摘要:

SUMMARYField‐caged and open‐plot populations of the aphidSitobion avenaeon winter wheat (cv. Maris Widgeon) were sampled approximately twice‐weekly in the summer of 1978. Cage populations began at growth stage 10.2 (Feekes scale) (Zadoks, G. S. 52); they were removed by spraying with pirimicarb at growth stages 10.54 (71), 11.1–11.2 (77) and 11.2–11.3 (85) respectively. All cage populations reduced mean weight per grain but the effect per aphid unit was lowest in the population of longest duration. Although the aphid index in the open plots was higher than that in the early cage treatment, yield was unaffected. Cage infestations affected the breadmaking quality of the grain: percentage flour extraction was reduced and there was an increase in colour, nicotinic acid content and thiamine (vitamin B1) content of the flour; percentage nitrogen in the flour was unaffected but there was a reduction in baking value and in the high molecular weight glutenin content; infestation also reduced α‐amylase activity. Different aspects of grain quality did not change in parallel with one another or with yield changes and thus damage thresholds will vary according to the yield/quality measure under c

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1981.tb00750.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

SUMMARYA study was made of the oviposition responses of the siricid woodwasps,Sirex noctilio, S. juvencus, S. cyaneusandUrocerus gigasto experimentally debilitated spruce and pine trees. Treatments included felled, fire‐burned, logged and girdled trees which were treated at intervals before bioassay with cages siricid females. In comparable treatments,S. noctilioshowed a distinct preference for pines although it was quite successful on spruce while the other siricid species displayed a preference for spruce. TheSirexspecies successfully attacked more recently treated material althoughU. gigasshowed a preference for timber in a more advanced state of debility. All species showed preferences for girdled trees although other treatments were also favoured by the different siricid species. Overall, the siricids preferred the area above the girdle irrespective of tree species and there was a higher rate of eggs above the girdle in treated spruce trees. A wide range of moisture content in the timber appeared to have no effect on drilling activity or the successful development of eggs. The response of a wild population ofS. juvencusto the treated spruce is also described. The results are discussed in relation to the ecology of Siricidae in Europe and the establishment ofS. noctilioin Australia. It is suggested that of the species studied, onlyS. noctilioposes a serious threat to the living pine stands of Australi

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1981.tb00751.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

SUMMARYTwo mathematical models are presented which describe progress of disease, caused by an unspecialised pathogen, in pure and mixed stands of cereal cultivars. One is a simple, discrete model utilising the parameters infection frequency (lesions/spore) and sporulation rate (spores/lesion/event) on each cultivar. The model predicts that, in most circumstances, the amount of disease in mixtures will be equal to or less than the arithmetic mean of the component pure stands. An increase in disease is only predicted in situations where the ranking of cultivars with respect to infection frequency and sporulation rate is opposed. In the other model differential equations are proposed. Solutions of these equations indicate that disease amelioration can usually be expected in such mixtures, confirming the conclusions of the discrete model and that disease levels will deviate around the geometric, rather than arithmetic, mean of the pure stands. However the advantages of this model lie in wider generality and conceptual, rather than practical, utility.

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1981.tb00752.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

SUMMARYEpidemic development ofSeptoria nodorumwas examined in pure stands and mixtures of two spring wheat cultivars, Kolibri and Maris Butler. In mixtures, disease development was reduced almost to that of the more resistant pure stand (Maris Butler). Disease levels, however, were low and no significant differences in grain yield per head were demonstrated. Disease development was further investigated for the host‐pathogen combinations winter wheat/S. nodorumand winter barley/Rhynchosporium secalis. With both combinations the mixed stands again reduced disease levels to almost that of the more resistant pure stands. In general, mixed stands were more effective againstR. secalis, although, withS. nodorum, disease levels were low. The yield response in mixed stands differed for the host cultivars and was not significantly changed by the presence of disease. The complexity of analysing such situations and the implications of these findings for mixtures of cultivars differing in resistance to non‐specialised pathogens are discus

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1981.tb00753.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

SUMMARYSeed tubers with severe (71% cover) or slight (12% cover) common scab selected from a badly infected crop of cv. Maris Piper were boxed for sprouting in November and February and planted in field experiments at Woburn and Rothamsted in 1980.The severely scabbed seed lost more moisture than the slightly scabbed seed during sprouting and sprouted from a larger number of eyes but total stem numbers were greater on plants from the slightly scabbed seed. Plant growth during the first 6–9 wk after planting was less from the severely scabbed than from the slightly scabbed seed. This was shown by records of ground cover, leaf area index, fresh weights of stems and leaves, total tuber yield (18% lower at 9 wk) and tuber number. Subsequently these differences became non‐significant. Seed with slight scab boxed in November produced smaller numbers of progeny tubers than that boxed in February but there were no other consistent effects of boxing date on growth and yield. Scab incidence on progeny tubers was very slight and unrelated to the severity of disease on the seed tubers.It seems unlikely that severe scab infection of seed tubers would significantly decrease tuber yields except perhaps in early potato product

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1981.tb00754.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

SUMMARYThe effects of direct drilling, shallow cultivation and ploughing on the infection of winter wheat roots by the take‐all fungus (Gaeumannomyces graminisvar.tritici) were studied on three field sites over a number of years. All three soil types were categorised by Cannell, Davies, Mackney&Pidgeon (1978) as suitable for sequential direct drilling. The results show that a smaller proportion of roots was infected at depth in the direct‐drilled plots in May/June. However by July these differences had all but disappeared and an estimate of infection in the top 7 cm of the roots (approximately equivalent to traditional hand sampling for take‐all) gave a reliable comparison of the total take‐all on plants grown under these different cultivation

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1981.tb00755.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

SUMMARYInoculation studies provided further evidence thatOncobasidium theobromaeis the cause of vascular‐streak dieback of cocoa and that the fungus can penetrate undamaged, soft leaves. Spores are forcibly ejected from basidia at night and dispersed by wind. They are rapidly killed by exposure to normal day‐time conditions and require free water for germination. Spore dispersal is probably very limited under plantation conditions and disease spread occurs along a steep gradient into new plantings. Disease increase in young plantations was found to occur at a rate of 0.31 per unit per month; in one block 100% of seedlings were infected and 55% were killed within 33 months of planting. The disease was most damaging in seedlings younger than 18 months old, although there was evidence that it also caused substantial yield loss in mature susceptible cocoa. Sporophore production and infection rate were correlated with rainfall incidence. The unusual grographical distribution of the disease and aspects of its control are discus

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1981.tb00756.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

SUMMARYA virus (isolate SYM) obtained from spinach plants in England with a severe yellow mottle disease induced symptoms resembling those of tobacco rattle virus (TRV) in several indicator species but caused systemic necrosis inChenopodium amaranticolorandC. quinoa. It was transmitted to bait plants grown in soil containing the nematodeTrichodorus primitivus.Purified virus preparations contained rod‐shaped particles that were predominantly of four modal lengths: 188 nm (L particles), 101 nm (S particles), 57 nm and 48 nm (together called VS particles), containing RNA with mol. wts of 2.4, 1.5, 0.7 and 0.6 million, respectively. L particles (s°20= 300 S) and S particles (230S) greatly outnumbered VS particles (c.150S). All particles contained a single polypeptide species with estimated mol wt of 24 700, slightly larger than those previously reported for tobraviruses. Purified L particles were infective but both L and S particles were needed to induce the production of virus nucleoprotein particles. VS particles were not infective and apparently had no qualitative or quantitative effect on infection by L or by L plus S particles. S particles carried determinants for serological specificity and ability to invadeC. amaranticolorsystemically.Isolate SYM produced pseudo‐recombinants with isolate PRN of TRV. Also, isolates CAM, OR and PRN of TRV, and isolate SYM, were found to be distantly related by three kinds of serological test. No relationship was detected between these isolates and pea early‐browning virus in gel‐diffusion precipitin tests or electron microscope serological tests, but a distant relationship between isolate SYM and pea early‐browning virus was found by micro‐precipitin tests. Isolate SYM therefore has closer affinities with TRV than with pea early‐browning virus and is considered to be a distinctive

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1981.tb00757.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

SUMMARYA virus, now named peanut green mosaic virus (PGMV), was isolated from groundnut (Arachis hypogaea) in India and identified as a member of the potato virus Y group by electron microscopy, aphid transmission, and its chemical properties. It was sap transmissible to 16 species of the Leguminosae, Solanaceae, Chenopodiaceae, Aizoaceae and Pedaliaceae;Phaseolus vulgariswas a good local lesion host. PGMV remained infective in buffered groundnut leaf sap at dilutions of 10‐3after 3 to 4 days at 25 °C, or heating for 10 min to 55 °C but not 60 °C. PGMV was transmitted in the non‐persistent manner byAphis gossypiiandMyzus persicaebut was not seed‐borne.Purified virus preparations contained flexuous filamentous particlesc.750 nm long which sedimented as a single component with a sedimentation coefficient (S°20w) of 171S, and contained a single polypeptide (mol. wt 34 500 daltons) and one nucleic acid species (mol. wt 3.25 × 106daltons). PGMV is serologically unrelated to peanut mottle virus (PMV) and other viruses infecting leguminous crops. Infected leaves contained cylindrical, cytoplasmic

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1981.tb00758.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

SUMMARYEnzyme‐linked immunosorbent assay (ELISA) was adapted for the efficient detection and assay of potato leafroll virus (PLRV) in aphids. Best results were obtained when aphids were extracted in 0.05 M phosphate buffer, pH 7.0, and the extracts incubated at 37 °C for 1 h before starting the assay. Using batches of 20 green peach aphids (Myzus persicae), about 0.01 ng PLRV/aphid could be detected. The virus could also be detected in single aphids allowed a 1‐day acquisition access period on infected potato leaves. The PLRV content of aphids depended on the age of potato source‐plants and the position of source leaves on them. It increased with increase in acquisition access period up to 7 days but differed considerably between individual aphids. A maximum of 7 ng PLRV/aphid was recorded but aphids more usually accumulated about 0.2 ng PLRV per day. When aphids were allowed acquisition access periods of 1–3 days, and then caged singly onPhysalis floridanaseedlings for 3 days, the PLRV content of each aphid, measured subsequently, was not strongly correlated with the infection ofP. floridana.The concentration of PLRV in leaf extracts differed only slightly when potato plants were kept at 15, 20, 25 or 30 °C for 1 or 2 wk, but the virus content of aphids kept on leaves at the different temperatures decreased with increase of temperature. PLRV was transmitted readily toP. floridanaat all temperatures, but by a slightly smaller proportion of aphids, and after a longer latent period, at 15 °C than at 30 °C.The PLRV content ofM. persicaefed on infected potato leaves decreased with increasing time after transfer to turnip (immune to PLRV). The decrease occurred in two phases, the first rapid and the second very slow. In the first phase the decrease was faster, briefer and greater at 25 and 30 °C than at 15 and 20 °C. No evidence was obtained that PLRV multiplies inM. persicae.These results are compatible with a model in which much of the PLRV in aphids during the second phase is in the haemocoele, and transmission is mainly limited by the rate of passage of virus particles from haemolymph to saliva.The potato aphid,Macrosiphum euphorbiae, transmitted PLRV much less efficiently thanM. persicae. Its inefficiency as a vector could not be ascribed to failure to acquire or retain PLRV, or to the degradation of virus particles in the aphid. Probably only few PLRV particles pass from the haemolymph to saliva in this species.The virus content ofM. euphorbiaecollected from PLRV‐infected potato plants in the field increased from early June to early July, and then decreased. PLRV was detected both in spring migrants collected from the plants and in summer migrants caught in yellow water‐traps. PLRV was also detected inM. persicaecollected from infected plants in July and August, and in trapped summer migrants, but their PLRV content was less than that ofM. euphorbiae, and in some instances was too small for uneq

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1981.tb00759.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY