tested were susceptible to the pathogen. There was no difference in the pathogenicity among the isolates. The present study first reveals that R. solani AG-l(IA) causes web blight of soybean in Korea.

Plant Spacing Effects on Microclimate and Rhizoctonia Web Blight Development in Container-grown Azalea

Rhizoctonia web blight, caused by Rhizoctonia solani and binucleate Rhizoctonia spp., is an annual problem in compact cultivars of container-grown azalea (Rhododendron spp.) in the Gulf Coast states. Increasing the space between plants is commonly recommended for suppression of the disease, but experimental evidence for the effectiveness of this cultural practice in container-grown azalea is lacking. During the summers of 2002 and 2003, disease severity was measured weekly in the inoculated center plant of plots consisting of 49 potted `Gumpo White' azalea plants growing in 3.8-L containers and having a canopy diameter of about 30 cm. Plant spacing within plots was 0, 6, 12, 18, or 24 cm, and plots were arranged in three randomized complete blocks. Evaporation, leaf wetness (LW), relative humidity (RH), and temperature were monitored in each plot. Disease severity increased steadily from mid-July to late August or early September, after which it leveled off or declined. Evaporation increased and the number of hours within the temperature range favorable for disease development (25 to 30 °C) decreased significantly with plant spacing (P &lt; 0.05), but LW and RH were not significantly different among treatments. Plant spacing also had no significant effect on disease severity. Daily overhead irrigation and compact plant form likely contributed to the lack of effect of spacing on disease development.

Herencia de la resistencia a la mustia hilachosa de la habichuela.

Para lograr una mayor producción de frijol común (Phaseolus vulgaris L.) en los trópicos húmedos, se requiere el desarrollo de cultivares con mayores niveles de resistencia a la mustia hilachosa, una enfermedad destructiva causada por Thanatephorus cucumeris (anamorph: Rhizoctonia solani Kuhn). Los objetivos de este estudio fueron 1) estimar la heredabilidad de la resistencia a la mustia hilachosa del frijol común y 2) medir el efecto de la mustia hilachosa en el rendimiento y en la calidad de la semilla. Sesenta líneas del cruce MUS 83/DOR 483//Tío Canela 75 se evaluaron en el campo y en el laboratorio para reacción a la mustia hilachosa. Los estimados de heredabilidad para lecturas de la magnitud del daño en las hojas (DH) y el porcentaje de daño en la semilla (DS) fueron intermedios con un rango de 0.32 a 0.53. Aunque el DH en muchas líneas fue severo, el DS fue menos de 10% para la mayoría de las líneas avanzadas evaluadas. Estos resultados sugieren que la resistencia a DH y a DS podría tener una herencia independiente. Las evaluaciones en el campo fueron sencillas ya que permitieron la evaluación preliminar para resistencia a la mustia hilachosa. La técnica de inoculación en el laboratorio indujo síntomas más severos, por lo cual es apropiada para identificar líneas con altos niveles de resistencia fisiológica en las hojas.

Controle químico da queima de folhas e da mela de estacas de eucalipto, causadas por Rhizoctonia spp.

Visando ao controle químico da queima de folhas e mela de estacas de eucalipto (Eucalyptus spp.) em viveiros florestais, avaliou-se a eficiência de 12 fungicidas em inibir in vitro o crescimento micelial de um isolado epifítico de Rhizoctonia solani AG1-IB (RH-2). Sete fungicidas que inibiram totalmente o crescimento micelial do fungo, a concentrações inferiores a 100 ppm, foram pré-selecionados: methyl-tolclophos, benomyl, pencycuron, iprodione, thiabendazol, thiram e captan. Avaliou-se, ainda, a sensibilidade (EC50 = dose provável que inibe o crescimento micelial em 50%) aos fungicidas methyl-tolclophos, benomyl, iprodione e pencycuron de mais oito isolados patogênicos ao eucalipto, que diferem entre si quanto a virulência, morfologia, grupo de anastomose, número de núcleos por célula vegetativa e padrões eletroforéticos de proteínas e isoenzimas. Embora variações nos valores de EC50 entre algumas combinações de fungicidas e isolados tenham ocorrido, todos os isolados foram sensíveis aos quatro fungicidas testados (EC50 &lt; 11 ppm). Sob condições controladas, pulverizações com iprodione (1,5 g/l), benomyl (1 g/l), methyl-tolclophos (1,5 g/l), thiram ( 2,1 g/l), captan (2 g/l) e pencycuron (2 g/l) reduziram significativamente (alfa=5%) a incidência de folhas lesionadas por R. solani AG1, em brotações de mudas envasadas. Associadas à poda de limpeza, pulverizações de brotações de eucalipto em jardim clonal (no campo) com iprodione (1 g/l) ou com mistura de benomyl (0,5 g/l) + captan (1 g/l), alternada com mistura de benomyl (0,5 g/l) + thiram (1 g/l), reduziram a incidência da mela de estacas na casa de vegetação (alfa=5%).

Genetic Variation Among Isolates of the Web Blight Pathogen of Common Bean Based on PCR-RFLP of the ITS-rDNA Region.

Variability of 45 isolates of Rhizoctonia solani (teleomorph Thanatephorus cucumeris) causing web blight (WB) of common bean, Phaseolus vulgaris, was examined by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis of the internal transcribed spacer regions (ITS1 and ITS2) and the 5.8S subunit (5.8S) of the nuclear ribosomal DNA repeat (ITS-5.8S-rDNA). Isolates were collected from diseased bean leaves from Argentina, Costa Rica, Cuba, Dominican Republic, Honduras, Panama, and Puerto Rico. These WB isolates belong to AG-1 and AG-2 based on anastomosis reaction. Isolates of AG-1 that cause WB were separated into three distinct groups of RFLP patterns from enzymatic digestion of a 740-bp PCR fragment. Microsclerotia-producing isolates (<1 mm) were differentiated from macrosclerotia-producing isolates (5 to 20 mm) based on PCR-RFLP patterns even though they are placed in the same AG1-1B subgroup by anastomosis reaction. WB isolates of AG-2 were separated into two distinct PCR-RFLP groups as previously reported. AG-1 macrosclerotial-producing isolates were the most virulent, whereas isolates of AG-2 were the least virulent. Genetic variability of the WB pathogen may have influenced the failure or success of management practices implemented in the past in Latin America.

Contributions of the Bean/Cowpea CRSP to cultivar and germplasm development in common bean

Disease and abiotic stress are important factors limiting bean production wherever beans are grown. The development of bean cultivars having resistance to these stresses is a cost-effective and sustainable means to address these constraints. During the past 20 years, the Bean/Cowpea Collaborative Research Support Project (B/C CRSP) has supported common bean cultivar development and germplasm improvement programs in the USA and developing countries. Plant breeders have developed and released in Central America and the Caribbean bean cultivars and germplasm with one or more of the following traits; resistance to bean golden yellow mosaic virus (BGYMV), bean common mosaic necrotic virus (BCMNV), rust, web blight and common bacterial blight (CBB) and greater tolerance to high temperatures. In the highlands of Mexico and Ecuador bean cultivars with resistance to anthracnose, rust, root rots and bean common mosaic virus (BCMV), greater biological nitrogen fixation and improved adaptation to intermittent drought have been released. The bean breeding programs in East Africa have developed and released bean cultivars and germplasm with resistance to BCMNV, rust and bruchid seed weevils. Participation in the B/C CRSP has permitted USA bean breeding programs to develop and release bean cultivars and germplasm with resistance to BGYMV, BCMNV, anthracnose, rust, CBB, architectural avoidance to white mold and greater yield potential. Numerous plant breeders, plant pathologists and agronomists from developing countries have received advanced degree training in the USA, which has enhanced the capacity to develop improved bean cultivars for Latin America and Africa. The lack of sustainable seed production and delivery systems continues to limit the impact of the release of improved bean cultivars in many parts of Latin America and East Africa.

Contributions of the Bean/Cowpea CRSP to management of bean diseases

Diseases are a major constraint to production of beans in developing countries, reducing yields and seed quality. Contributing factors include poor disease management, lack of resistant cultivars, and the limited availability of certified disease-free seed. From the outset the Bean/Cowpea Collaborative Research Support Program (CRSP) has emphasized integrated disease management, and breeding for resistance to bean rust ( Uromyces appendiculatus), common bacterial blight ( Xanthomonas campestris (= X. axonopodis) pv. phaseoli) (CBB) and web blight ( Thanatephorus cucumeris (anamorph Rhizoctonia solani)) (WB). Later in the 1990s, angular leaf spot ( Phaeoisariopsis griseola) (ALS), and virus induced bean common mosaic (BCM) and bean common mosaic necrosis (BCMN) became more widespread and epidemic. The research included development of disease-resistant germplasm and studies of pathogenic variation and epidemiology. During the early 1990s bean golden yellow mosaic (BGYM), became a major disease of common bean in the Caribbean and Central America. No cultivar resistant to the gemini virus causing BGYM was available in the Dominican Republic (DR) and pesticides were used to reduce transmission of the virus by white flies ( Bremisia tabaci) biotype A. A change in the bean-production system to limit reproduction of the vector through a host-free period and concentrate the bean growing season to a four month period reduced BGYM significantly. Improved resistant cultivars and disease management resulted in high yields and self-sufficiency of beans in the DR. Evidence for co-evolution of the pathogens causing ALS, CBB, and rust in the two major bean gene pools (Andean and Middle American) was found. Variation in the WB pathogen on bean indicated independent genetic populations and the presence of different anastosmosis subgroups. WB disease management practices should be designed for the sub-group of the pathogen. Seed transmission was found to be a significant source of R. solani inoculum. Specific ( Ur-9 gene) and adult-plant resistance ( Ur-12 gene) to rust was identified for PC-50 (Andean origin) in the DR, and the genes were mapped. A mobile rust nursery was developed to monitor races of rust in a region and assist in resistance-gene deployment in bean germplasm and varieties. DNA based diagnostic methods were developed to differentiate X. campestris pv. phaseoli from X. campestris pv. phaseoli var fuscans (prevalent in East Africa) and P. griseola isolates. DNA hybridization methods also were developed to identify BGYM and bean golden mosaic viruses. Research in USA and East Africa has helped define bean common mosaic virus (BCMV) and bean common mosaic necrotic virus (BCMNV) as separate viruses and has demonstrated that BCMNV appears to be indigenous to Africa. Serological tools were developed that assist in the detection and identification of potyviruses, BCMV and BCMNV. These tools are now used worldwide.

First Report of Web Blight Caused by Rhizoctonia solani on Catharanthus roseus in Louisiana.

Web (aerial) blight was observed in field plots of Catharanthus roseus (L.) G. Don (Madagascar periwinkle) during three consecutive summers at the Burden Research Center in Baton Rouge. Leaf spots formed first, followed by a general blighting of leaves and stems that resulted in circular areas of dead plants in the plots. Dead leaves were matted together but remained attached to plants. Mycelia, and occasionally small, brown sclerotia (1 to 3 mm) were observed on blighted foliage. During the first year, only prostrate-growing cultivars belonging to the Mediterranean series of C. roseus were infected, but in 2001 and 2002 upright-growing cultivars as well as those with prostrate growth habit became infected. The disease occurred in July and August during periods of hot, humid, and rainy weather. Among 52 cultivars in the 2001 trial, only 'Tropicana Pink', 'Tropicana Rose' and 'Stardust Orchid' were disease free. A Rhizoctonia sp. was consistently isolated from diseased plants and further characterized as R. solani Kühn AG-1 based on its multinucleate cells and hyphal anastomosis with several AG-1 tester isolates. On potato dextrose agar, colonies displayed morphologies with characteristics of AG-1 IA and AG-1 IB, therefore, identification to AG subgroup was not made. Mature colonies ranged from light tan to brown and produced sclerotia, individually or in clumps, at the edge of the culture dish. Pathogenicity tests were performed by placing agar blocks, taken from the margins of 7-day-old cultures, on stems of eight healthy Madagascar periwinkle plants (15 to 20 cm tall). Inoculated and noninoculated control plants were held in a dew chamber at 26°C for 3 days and then moved to a greenhouse. Leaves on all inoculated plants developed water-soaked spots that turned dark brown or black prior to death, whereas noninoculated plants remained healthy. R. solani was reisolated from inoculated plants and its cultural characteristics were similar to those of the original isolate. Web blight occurs in Louisiana on Madagascar periwinkle used as landscape bedding plants, but has not been observed on container-grown plants. Web blight caused by R. solani AG-1 was previously reported on Madagascar periwinkle from Alabama (1). R. solani AG-1 has been reported previously as causing web blight in Louisiana on rosemary (2), dianthus (4), and verbena (3). To our knowledge, this is the first report of web blight on Madagascar periwinkle (C. roseus) in Louisiana.

Rhizoctonia leaf web blight and collar rot, two new diseases of Cinnamomum camphora

Two new diseases of Cinnamomum camphora namely leaf web blight and collar rot both caused by Rhizoctonia solani were recorded from Central Silviculture Nursery at New Forest, Dehra Dun and described. Two biotypes of the pathogen were associated with leaf web blight, one of which produced typical greyish brown to brown, hard, spherical to oval sclerotia in culture whereas the other biotype produced sclerotial bodies which were not so compact and hard and moreover, they did not have any definite shape and size. It is the latter biotype which occurred predominantly and was highly damaging to young seedlings. It caused defoliation to the extent of 60-100% at the peak of the growing season.Control of the disease included 1) reduction in the quantity of farm yard manure in the potting mixture to half, 2) sowing of seeds in late March or early April instead of June, and 3) application of Dithane M-45 (0.3%) @ 15 ml per tube as soil drench prior to seed sowing. Bayleton (0.1%) may be applied as foliar spray if the disease appears despite the above measures.Collar rot of C. camphora caused by R. solani was a serious disease problem as it wiped out 95-100% seedlings when they were attempted to be pricked in root trainers in July - August. Wrong timing of pricking of seedlings and large quantity of farm yard manure in the potting mixture are attributed to be the cause of the malady. Reduction in the quantity of farm yard manure in the potting mixture to half, pricking of seedlings in May instead of pricking them in July - August by timely sowing of seeds in late March or early April and application of Dithane M-45 (0.3%) @ 15 ml per tube before or at the time of pricking of seedlings are recommended to control collar rot.

Observations on the geographic distribution, ecology and conservation status of several phaseolus bean species in Costa Rica

Interest in bean genetic resources of Central America has resumed because of disease pressures (e.g., web blight, BGMV) and limitations of current bean varieties. As most of the diversity in landraces has been explored, focus is now on the exploration of wild forms of the primary gene pool and wild species of the secondary gene pool. A germplasm collection was carried out in the field and resulted in the collection of 29 wild populations for six Phaseolus species; it complemented field work done in 1987. Nine more populations were found for P. costaricensis, 10 for wild P. lunatus, one for P. oligospermus, one for P. tuerckheimii, four for wild P. vulgaris and four for P. xanthotrichus. Ninety-three herbarium voucher specimens were collected for 19 populations of the six species (deposited at CR). These results confirm the presence of wild P. vulgaris on both slopes of the central valley of Costa Rica, namely in the life zones bh-MB and bmh-P, and of P. costaricensis in the life zone bmh-MB. These life zones of limited range in Costa Rica have been heavily modified, thus fully justifying the germplasm collection for ex situ conservation. For both species the range of distribution in Costa Rica has been almost completely sampled. The life characteristics of each species that are relevant for their conservation in situ are briefly reviewed. Distribution ranges of each wild bean species are compared with the present extension of national parks, protected areas and fauna/flora sanctuaries, and suggestions for expanding such protected areas are made.

Web Blight of Fan Columbine, Aquilegia flabellata, Caused by Rhizoctonia solani AG1-IB : A New Disease Record

A new disease, found on fan columbine in August of 1997, first appeared as necrotic spots on leaves and within a week caused wilting of all the leaves. Fungal mycelia bound aerial parts of the plants together, formed mats of mycelia and eventually killed the plants. The pathogen, isolated from the infected leaves and stalks, was identified as Rhizoctonia solani AG1-IB in respect to hyphal anastomosis and culture types. The common name of web blight is proposed for this new occurrence on fan columbine (Kumonosu-byo in Japanese).

The effect of slash/mulch and alleycropping bean production systems on soil microbiota in the tropics

Mulch applied to tropical soil may increase soil health both through stimulation of microbiota beneficial to plant nutrient uptake and the suppression of plant disease. To test this hypothesis, we compared beans in three cropping systems: (1) mulched with secondary vegetation (slash/mulch), (2) mulched with foliage from alleycropped nitrogen-fixing trees, Calliandra calothyrsus, Gliricidia sepium and Inga edulis (alleycrop/mulch), and (3) unmulched, for their effect on positive microbial interactions (nodulation with Rhizobium and arbuscular mycorrhiza (AM)), and pathogenic associations (foliar and root diseases) in long-term field experiments in Costa Rica. In some dry years slash/mulched beans nodulated significantly more than the unmulched, though in wet years the nodule biomass was not significantly different between treatments. Beans grown in alleycropping systems mulched with foliage from Calliandra and Inga had lower nodule biomass than beans in slash/mulched and Gliricidia mulched plots at 3 and 5 weeks, probably due to high nitrogen levels from the decomposing vegetation of the tree mulch. Roots of bean plants were 95–98% colonized by AM fungi and there were no differences between the slash/mulched beans and the unmulched treatments. Slash/mulch had no effect on the foliar diseases angular leaf spot and web blight, due to their naturally low incidence in that year; slash/mulched decreased the severity of anthracnose (caused by Colletotrichum lindemuthianum). Slash/mulch decreased severity of a Fusarium-type root rot, but increased a Rhizoctonia-type. In the laboratory, microbial activity as determined by measurements of CO 2 respiration was greater in the slash/mulched than the unmulched system due to the high respiration of the mulch material. Our hypothesis that the use of mulch would favor mutualistic symbionts was not supported, which may be due to weather and nutrient interference. However, the hypothesis that mulch would reduce disease incidence was supported for certain diseases. A comparison of the microbial community with those of nearby natural systems can help provide a standard for the assessment of microbial health in agroecosystems and allow us to avoid the circularity of defining microbial communities as ‘healthy’ because they are found in cropping systems whose plants appear without disease.

First Report of Web Blight on Verbena Caused by Rhizoctonia solani.

Web blight was observed on verbena (Verbena × hybrida) during July 1999 in a cultivar trial planting at Burden Research Plantation in Baton Rouge, LA. Foliage blight, stem lesions, and branch death were common symptoms on 12 of 24 cultivars in the trial. Plant death occurred in cvs. Babylon Florena (one of four plants), Purple Princess (two of four plants), and Taylortown Red (two of four plants). Isolations from infected leaves and stems on acidified water agar consistently yielded a fungus with the mycelial and cultural characteristics of Rhizoctonia solani. Pathogenicity tests were carried out by placing 5-day-old fungal mycelial plugs, grown on acidified potato dextrose agar, at the base of healthy verbena stems and holding plants in a dew chamber at 26°C. After 3 days, foliage blight and stem lesions appeared on inoculated plants, and plants were moved to a greenhouse where temperatures ranged from 23 to 32°C. Seven of nine inoculated plants died after 7 days; noninoculated plants remained healthy. The fungal pathogen was reisolated from all inoculated plants. The fungus was identified as R. solani anastomosis group (AG)-1 IB based on multinucleate condition, type of sclerotia produced, and ability to anastomose with R. solani tester isolates of AG-1 IB. This is the first report of web blight on verbena.

METHODOLOGY FOR SCREENING COMMON BEAN RESISTANCE TO WEB BLIGHT

METHODOLOGY FOR SCREENING COMMON BEAN RESISTANCE TO WEB BLIGHT

A New Foliar Blight of Douglas-fir, Grand Fir, and Noble Fir Caused by a Binucleate Rhizoctonia-like Fungus.

The Oregon State University Plant Clinic received samples in 1996 and 1997 of Douglas-fir (Pseudotsuga menziesii) with web blight from Christmas tree plantations in Douglas, Lane, and Benton counties. Affected needles were 1 or more years old, uniformly brown, with no chlorosis or banding, and had a tendency to hang down from the branches and cling together, bound by mycelial webbing visible at ×10 magnification. In 1998, the disease was found on grand (Abies grandis) and noble (A. procera) firs in Christmas tree plantations as well as in native stands of Douglas-fir. Twigs of grand fir were killed, but not those of Douglas-fir. Brown needles may encompass half the side of a tree, destroying its value. This disease has been severe in Christmas tree plantations. A binucleate Rhizoctonia-like species has been consistently isolated from affected needles from all three tree species. The fungus has hyphae averaging 6.0 μm in diameter, with dolipore septa, slight constriction at the branch points, and acute to right angle branching. No sclerotia are formed, but monilioid cells develop after 2 weeks. Cell nucleation was determined with cotton blue or ethidium bromide. Koch's postulates were fulfilled by inoculating with an isolate obtained from grand fir onto 2-year-old Douglas-fir trees. Plugs (7 mm in diameter) of mycelium growing on potato dextrose agar plates were removed and placed onto the foliage of three trees. Plugs from uninoculated plates were used as a control on three additional trees. Inoculated trees were placed under mist on a greenhouse bench. Symptomatic needles were observed 5 weeks after inoculation. A fungus identical to the inoculum was obtained from symptomatic needles. No Rhizoctonia-like fungi were obtained from needles of the control plants. Further characterization of the fungus is underway. This new disease of Douglas-fir and true firs is potentially devastating, since Douglas-fir is a major component of managed timber forests in western Oregon, and the disease has been found at numerous sites in timber trees in the region.