Quantitative assay of resting spores of Polymyxa graminis on barley roots

A procedure was developed to examine amounts of Polymyxa graminis on eleven barley cultivars from a field experiment on a site infested with barley yellow mosaic virus (BaYMV) and which differed in field response to the virus. Powder produced from dried barley roots infected with P. graminis was soaked overnight at 4°C in a solution of 1 % sodium metaphosphate and 0.25% Tween 20. This was followed by high speed homogenisation, filtering, ultrasonic treatment of the residue and differential centrifugation. A suspension of individual resting spores free from other recognisable fungi was obtained, which ranged in concentration from 0.4 to 7.3 × 10⁷spores per g root. Repeated extraction of the residues suggested that most spores were liberated by the first cycle of treatment. The cultivar with the greatest incidence of BaYMV also had the most P. graminis; some cultivars resistant to BaYMV had less P. graminis but there was no general correlation between the incidences of virus and vector.     

Subtractive Cloning of DNA from Polymyxa graminis– an Obligate Parasitic Plasmodiophorid

A simple subtractive hybridization was applied for cloning of Polymyxa graminis deoxyribonucleic acid (DNA). Total DNA preparations from concentrated P. graminis resting spores and from roots of non‐infested (healthy) barley were digested with different restriction enzymes and batch hybridized, followed by cloning in a plasmid vector. Sequencing and blast analysis of coincidentally selected clones enabled construction of polymerase chain reaction primers specific to P. graminis DNA. Four Polymyxa‐specific primers showed different affinities to DNA of type I and type II P. graminis and to DNA of P. betae.     

A comparative study of Phialophora radicicola, an avirulent fungal root parasite of grasses and cereals

The biology and infection-behaviour of a typical isolate of Phialophora radicicola Cain have been compared with those of a representative isolate of Ophiobolus graminis (Sacc.) Sacc. Both species can utilize a nitrate source of nitrogen and both require thiamine and biotin for growth on inorganic nitro-gen; P. radicicola, but not O. graminis, was able to synthesize biotin when grown on asparagine as a nitrogen source. The pH range for good growth of P. radicicola in nutrient solution was narrower than that for O. graminis, and its growth rate on agar was only one-third. P. radicicola was the more active decomposer of cellulose, and its cellulolysis adequacy index was I.66 as com-pared with a value of 0.33 for 0. graminis. In agreement with prediction from Garrett's (I966) hypothesis on the cellulolysis adequacy index, saprophytic survival of P. radicicola in wheat straw was shortened by additional soil nitrogen, which prolongs survival of O. graminis.P. radicicola was found to spread ectotrophically over the roots of wheat, oats and barley by runner hyphae indistinguishable from those of O. graminis, but cortical infection caused no necrosis and no discernible check to growth of the infected cereals, nor any significant decrease in grain yield of inoculated wheat grown to maturity. Pre-existing infection of wheat roots by P. radicicola retarded spread of infection by O. graminis; inoculation of several grass species with P. radicicola reduced the extent of infection by O. graminis of wheat following the grasses.     

The invasion of root tips of cereals by the cereal cyst nematode Heterodera avenue

Newly germinated seedlings of susceptible cultivars of oats, wheat, barley and rye were inoculated with second-stage juveniles of Heterodera avenae in pots of sand. Subsequent examination showed oat root tips to be more commonly invaded, and by a greater range of nematode numbers than the other cereals. A comparison of oats and barley showed that lower nematode numbers in barley were not due to a higher emigration from barley; invasion, establishment and emigration by nematodes all being greater in oats. Second-stage juveniles were more likely to migrate prior to establishment in barley than in oats.     

The effect of barley yellow dwarf virus on field populations of the cereal aphids, Sitobion avenae and Metopolophium dirhodum

The numbers of cereal aphids, especially Metopolophium dirhodum in 1979, and Sitobion avenae in 1980, were significantly increased on BYDV infected wheat and oats in 1979, and wheat, barley and oats in 1980. The differences were probably caused by attraction of alates of each species to virus infected plants which had changed colour as a result of their infection. Significantly more alates of M. dirhodum were found on virus infected oats in 1979, and of S. avenae on oats and barley in 1980, although not on wheat in either year. probably because the colour contrast in wheat was less intense than in the other crops. Flight chamber experiments with alates of both species confirmed their visual attraction to virus-infected leaves. The interaction between virus, vector and host plants is discussed with reference to the ecology of virus spread.     

Use of Polymerase Chain Reaction To Detect the Take-All Fungus, Gaeumannomyces graminis, in Infected Wheat Plants

Gaeumannomyces graminis, the causative agent of take-all disease of wheat, barley, and oats, was detected in infected wheat seedlings by using the polymerase chain reaction to amplify Gaeumannomyces-specific DNA fragments. Nested primers and two rounds of amplification were used to amplify two fragments, approximately 287 and 188 bp in size, from G. graminis-infected wheat seedlings. The use of nested primers greatly decreased the number of nonspecific amplification products. Polymerase chain reaction products were not obtained with DNA from seedlings infected with several other phytopathogenic fungi or with DNA from uninfected seedlings. Amplified products were visualized on agarose gels, and their identities were confirmed by DNA hybridization. This method did not require culturing the fungus and has potential for detecting G. graminis in infested wheat, barley, or oat fields.     

Host range,mating type and population structure of Magnaporthe sp. of a single barley field in São Paulo state,Brazil

Brazil is blast disease hot spot because severe epidemics have occurred among wheat, triticale, rye, barley and oat crops. Although the first outbreak of barley blast appeared in 1998, little information is available. Therefore, this study aimed to examine host range, mating type composition and population structure of Magnaporthe sp. from a single barley field in São Paulo, Brazil. To examine pathogenicity, 25 Magnaporthe isolates were inoculated on five, three, two and two cultivars of barley, wheat, oat and rice, respectively, and one cultivar each of rye, corn, sorghum, triticale and certain weeds (Cenchrus echinatus, Setaria geniculata, Brachiaria plantaginea and Eleusine indica). Mating type distribution of 33 isolates was investigated by molecular tools. The genotypic divergence of 41 barley and five wheat isolates was investigated by 15 random amplified polymorphic DNA primers and unweighted pair group method with arithmetic mean. The host range of the barley blast pathogen included wheat, oat, rye and triticale but not rice and weeds. Sexual reproduction appeared to not be involved in the high genotypic diversity because only a single isolate, MAT1‐2, was identified. The majority of barley isolates clustered together with wheat blast, except for four, suggesting a different origin.     

Interaction between barley yellow dwarf virus and rust in wheat, barley and oats, and the effects on grain yield and quality

In three separate experiments, the upper leaf surface of the fifth formed leaf of wheat cv. Highbury, the fourth and fifth leaves of barley cv. Julia and the third and fourth leaves of oat cv. Mostyn were inoculated in a spore settling tower with wheat brown rust (Puccinia recondita f. sp. tritici), barley brown rust (P. hordei) or oat crown rust (P. coronata f. sp. avenae), respectively. Fewer pustules developed on distal portions of leaves of plants infected with barley yellow dwarf virus (BYDV) than on similar portions of leaves from virus-free plants. There were no significant differences in the number of pustules on proximal leaf portions. In barley and oats, the number of pustules on distal leaf portions was negatively correlated with the amount of yellowing of the leaf areas scored. In wheat, symptoms of BYDV were mild and leaves were little affected by yellowing. The latent period of rust on wheat and oats was not affected by BYDV. In barley, BYDV reduced the latent period of rust on leaf 5, but not on leaf 4, and reduced it on proximal, but not distal, leaf portions. In other experiments, BYDV reduced the yield of wheat and oats by 44% and 66%, respectively, while BYDV-infected barley was almost sterile. The appropriate rust reduced the yield of wheat, barley and oats by 33%, 13% and 86%, respectively. When infected with both BYDV and rust, yield of wheat and oats was reduced by 63% and 91%, respectively. Neither BYDV nor rust affected the percentage crude protein content of wheat grain, nor did rust affect that of barley. In oats, BYDV and rust each significantly increased crude protein of grain, but rust infection of BYDV-infected plants tended to reduce it.     

Development of powdery mildew on leaves of several barley varieties at different growth stages

On detached leaves and intact plants of several barley varieties at different growth stages, lower percentages of germinated conidia of Erysiphe graminis f.sp. hordei penetrated the host and initiated infection on the abaxial than adaxial surface. More and larger E. graminis colonies developed on the adaxial surface and these comprised more densely packed hyphae and produced more conidiophores than did colonies on the abaxial surface. These results are consistent with the observation that there is usually more powdery mildew on the adaxial than abaxial surface of barley leaves in the field. Smaller proportions of germinated E. graminis conidia penetrated and infected the host on leaves of adult or near-adult plants than on those of seedlings or juvenile plants. Older plants also supported fewer, smaller and less dense colonies with less sporulation than young plants. The effects of growth stage of the host plant on development of powdery mildew were much greater in some barley varieties, and with some E. graminis isolates, than others.     

Confirmation of the transmission of barley yellow mosaic virus (BaYMV) by the fungus Polymyxa graminis

Resting spores (cystosori) of Polymyxa graminis, selected from roots of barley plants infected with barley yellow mosaic virus (BaYMV), were used to start mono-fungal sand cultures. Out of 20 attempts using over 800 cystosori, P. graminis became established in 12, and in two of these BaYMV symptoms also occurred. BaYMV was detected by ELISA in extracts of dried roots heavily infected with cystosori and in zoospores of P. graminis. Calculations suggested that, on average, each zoospore carried less than 100 virus particles. In two virus acquisition experiments, non-viruliferous isolates of P. graminis failed to acquire BaYMV from roots of mechanically-inoculated plants. In two further experiments, non-viruliferous isolates were grown on rooted tillers produced from healthy plants and those infected with BaYMV by either vector or mechanical inoculation. Zoospores and cystosori of P. graminis subsequently transmitted the virus, but only from plants where it had been introduced by the vector. Repeated mechanical transmission appeared to have selected a strain of virus that could not be acquired and/or transmitted by the vector. The results provide convincing evidence that P. graminis is a vector of BaYMV but suggest that, in natural populations, only a small proportion of spores may be viruliferous.     

Production of a Chlorosis-Inducing, Host-Specific, Low-Molecular Weight Toxin by Isolates of Pyrenophora tritici-repentis Cause of Tan Spot of Wheat

An extraction procedure was developed to isolate toxin from cell-free culture filtrates of Pyrenophora tritici-repentis, cause of tan spot disease of wheat (Triticum aestivum). Designated Ptrtoxin, this low-molecular weight toxin (MW 800–1800) induces the characteristic chlororis associated with tan spot disease when inoculated to healthy wheat leaves and differentially inhibits the elongation of wheat seedling coleoptiles. The sensitivity of seven wheat, one barley, and non-host (corn) cultivars to Ptr-toxin corresponds with reported field reactions to the pathogen. Seven isolates of P. triticirepentis from wheat in Oklahoma and four from smooth bromegrass Bromus inermis, an alternative host of P. tritici-repentis), in North Dakota were evaluated for production of Ptr-toxin in vitro. Results indicate that Ptr-toxin may be involved in the expression of disease symptoms in tan spot of wheat, but is not the sole determinant of P. tritici-repentis pathogenicity. Instead, toxin production may contribute to the virulence of individual isolates of the pathogen. Isolates from the alternative host, smooth bromegrass, appear at least as toxigenic as those from wheat, with the exception of one which appeared more toxigenic than four of the wheat isolates tested. Possible relationships of Ptr-toxin to high-molecular weight toxins reported to be produced by P. triticirepentis in vitro (Ptr-necrosis-toxin) are not understood.     

Virulence of Septoria nodorum as Affected by Passage Through Detached Leaves of Wheat and Barley Cultivars1

Abstract Three genetically marked, single–spore isolates of Septoria nodorum from wheat were passed through detached leaves of wheat cvs Blueboy and Coker 747 and the barley cv. Boone to produce three sub–isolates per original isolate. Each sub–isolate was cultured for three pycnidiospore generations on its respective host. Virulence of each sub–isolate on detached leaves of Blueboy, Caldwell, Coker 747, and NK81W701 wheat, and Boone and Surry barley was compared with that of the original single–spore isolate from which it was derived. In most cases, sub–isolates passed through wheat were significantly more virulent than the originals on wheat cultivars. They also were more virulent to barley than the original isolates but they were less virulent to barley than to wheat cultivars. Isolate × cultivar interactions were statistically significant (P < .0001) for isolates passed through wheat or barley and were greater than isolate × cultivar interactions among the original isolates. In seven of eight isolates passed through wheat or barley, only the original genetic marker was recovered after three generations, indicating that cross–contamination could not account for the observed change of virulence. In the single case of apparent contamination, of a sub–isolate, virulence declined.     

Pathogenicity of Fusarium graminearum Schwabe isolates from Poland towards seedlings of cereal species

Ten isolates ofFusarium graminearum Schwabe originating from diseased cereal plants and kernels were tested for pathogenicity to various cultivars of wheat, rye, triticale and oats. The isolates varied greatly in their pathogenicity to the seedlings of the species, and were most pathogenic to rye and triticale, less pathogenic to barley and wheat and least pathogenic to oats.     

Germination of Erysiphe gratninis f.sp. hordei conidia on barley leaves

Germination of Erysiphe graminis f.sp. hordei conidia on leaves of several barley cultivars was studied in the laboratory. On both detached leaves and intact plants, within 48 h of inoculation a higher proportion of conidia had germinated on the basal and middle portions of the adaxial leaf surface than on the corresponding portions of the abaxial surface. Such differences between surfaces were not observed near the leaf tip. Similar results were obtained with all the cultivars and growth stages tested, and with five isolates of E. graminis, and are consistent with the observation that there is usually less powdery mildew on the abaxial than the adaxial surface of barley leaves. With most of the barley genotype/mildew isolate combinations tested, within 48 h of inoculation higher proportions of conidia germinated on seedlings and juvenile plants than on older plants. Inherited characteristics which affect spore germination on the leaf surface may be important factors in the development of adult-plant resistance of barley to powdery mildew, particularly in certain genotypes.     

Bionomics of aphids reared on cereals and some Gramineae

In controlled temperature, light and relative humidity, Metopolophium dirhodum and Sitobion avenae multiplied more on young Proctor barley than on Blenda oats, and less on Cappelle wheat. Rhopalosiphum padi increased in number fastest on barley and slowest on oats. More survived, and generation lengths seemed shorter, on barley for M. dirhodum and S. avenae and on wheat for R. padi. Tests with young cereals outdoors generally agreed with those in controlled conditions. On mature plants, there were more M. dirhodum on barley, more R. padi on wheat and more S. avenae on oats than on the other cereals. Given a free choice in large cages outdoors, most aphids were found on barley. When allowed to choose between grasses, more M. dirhodum were on Dactylis glomerata, Poa pratensis and Festuca pratensis, more R. padi on Lolium perenne and F. pratensis, and more S. avenae on D. glomerata and L. perenne. Most aphids of all species combined were on F. pratensis, Lolium and Phleum, and fewest on Festuca rubra and Holcus mollis.     

Reduction of Take-all Inoculum by Rotation with Lupins, Oats or Field Peas

The feasibility of use of lupins, oats and field peas as alternative rotation crops to reduce inoculum of the take-all fungus (Gaeumannomyces graminis var. tritici) (under Western Australian field conditions) and disease in following wheat was investigated with a one year field trial, the soil from which was used in two succeeding pot experiments. The possible mechanisms of reduction of inoculum and disease by these crops were examined testing the soil for pathogen and disease suppression. Rotation with lupins or oats for two seasons reduced (P <0.05) inoculum of the take-all fungus and lupins, oats or field peas reduced (P <0.05) disease in following wheat. Lupins alone reduced inoculum and disease, (P <0.1) after one season. No apparent suppression of the pathogen in the absence of host plants was recorded after one season of rotation, but after two seasons, lupins, oats or field peas all suppressed (P <0.02) growth of the pathogen within soil. However only field pea soil suppressed take-all in comparison with the wheat control. Although after two seasons all rotation crops were effective in reducing inoculum and disease the mechanisms of reduction appear to differ between the rotation crops used in this study.     

II. Cereal disease: the interplay between resistance and pathogenicity: Inheritance of pathogenicity in Gaeumannomyces graminis var. tritici

Variation in host response of isolates of the eyespot pathogen from different sources was examined over a number of years. Pathogen types were found in intensively-cropped couch-infested cereal sites that were almost as virulent on Agropyron repens (couch) as on wheat or barley. The commonly occurring wheat (W) type isolates from couch-free cereal crops were virulent on wheat and barley but avirulent on couch. Couch (C) types were isolated not only from couch but also from wheat, barley and oat crops with couch infestation. In pathogenicity tests on rye, C. types did not differ in virulence from the more commonly occurring W types. Aegilops ventricosa was equally resistant to both types. W type isolates from wheat and barley were examined to assess differential pathogenicity on wheat and barley. Sequential cropping with single cereal crops was used to separate out possible specific types. Isolates from fourth wheat and fourth barley crops were more pathogenic on the original than on the alternative host. When comparisons were made between isolates from third and fifth consecutive wheat and barley crops only those from barley showed a preference for the original host. An experiment comparing isolates from third and seventh consecutive wheat and barley crops showed a decline in virulence from the short to the longer sequences on the alternative but not on the original host.     

Studies on transmission of Indian peanut clump virus disease by Polymyxa graminis*

The plasmodiophoromycete fungus, Polymyxa graminis was observed in the roots of Sorghum bicolor, S. sudanense, Pennisetum glaucum, Triticum aestivum, Cyperus rotundus, Eleucine coracana, Zea mays, Tridax procumbens and Arachis hypogaea collected from Indian peanut clump virus (IPCV)-infested fields. Examination of roots of IPCV-infected S. bicolor, S. sudanense, P. glaucum and T. aestivum grown in previously air dried field soil also showed the presence of cystosori of P. graminis. IPCV-infested soil stored at room temperature for 3 years transmitted the virus to A. hypogaea, T. aestivum and S. bicolor. Roots extracted from IPCV-infected P. glaucum and S. bicolor containing cystosori, and dried root fragments incorporated into sterile soil, transmitted the virus to A. hypogaea and T. aestivum. The root extracts contained primary zoospores of the fungus, presumably arising from cystosori. Utilising root fragments of S. sudanense containing cystosori as inoculum P. graminis was shown to infect both monocotyledonous and dicotyledonous plants. Profuse cystosorus production in rootlets only occurred in monocotyledonous plants. In dicotyledonous plants, in general, only few rootlets showed cystosori. Indian isolates of P. graminis appear to differ from isolates from temperate soils in that they can infect dicotyledonous plants and have a much wider host range.     

Influence of Seed‐borne Cochliobolus sativus (Anamorph Bipolaris sorokiniana) on Crown Rot and Root Rot of Barley and Wheat

The effect of seed‐borne pathogens of wheat and barley on crown and root rot diseases of seven barley cultivars (Jimah‐6, Jimah‐51, Jimah‐54, Jimah‐58, Omani, Beecher and Duraqi) and three wheat cultivars (Cooley, Maissani and Shawarir) was investigated. Bipolaris sorokiniana and Alternaria alternata were detected in seeds of at least eight cultivars, but Fusarium species in seeds of only two barley cultivars (Jimah‐54 and Jimah‐58). Crown rot and root rot symptoms developed on barley and wheat cultivars following germination of infected seeds in sterilized growing media. Bipolaris sorokiniana was the only pathogen consistently isolated from crowns and roots of the emerging seedlings. In addition, crown rot and root rot diseases of non‐inoculated barley cultivars correlated significantly with B. sorokiniana inoculum in seeds (P = 0.0019), but not with Fusarium or Alternaria (P > 0.05). These results indicate the role of seed‐borne inoculum of B. sorokiniana in development of crown rot and root rot diseases. Pathogenicity tests of B. sorokiniana isolates confirmed its role in inducing crown rot and root rot, with two wheat cultivars being more resistant to crown and root rots than most barley cultivars (P < 0.05). Barley cultivars also exhibited significant differences in resistance to crown rot (P < 0.05). In addition, black point disease symptoms were observed on seeds of three barley cultivars and were found to significantly affect seed germination and growth of some of these cultivars. This study confirms the role of seed‐borne inoculum of B. sorokiniana in crown and root rots of wheat and barley and is the first report in Oman of the association of B. sorokiniana with black point disease of barley.     

Fungal communities and health status of roots of winter wheat cultivated after oats and oats mixed with other crops

Health status of winter wheat roots and thecomposition of wheat root fungi were studiedover 1996-1999 following the cultivation ofoats in a pure stand and mixed with otherplants as forecrops. The infection of wheatroots by >Gaeumannomyces graminis wasobserved to be largely dependent on the kind offorecrop; the best being oats in a pure stand,and then oats with pea or lupin mixtures. Inthe emergence and shooting phases, saprophyticfungi were dominant, while in the stage of harddough stage mainly pathogenic fungi, especially>G. graminis were common. The pathogenicfungi were mostly represented by >G.graminis and >Fusarium spp., while >Rhizoctonia spp. were much less frequent.The composition of the fungal communitydepended considerably on the forecrop anddevelopment phase of the plant. The kind offorecrop significantly affected the frequencyof infection by >G. graminis. The highestnumber of isolates was obtained from wheat rootsof crops grown after a mixture of oats andbarley.