Showing papers in "Plant Disease in 1996"

Variation in Fusarium graminearum and cultivar resistance to wheat scab.

Abstract: Understanding variation in pathogen virulence and cultivar resistance is important for development of effective strategies for breeding wheat cultivars resistant to scab. Six isolates of Fusarium graminearum from China and the United States were compared for variation in cultural characteristics and virulence on nine wheat cultivars with different degrees of resistance to scab. The isolates varied in their cultural characteristics and ability to cause scab, but there was no consistent specificity of cultivar resistance or pathogen virulence. Therefore, a mixture of local isolates is an appropriate inoculum to screen for scab resistance. Subculturing the fungus on potato dextrose agar for eight generations did not reduce virulence. In the greenhouse, eight cultivars were tested five times over 3 years by inoculating one central floret in a spike with an Indiana isolate of the fungus. Cultivars Ning 7840, Sumai 49, Fu 5114, and Sumai 3 were consistently resistant. The fungus spread from the inoculated spikelet to noninoculated spikelets of resistant cultivars in less than 20% of the plants, and spread was not evident until 12 days after inoculation. All plants of susceptible cultivar Clark showed spread of infection, and symptoms appeared on noninoculated spikelets by 8 days after inoculation. Sudden blight on the top part of the spike may be an important characteristic of highly susceptible cultivars. Measurement of spread of scab within a spike is a stable and reliable estimate of cultivar resistance.

Induced systemic resistance in cucumber and tomato against cucumber mosaic cucumovirus using plant growth-promoting rhizobacteria (PGPR).

Abstract: Plant growth-promoting rhizobacteria (PGPR) strains 89B-27 (Pseudomonas fluorescens) and 90-166 (Serratia marcescens), which previously demonstrated induced systemic resistance in cucumber against some fungal and bacterial diseases, were tested for their capacity to protect Cucumis sativus L. cv. Straight 8 from disease development of cucumber mosaic cucumovirus (CMV). Seed treatment with both PGPR strains significantly and consistently reduced mean numbers of symptomatic plants when CMV was inoculated onto cotyledons. Plants treated with PGPR did not develop initial symptoms 14 days after CMV inoculation and remained symptomless throughout the experimental period. In a comprehensive study with cucumber, no viral antigen could be detected by enzyme-linked immunosorbent assay (ELISA) in any asymptomatic PGPR-treated plants ; whereas CMV was evident in every leaf of symptomatic plants. The same two PGPR strains were evaluated for effects on CMV symptom development in tomato in three experiments by measuring the disease severity at six observation dates. In all experiments, the area under the disease progress curve (AUDPC) was significantly lower with strain 89B-27 than in the nonbacterized control. The AUDPC with strain 90-166 was also significantly lower than with strain 89B-27. These results suggest that PGPR should be further evaluated for their potential to contribute toward management of viral plant diseases.

Diurnal release of ascospores by Gibberella zeae in inoculated wheat plots

Abstract: The daily pattern of ascospore release by Gibberella zeae (= Fusarium graminearum), the causal agent of Fusarium head blight of wheat, was investigated in artificially inoculated wheat plots. Mature perithecia and ascospores appeared on corn colonized by G. zeae 2 to 3 weeks after being placed in the plots (mid June). Ascospores over the plots were sampled with a Burkard continuous 7-day spore sampler. Temperature, relative humidity (RH), leaf wetness, and rainfall were also recorded in the plots on an hourly basis. Ascospores were released during the first three weeks of July in 1992 and 1993, with hourly concentrations of 600 to 9,000 asco-spores/m 3 . Ascospore release typically showed a diurnal pattern. Release began around 1600 to 1800 hours, reached a peak usually before midnight, and declined to low levels by 0900 hours the following morning. The beginning of ascospore release was correlated with a rise in RH during early evening hours. Ascospore release occurred before leaf wetness was detected and was not correlated with rainfall or continuous high RH during the preceding daylight hours. Peak ascospore releases occurred 2 to 4 days after major rainfalls. Ascospore release was diminished on days with continuous RH >80% or rainfall >5 mm. Light rain during a spore release event temporarily washed ascospores from the air; however, heavy rain (>5 mm) stopped spore release. This data suggests rainfall may be needed for perithecial and ascospore formation and maturity on crop debris, but not to trigger the actual release of ascospores. Perithecial drying during the day, followed by sharp increases in RH, may provide the stimulus for release of ascospores.

Inheritance of resistance to scab in two wheat cultivars from Brazil and China

Abstract: One of the major diseases that reduces the quality of wheat is scab. Quality is reduced by the production of toxins in grain by the pathogen Fusarium graminearum. Various genetic sources of resistance have been identified, but the degree of resistance in most cultivars is insufficient. Although both South American and Chinese germ plasm are being used in breeding programs around the world, it is not known whether the resistance genes contributed by these sources are different. The purpose of this study was to determine the mode of inheritance and number of genes involved in the scab resistance of a wheat cultivar from Brazil and one from China, both known to possess intermediate to high resistance in regard to spread of the disease in the wheat head. The resistant Brazilian cultivar Frontana was compared to the highly resistant Chinese cultivar Ning 7840 in crosses with CNO79, a highly susceptible Mexican wheat, as the susceptible parent. Random F2-derived F7 lines from the six crosses possible (including reciprocals) among the parents were studied for their field response following inoculation with F graminearum on two inoculation dates. Three methods of disease assessment were compared. The two resistant parents were shown to possess two unique dominant genes each, with all four genes being different. Combining their resistance genes may produce higher levels of resistance. The two inoculation dates provided the same gene postulations. The three disease assessment methods essentially measured the same basic process of disease spread within the head, based on the calculation of the genotypic correlations between the methods and the similarity in gene postulations. The easiest and most relevant method should therefore suffice for assessing the spread of scab through the wheat head.

Pathogenic fungi associated with Fusarium foot rot of winter wheat in the semiarid Pacific Northwest

Abstract: Winter wheat plants and soil were collected from 288 nonirrigated fields in the semiarid Pacific Northwest during 1993 and 1994. Fungi associated with 5,390 crown and subcrown internodes from 10 Oregon and nine Washington counties were identified. Fusarium graminearum Group 1 was most widespread and the dominant pathogen associated with a crown and root rot named Fusarium foot rot or dryland root rot. E culmorum was widely distributed in soil but was detected in plants in only half as many locations as F. graminearum. Other pathogens included Bipolaris sorokiniana, Microdochium nivale, and F. avenaceum. Highly variable isolation frequencies for all five pathogens were presumed related to a very dry and a very wet survey year. Each pathogen was considered dominant or co-dominant at one or more sites during one or more years. All five species and F acuminatum and F. oxysporum included isolates capable of killing wheat seedlings in the greenhouse.

Characterization of the genetic resistance to Colletotrichum lindemuthianum in common bean differential cultivars

Abstract: The inheritance of genetic resistance to bean anthracnose in genotypes Catrachita and SEL 1360 derived from two anthracnose differential cultivars, AB 136 and G 2333, respectively, is described. Segregation data from three different F2 populations and their respective F 2:3 families indicated that a single dominant gene is responsible for the anthracnose resistance in Catrachita. In the test for allelism, chi-square test confirmed that the single dominant resistance gene in Catrachita was situated at a different locus from previously characterized resistance genes A, Are, Mexique 1, Mexique 2, and Mexique 3. It is proposed that the single dominant resistance gene present in Catrachita be assigned the genetic symbol Co-6, Co for Colletotrichum and 6 because it is the sixth major anthracnose resistance gene characterized and reported in the literature. Segregation in the three F2 populations where SEL 1360 was used as the resistant parent fitted a 3:1 (R-:rr) ratio and a 1:2:1 (RR:Rr:rr) ratio in the F 2:3 families. Segregation data suggested that a single dominant gene was conditioning resistance to anthracnose in SEL 1360. The test for allelism involving SEL 1360 indicated that the single dominant gene in SEL 1360 is independent from A (Co-1), Are (Co-2), Mexique 1 (Co-3), and Mexique 2 (Co-4) genes. However, the dominant gene in SEL 1360 did not segregate independently from the resistance gene Mexique 3 in the differential cultivar TU, demonstrating that both dominant alleles are located at the same locus. Deployment of major genes of Middle American origin, such as Co-6 and Mexique 3 (Co-5), in different combinations with other characterized genes of Andean origin is possible and should contribute to more durable anthracnose resistance in common bean.

Etiology, importance, and distribution of Verticillium wilt of cotton in southern Spain.

Abstract: Surveys for Verticillium wilt in 142 cotton fields in 1981 to 1983 and 1985 indicated that the disease is widespread in the Guadalquivir Valley, southern Spain, where it occurs in 80.0 to 82.5% of the fields. Verticillium wilt was most prevalent in the upper valley in the 1981 to 1983 surveys and in the lower valley in the 1985 survey. Analyses of dry soil samples collected in spring 1985 by means of an Andersen sampler detected Verticillium dahliae propagules in 35.1% of the fields surveyed. Inoculum density in soil was much higher in the lower valley (average 37.1 CFU/g) than in the upper and central areas of the valley (average 4.7 CFU/g). The increasing importance of the disease in the lower valley corresponds to the spread of a defoliating pathotype of V. dahliae in this area during the early 1980s. Two groups of isolates, including 100 mild nondefoliating and 90 severe defoliating, were distinguished among 191 isolates by means of morphological, physiological, and pathogenicity tests. The remaining isolate was identified as nondefoliating with intermediate virulence. All defoliating isolates were obtained from the lower valley, while nondefoliating isolates were widespread. Isolates of the defoliating pathotype produced elongated and rounded microsclerotia on water agar, were able to grow on sanguinarine-amended potato dextrose agar, and fluoresced under UV light. Isolates of the nondefoliating pathotype formed only rounded microsclerotia on water agar, were inhibited greatly on sanguinarine-amended PDA, and did not fluoresce under UV light. Furthermore, the defoliating isolates had an optimum temperature for in vitro growth of 24 to 27°C compared with 21 to 24°C for the nondefoliating isolates.

Diseases of wheat in long-term agronomic experiments at Pendleton, Oregon.

Abstract: Diseases of winter wheat were evaluated over 3 years in four long-term (27- to 60-year) cropping system experiments. Disease incidence and severity were evaluated with respect to seasonal precipitation and soil chemical and microbiological parameters. Take-all and eyespot were associated with increasing precipitation, and Rhizoctonia root rot and Fusarium crown rot were favored by drought. Eyespot and crown rot increased with rate of applied nitrogen and were inversely proportional to soil pH. Surface residue from previous crops had variable effects on diseases. Crown rot increased with amount of surface residue and was directly correlated with soil organic nitrogen and carbon. Surface residue also had a variable effect on Rhizoctonia root rot, depending on the magnitude of soil microbial respiration; root rot increased directly with amount of residue in a wheat-summer fallow rotation and was unaffected by residue or tillage in a wheat-pea rotation. Repeated burning of wheat stubble caused variable disease response, depending on precipitation and nitrogen rate. At high fertility, burning suppressed Pythium root rot and Rhizoctonia root rot, and enhanced eyespot and take-all. Effects of crop rotations on diseases appeared related to soil microflora effects on pathogen survival or virulence. Rhizoctonia root rot was most damaging in wheat-fallow rotation, Pythium root rot in wheat-fallow and annual wheat, and eyespot and crown rot in annual wheat. Diseases were collectively least prevalent where nitrogen in a wheat-fallow rotation was applied as pea vines or manure, rather than as inorganic fertilizer. Diseases also were generally less damaging in a wheat-pea rotation than in an annual wheat or wheat-fallow rotation. Soilborne plant pathogenic fungi appeared to suppress wheat yield by 3 to 12%. Long-term experiments provided insights to crop management and seasonal effects that are unlikely to be identified in short-term experiments.

Response of transgenic cucumber and carrot plants expressing different chitinase enzymes to inoculation with fungal pathogens.

Abstract: Three lines of cucumber cv. Endeavor, each transformed with a chitinase gene originating from petunia (acidic), tobacco (basic), or bean (basic) using Agrobacterium tumefaciens, were compared with nontransgenic plants for response to inoculation with Alternaria cucumerina, Botrytis cinerea, Colletotrichum lagenarium, and Rhizoctonia solani. In both growth chamber studies using whole plants and in vitro inoculations conducted with detached leaves, no differences in disease development (rate and final levels) were detected between transgenic and nontransgenic plants. Carrot cvs. Nanco and Golden State transformed with two chitinase genes (from petunia and tobacco) were also evaluated for response to inoculation with the pathogens Alternaria radicini, B. cinerea, R. solani, Sclerotium rolfsii, and Thielaviopsis basicola. A detached petiole inoculation method was used to compare nontransgenic and transgenic plants. The rate and final extent of lesion development after 7 days were significantly (P = 0.01) lower in the transgenic plants expressing the tobacco (basic) chitinase gene upon inoculation with B. cinerea, R. solani, and S. rolfsii, but not in plants expressing the petunia (acidic) chitinase gene. There were no detectable differences with A. radicini or T. basicola in either group of transgenic punts. These results demonstrate the in planta efficacy of a basic chitinase protein in providing enhanced tolerance of carrot to three fungal pathogens; however, the efficacy of chitinase gene transformation as a strategy for enhancing disease tolerance in plants can be influenced by the plant species used, the type of chitinase protein expressed, and the characteristics of the fungal pathogen.

Relative resistances of potato clones in response to new and old populations of Phytophthora infestans

Abstract: Potato cultivars and clones were evaluated in Washington and New York in 1993 and 1994 for field reaction to recent immigrant genotypes of Phytophthora infestans. Plants were visually evaluated at regular intervals for percent blighted foliage. Relative cultivar susceptibilities were compared by ranking the values obtained for areas under disease progress curves (AUDPC) of each line tested. Cultivar rankings in response to infection by new, immigrant isolates of P infestans were nearly identical to rankings obtained previously with isolates prevalent prior to 1990. The cultivars Norchip, Hilite, Russet Norkotah, Goldrush, Superior, and Shepody were more susceptible than Russet Burbank. White Rose and Ranger Russet were similar in susceptibility to Russet Burbank. C008-3008-1, ND-2438-7R, Kennebec, and Elba were less susceptible than Russet Burbank. Comparisons of the number of blighted tubers at harvest showed that foliage and tuber susceptibilities differed among cultivars. Shepody and Russet Norkotah tubers were most susceptible to tuber blight of those tested.

Aspergillus species and mycotoxins in figs from California orchards.

Abstract: Although 23 different Aspergillus spp. decayed figs in California orchards, only A. niger occurred in more than 0.2% of the figs. The black-spored Aspergillus isolates that caused the disease fig smut were classified as A. niger var. niger, A. niger var. awamori, A. japonicus, and A. carbonarius. Different fungi differed in their association with Aspergillus Section Nigri (causal agents of fig smut) infections in figs. For example, most figs infected with Aspergillus Section Flavi (potential aflatoxin producers) also had infections by Section Nigri. For other fungi, there was either no significant relationship between fig infections by these fungi and Section Nigri or simultaneous infections by Section Nigri were fewer than expected. Insect damage to the fig fruit, predominantly by navel orangeworm (Amyelois transitella), did not significantly increase the colonization of figs by Aspergillus spp. The incidences of infection by Aspergillus (Sections Nigri, Aspergillus, Flavi, and Circumdati) in figs differed little for different harvests. Figs naturally infected with A. alliaceus, A. melleus, A. ochraceus, and A. sclerotiorum of Aspergillus Section Circumdati contained ochratoxin up to 9,600 ng/g, although only 40% of the figs with these fungi had more than a trace amount of ochratoxin. Aflatoxin contamination in figs naturally infected with Aspergillus Section Flavi varied according to the species involved. No aflatoxins were detected in all figs infected with A. tamarii and in most figs infected with A. flavus. High levels of aflatoxin (>100 ng/g) were detected in 83% of the figs infected by A. parasiticus, but in only 32% of the figs infected by A. flavus. Section Flavi isolates from fig orchard soils were tested for their ability to produce the mycotoxins aflatoxin and cyclopiazonic acid. Aspergillus parasiticus isolates always produced aflatoxin but never cyclopiazonic acid; A. flavus strain S (producers of small sclerotia) isolates always produced both aflatoxin and cyclopiazonic acid, but strain L (producers of large sclerotia) isolates frequently did not produce aflatoxin or cyclopiazonic acid; and A. tamarii isolates never produced aflatoxin but always produced cyclopiazonic acid. Aspergillus flavus was recovered from the soil, at fewer than 6 CFU/g of dry soil of every fig orchard assayed in 1992 and 1993. Although A. parasiticus was rarer in fig fruit than was A. flavus for each year, in orchard soil A. parasiticus was more frequent than A. flavus. Isolates of A. flavus strain L were much more common in the orchard soil and fig fruit than those of strain S. Figs in commercial orchards seem to be a favorable substrate for infection by and growth of Aspergillus spp.

Methyl iodide, an ozone-safe alternative to methyl bromide as a soil fumigant.

Abstract: Methyl iodide was tested as a possible replacement for methyl bromide as a soil fumigant due to the scheduled removal of methyl bromide from the market. Methyl iodide is a better methylating agent than methyl bromide; it is rapidly destroyed by UV light and therefore unlikely to be involved in stratospheric ozone depletion. In laboratory and field trials, we tested methyl iodide alone or in comparison with methyl bromide for effectiveness in controlling the fungi Phytophthora citricola, P. cinnamomi, P. parasitica, and Rhizoctonia solani; the nematode Heterodera schachtii; and the plants Abutilon theophrasti, Chenopodium album, C. murale, Convolvulus arvensis, Cyperus rotundus, Poa annua, Portulaca oleracea, and Sisymbrium irio. In addition, we compared methyl iodide for biocidal effectiveness with seven other alkyl iodides. In both laboratory and field trials, when compared at equivalent molar rates, methyl iodide was equal to or better than methyl bromide in controlling the tested soilborne plant pathogens and weeds. When compared with other alkyl iodides, methyl iodide was the most effective fumigant.

Root and stem rot of cucumber caused by Fusarium oxysporum f.sp. radicis-cucumerinum f.sp. nov.

Abstract: Root and stem rot of cucumber (Cucumis sativus) caused by a new Fusarium oxysporum forma specialis is reported for the first time. Of 18 cultivated species from six different botanical families that were artificially inoculated, two species of Cucurbitaceae-mdon (C. melo) and sponge gourd (Luffa aegyptiaca)-showed root and stem rot similar to that caused by isolates of F. oxysporum that attack cucumber. Optimum temperature for disease development was close to 17°C. Disease symptoms and pathogenicity tests confirm that a new forma specialis of F. oxysporum is the incitant of the disease. The author proposes to name this forma specialis as F. oxysporum f. sp. radicis-cucumerinum f. sp. nov.

Quantitative-genetic basis of aggressiveness of 42 isolates of fusarium culmorum for winter rye head blight

Abstract: Forty-two isolates of Fusarium culmorum obtained from diseased plant parts collected from fields in nine European countries and Australia were tested on a synthetic winter rye population. A spore suspension of each isolate was sprayed during midanthesis in five environments (location-year combinations) onto the heads. All isolates were pathogenic as judged by head blight rating scored on a 1 to 9 scale and grain weight relative to the noninoculated control. Isolates differed, however, in their ability to cause disease (aggressiveness). Quantitative geno-typic variation for aggressiveness occurred, while isolate-environment interaction variance, although significant, accounted for only 14% of total variance averaged over both traits. Correlations for aggressiveness across environments ranged from 0.6 to 0.8 (P = 0.01). Estimates of heritability on an entry-mean basis were high (h 2 = 0.9) for both traits, indicating that a substantial proportion of the phenotypic variation was caused by genetic effects. It is concluded that aggressiveness of F. culmorum is inherited as a complex trait.

Association of a badnavirus with citrus mosaic disease in India.

Abstract: A previously unreported badnavirus was found to be associated with a mosaic disease occurring commonly in orchard trees and nursery plants of citrus in India. The virus had 30 x 150 nm nonenveloped bacilliform particles typical of badnaviruses, and was found by immunosorbent electron microscopy to be related serologically to sugarcane bacilliform and to eight other badnaviruses. Upon PCR amplification using degenerate oligonucleotide primers based on conserved badnavirus genomic sequences, the viral genomic nucleic acid yielded a product similar in size to that obtained with other badnaviruses which have been shown to contain dsDNA genomes. The virus, named citrus mosaic badnavirus (CMBV), was graft- and dodder-transmitted to 13 of 14 citrus species and cultivars, and was transmitted by mechanical inoculation from symptomatic citrus to healthy Citrus decumana.

Development of leaf anthracnose and its effect on yield and grain weight of sorghum in West Africa

Abstract: Leaf anthracnose (Colletotrichum graminicola) development and its effect on yield and grain weight were studied for three seasons between 1990 and 1992 on two inbred sorghum cultivars exposed to natural inoculum in Mali, West Africa. Infection on the susceptible, medium-maturing (70 to 80 days to 50% flowering) cultivar IS 18696 (race caudatum) increased gradually until anthesis, then increased rapidly so that most leaves were killed at physiological maturity. Development of the disease on the moderately resistant early-maturing (<70 days to 50% flowering) cultivar IS 25105 (race guineense) was slower even after anthesis than on IS 18696, and leaves were not killed at physiological maturity. Mean disease severities for 15 plants on a 1 to 6 scale at about 50% flowering for IS 18696 were 3.1, 4.0, and 3.4 in 1990, 1991, and 1992, respectively. In contrast, disease severities for IS 25105 at 50% flowering were 2.1 in 1990 and 1991, and less than 3.3 at 11 days after 50% flowering in 1992. Areas under the disease progress curve (AUDPC) for the three years were 61, 127, and 121 for IS 18696, and 47, 112, and 122 for IS 25105. Yield loss calculated from fungicide-treated and unsprayed plots was as high as 67% in 1990, 55% in 1991, and 57% in 1992 for the susceptible cultivar IS 18696. The disease had no effect on yield for the moderately resistant cultivar IS 25105 in 1990 and 1991, and caused only 4% loss in 1992. Kernel weight losses were between 18 and 36% for IS 18696 and 15% for IS 25105 in 1991 only.

Effect of sheath blight on yield in tropical, intensive rice production system

Abstract: Sheath blight (Rhizoctonia solani) of rice (Oryza sativa) is associated with intensive and high-input production systems. The effect of sheath blight on yield, the effect of high nitrogen (N) rate on sheath blight incidence, and the stages of crop that are most susceptible to the disease and vulnerable to yield losses were investigated. Grain yield data from a long-term experiment showed a quadratic polynomial curve in response to N input. An initial increase in N supply corresponded to an increase in yield, but at the highest N level, a reduction in yield was observed. Sheath blight incidence also increased with increasing N level. The estimated yield reduction from sheath blight in plots receiving the highest N rate ranged from 20 to 42% in artificially inoculated plots. The highest sclerotial population recorded was only 2.02 sclerotia per 500 g of oven-dried soil or about 1.23 sclerotia per liter of puddled paddy soil. This low sclerotial density in our studies suggested that sclerotia may not be the primary source of inoculum in a tropical lowland rice system. Crop residues colonized by the pathogen may play an important role in sheath blight epidemics in this intensive rice production system. Screenhouse and field experiments indicated significant yield losses when sheath blight infection started at panicle initiation, booting, or flowering. The effect of sheath blight on yield resulted primarily from a reduction in mean seed weight and a lower percentage of filled spikelets. No yield loss or decrease in yield components was observed when infection started at tillering or grain filling.

Bacterial diseases of rice I. Pathogenic bacteria associated with sheath rot complex and grain discoloration of rice in the Philippines

Abstract: Surveys were made to determine if bacterial pathogens were associated with grain discoloration and leaf sheath rot of rice in the major rice-growing districts in the Philippines. In 1988 and 1989, 304 diseased plant and grain samples were collected from 16 different provinces, and bacterial pathogens were found in 64 samples representing 12 of the provinces. In other cases, sheath rot or grain discoloration was attributed to fungal disease, insects, or abiotic factors. Pathogenic bacteria (204 strains) represented 3.6% of the total number of strains isolated and tested. Inoculations into the leaf sheaths of seedlings or the flag leaf sheaths produced symptoms characteristic of infection by Burkholderia glumae (formerly Pseudomonas glumae) and Pseudomonas fuscovaginae. None of the pathogens could be associated with distinctive symptoms. Strains of putative P. fuscovaginae were isolated from samples collected from the tropical lowland provinces of Laguna, Palawan, and Davao, and from the tropical highland (above 950 m) province of Ifugao. Since bacterial pathogens were isolated from 21% of the collections with sheath rot or grain discoloration, bacteria appear important in these disease complexes in the Philippines.

Characterization and pathogenicity of Rhizoctonia from soybean.

Abstract: Of 102 isolates of Rhizoctonia recovered from roots and stems of soybean, 98 were R. solani and were identified to four anastomosis groups (AG): AG-2-2 (3.1%), AG-3 (2.0%), AG-4 (45.9%), and AG-5 (37.8%); 10.2% of the isolates did not consistently anastomose with any of the tester isolates (AG-1 to 9 and AG-BI). Four isolates from roots were binucleate Rhizoctonia. AG-2-2, AG-4, and AG-5 were virulent on soybean seedlings and adult plants, whereas AG-3 caused small lesions only on tap roots of adult plants. The binucleate Rhizoctonia were not pathogenic on soybean. AG-5 was generally less virulent on soybean than AG-2-2 and AG-4, but when inoculum was placed in direct contact with seeds, AG-5 caused high levels of pre- and postemergence damping-off. AG-5 also caused high disease severity ratings on adult soybean when the inoculum level was increased. Sugar beet seedlings were highly susceptible to AG-2-2 and AG-4, but only slightly susceptible to AG-5. Dry bean, mustard, and flax seedlings were susceptible to AG-2-2 and AG-4, and dry bean and flax were slightly susceptible to AG-5. AG-4 and AG-2-2 caused moderate reductions in emergence of sunflower, and AG-2-2 caused a root rot on corn seedlings. These results indicate that AG-5 could be an important soybean pathogen and that other rotational crops are hosts to R. solani recovered from soybean.

Two genetically distinct populations of Fusarium oxysporum f.sp. lycopersici race 3 in the United States.

Abstract: Isolates of Fusarium oxysporum f. sp. lycopersici from the United States, Australia, and Mexico were examined for vegetative compatibility, pathogenicity on a set of differential tomato cultivars, mitochondrial DNA (mtDNA) restriction fragment length polymorphisms (RFLPs), and isozyme polymorphisms. The collection of 113 isolates included representative isolates of races 1, 2, and 3, and the three previously described vegetative compatibility groups (VCGs). A new and previously undescribed VCG, VCG 0033, was identified that contained race 3 isolates of F. oxysporum f. sp. lycopersici from locations in Arkansas, North Carolina, and Florida. Previously described race 3 isolates belonged to VCG 0030. Recently recovered race 3 isolates from Mexico also belonged to VCG 0030. VCGs 0030 and 0033, which both contain race 3 isolates, represent genetically distinct populations based on vegetative compatibility, mtDNA RFLPs, and isozyme differences. These data support the hypothesis that isolates of F. oxysporum pathogenic on tomato may represent two genetically distinct evolutionary lineages. No differences in pathogenicity or virulence (aggressiveness) were detected among race 3 isolates from each of the two VCGs in greenhouse inoculation tests.

A new disease of maize and wheat in the high plains.

Abstract: Dent corn, sweet corn, and blue corn showing severe viruslike symptoms were found in Texas, Kansas, Colorado, Idaho, Nebraska, and Utah in 1993 and 1994. The disease can be devastating to susceptible genotypes, and was also found in wheat. Pathogen nucleoproteins from the infected tissue were concentrated by ultracentrifugation and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealing the coat protein of wheat streak mosaic virus (WSMV) and a unique, approximately 32-kDa protein. Extraction of symptomatic tissue and purification by density gradient centrifugation yielded a product with a nearly pure 32-kDa protein but no defined viruslike structure. Neither electron microscopy of leaf dip preparations nor density gradient purification trials consistently revealed particles other than WSMV. An antiserum has been prepared to the 32-kDa antigen. The pathogen, which we have termed the high plains virus, is transmitted by eriophyid mites.

Fungi associated with soybean seedling disease in Iowa.

Abstract: Surveys were conducted over a 2-year period to identify fungi associated with soybean seedling disease in Iowa. Fungi were isolated from diseased soybean seedlings collected from 52 and 66 locations in 1993 and 1994, respectively. The percentages of major fungal taxa isolated from soybean seedlings in the 2 years were Rhizoctonia solani, 27.5% in 1993 and 27.3% in 1994; Fusarium spp., 11.9% in 1993 and 13.7% in 1994; and Pythium spp. and Phytophthora sojae, cumulatively 60.5% in 1993, and 31.7% and 24.3% in 1994 respectively. Other isolated fungi were the seed decay pathogen Phomopsis longicolla, and the nonpathogenic Rhizopus stolonifer and Trichoderma viride. Species of Fusarium and Pythium were identified as F. acuminatum, F. equiseti, F. oxysporum, P. aphanidermatum, P. irregulare, P. myriotylum, P. sylvaticum, P. ultimum var. sporangiiferum, and P. ultimum var. ultimum. Repeated tests of pathogenicity confirmed that Pythium spp., Phytophthora sojae, and R. solani were the major causal fungi associated with the seedling disease complex of soybeans in Iowa.

Phenotypic characterization of the rice blast resistance gene Pi-2(t)

Abstract: The spectrum of resistance conditioned by the blast resistance gene Pi-2(t) in rice was characterized. Near-isogenic line (NIL) C101A51 carrying Pi-2(t) in the genetic background of the indica rice cultivar CO39 was resistant to 455 Philippine isolates of Pyricularia grisea when inoculated in greenhouse tests. The P. grisea isolates, collected mainly from 1983 to 1992, represented at least 18 lineages based on DNA fingerprinting data. The NIL C101A51 was planted at two sites used by the International Rice Research Institute for the evaluation of blast resistance, and for which the P. grisea populations had been analyzed by DNA fingerprinting. After initial resistance at the two sites, C101A51 eventually showed susceptible lesions. All isolates (n = 60) collected from C101A51 at the two sites were identified as belonging to a single lineage (more than 95% band similarity using the repetitive DNA probe MGR586), distinct (less than 80% band similarity) from the approximately 2,000 Philippine isolates previously subjected to DNA fingerprinting. The group of isolates, designated lineage 44, was inoculated to a set of NILs in the CO39 genetic background in greenhouse tests. Lineage 44 was found to be compatible to lines carrying Pi-2(t) (in C101A51 and 5173, the donor of Pi-2(t)), Pi-4 a (t) (in C102PKT), Pi-4 a (t) + Pi-? (in C105TTP-4-L23), and CO39. C101A51 was found to be resistant in nursery tests at 24 of the 40 sites in 16 countries in which it was tested during 1991 to 1993 through the International Network for the Genetic Evaluation of Rice. These results suggested that Pi-2(t) could condition resistance to many lineages of P. grisea and could be useful if pyramided or otherwise deployed with genes that condition resistance to lineage 44.

Biological control of turfgrass diseases with a rhizosphere competent strain of trichoderma harzianum

Abstract: Trichoderma harzianum strain 1295-22 is a commercially available biocontrol agent that is strongly rhizosphere competent and able to control several plant pathogenic fungi. Two formulations were tested for their ability to control brown patch caused by Rhizoctonia solani, dollar spot caused by Sclerotinia homoeocarpa, and Pythium root rot and blight caused by Pythium graminicola. In growth chamber trials, soils planted with creeping bentgrass were amended with the granular formulation to give 10 6 cfu/g. All three diseases were significantly reduced by this treatment. Populations of Pythium spp. were suppressed under laboratory conditions by strain 1295-22. In field trials conducted over 4 years, strain 1295-22 reduced dollar spot severity relative to untreated plots. Monthly applications of granular or peat-based formulations of T harzianum 1295-22 reduced initial disease severity by as much as 71% and delayed disease development by up to 30 days. The persistence of strain 1295-22 in soil core samples from treated creeping bentgrass greens was also measured. After application of strain 1295-22, soil populations of Trichoderma spp. increased 100-fold relative to populations in untreated plots. Population levels remained at least an order of magnitude greater in treated than in untreated plots. Even after overwintering, population levels remained at or above 105 cfu/g of dry weight of the sample.

Contribution of adult plant resistance gene Yr18 in protecting wheat from yellow rust

Abstract: Yellow, or stripe, rust of wheat (caused by Puccinia striiformis), an important disease in many wheat growing regions of the world, is best controlled through genetic resistance. The Yr18 gene is known to confer slow rusting resistance in adult plants. This study was conducted to quantify the effectiveness of Yr18 in reducing losses in grain yield and other traits under high yellow rust pressure. Fungicide-protected and nonprotected plots of two near-isogenic lines, Jupateco 73R with the Yr18 gene and Jupateco 73S without it, were sown on two planting dates during two crop seasons. Yellow rust epidemics were initiated by artificial inoculation. The area under the disease progress curve and final rust severity were significantly higher for both plantings of nonprotected plots of Jupateco 73S, confirming that the Yr18 gene does confer slow rusting in Jupateco 73R. Comparison between protected and nonprotected treatments showed that yellow rust infection caused grain yield losses of 31 to 52% in Jupateco 73R and 74 to 94% in Jupateco 73S. This indicates that the slow rusting resistance conferred by Yr18 protected grain yield in the range of 36 to 58%, depending on the year and sowing date. Grain yield losses in both cultivars were mainly associated with reductions in kernel weight and kernels per m 2 ; however, reductions in spikes per m 2 and kernels per spike also contributed to yield loss in Jupateco 73S. A reduction in plant height was observed in late plantings of Jupateco 73S, suggesting that early yellow rust infection may affect stem elongation on susceptible cultivars. Although deployment of Yr18 alone is not recommended in areas with high yellow rust pressure, previous studies have shown that the Yr18 pyramided with other slow rusting genes, a combination commonly known as the Yr18 complex, should provide effective control of yellow rust.

Evaluation of bacterial antagonists for reduction of summer patch symptoms in Kentucky bluegrass.

Abstract: Ten bacterial strains previously identified for their ability to control soilborne pathogens on agronomic crops were evaluated for their ability to suppress summer patch disease caused by Magnaporthe poae in Kentucky bluegrass (Poa pratensis L). Bacterial strains varied in the ability to inhibit the growth of M. poae in agar plate bioassays, although most strains inhibited the fungus to some degree. Three strains originally isolated from wheat, Pseudomonas fluorescens 2-79, P. fluorescens 13-79, and Bacillus subtilis D-39Sr, and two strains originally isolated from cotton, Enterobacter cloacae EcH-1 and EcCT-501, significantly reduced summer patch symptoms by between 29 and 46% compared to untreated control plants after a 5-week period in the growth chamber. The five strains did not reduce summer patch disease in field trials in 1990. However, in 1991, B. subtilis D-39Sr and E. cloacae EcH-I reduced summer patch severity by 53 and 49%, respectively, over the entire season compared to untreated control plots. In 1994, both B. subtilis D-39Sr and E. cloacae EcH-1 also reduced summer patch severity over the entire season by up to 39 and 34%, respectively, compared to the untreated control in field plots. All five bacteria that reduced summer patch in growth chamber studies were present in the rhizosphere of greenhouse/growth chamber grown turfgrass at 10 4 to 10 6 CFU/g of fresh weight sample 2 weeks after application to plants. A single sampling of field plots during 1991 and four separate samplings of field plots during 1994 indicated that introduced bacteria were present within the turf at populations above 10 3 CFU/g of sample.

Occurrence of resistance in Uncinula necator to triadimefon, myclobutanil, and fenarimol in California grapevines

Abstract: Uncinula necator subcultures from 19 vineyards in four regions in California were analyzed for sensitivity to triadimefon, myclobutanil, and fenarimol. The means of EC 50 values to triadimefon, myclobutanil, and fenarimol of U. necator subcultures from a vineyard without previous exposure to demethylation inhibition (DMI) fungicides were 1.40, 0.15, and 0.13 mg/liter, respectively. The highest means of EC 50 values were found in the Central Coast region, and frequency distributions were skewed most toward higher resistance to all three fungicides. Subcultures with high resistance levels also were present in the other regions examined. A time course study performed in one vineyard, where resistant strains were reported, demonstrated a steady and significant increase in EC 50 values for all three fungicides during the growing season after multiple applications of triadimefon. Increased resistance to triadimefon, but not to myclobutanil and fenarimol, was maintained in early-formed ascospores released after the growing season.