摘要:

Sudden‐death disease of cloves has been present and steadily increasing in both Zanzibar and Pemba for many years. The only premonitory symptom is a slight chlorosis followed by thinning of the foliage and reduction of the absorbing system. Death follows after a period, which may vary from only a few days to many months. Death occurs from lack of water caused by the disorganization of the absorbing roots.The outbreaks fall into three classes: (1) the sporadic, which ceases to spread after killing a few trees; (2) the ‘Pemba’ type showing clear peripheral spread; (3) the diffuse epidemic type.In Pemba some 500 small outbreaks are scattered through the clove areas and some seem to be passing from type (2) to type (3). The total number of trees affected there is less than in Zanzibar, where the situation approximates to a single outbreak involving half of the clove‐growing area of the island. The rate of spread of the disease varies, but it is accelerating.Various causes, physical, physiological, and pathological, have been suggested to account for the condition. The epidemiology suggests that all but a pathogenic hypothesis can be discarded. Of the possible pathogens, a virus carried by a lethargic vector is the most probable. Suspicion is attached to a scale insect which is tended and transported by the red tree‐ant,Oecophylla

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1949.tb06939.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Apparently healthy cacao seedlings were compared with those infected before planting with ‘swollen shoot’ viruses. The leaf area and the fresh and dry weights of each organ were measured. Infected plants were lower in dry weight, leaf area, relative growth rate and net assimilation rate; a smaller proportion of the dry matter was in the leaves and lateral roots, a larger proportion in stems and tap roots. Infection caused extensive necrosis of the lateral roots, and reduced the rate of depletion of reserves in the cotyledons and the water content of the plant. Many of these effects were apparent within a month of infection and plant

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1949.tb06940.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

In tobacco plants infected with mild strains of virusX, severe strains may arise as mutations which multiply locally.Several strains of virusXgradually lost infectivity for potato on continued culture in other hosts such as tobacco.Strains that are poorly infective or invasive for potato may take considerable time to move into the growing shoot from the tuber in detectable amounts.Pronounced and apparently spontaneous increases in the severity of the strain that dominates in potatoes may occur in field crops.

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1949.tb06941.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

The range and type of symptoms produced by strains of virusXare briefly described.Of ten strains examined, six were identical in serological cross‐absorption tests, and two (T.B.R.XandB) differed considerably from the others. The possibility of preparing strain‐specific antisera is indicated.The protection afforded by mild strains against severe strains in tobacco andDatura tatulawas complete for all strains tested except T.B.R.XandB, which gave only partial protection. Thus there is a correlation between wide serological differences and a breakdown in the protection afforded in the plant.There was no correlation between type of symptoms caused and serological relationships.There was some evidence to show that strains from the same source were more closely related than strains from different sour

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1949.tb06942.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Various workers have proposed formulae to express the spatial distribution of insect‐borne diseases. All the published data examined, as well as the Rothamsted data for the spread of rugose mosaic and leaf‐roll from point sources in potato crops, were fitted as well by the simple empirical expression logI=a + bxas by more complex expressions (I= number of infective punctures at a distancexfrom the source after a given time, andaandbare constants for any one given set of field conditions). It is suggested that distances should always be given in metres, in order to give comparable results from one investigation to another. In the analysis of data on rugose mosaic and leaf‐roll in'different years, it is shown thataandbvary independ

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1949.tb06943.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

The development of cankers caused byNectria galligenaon apple is described; the pathogen exploits all the tissues outside the xylem and will also penetrate the xylem to an appreciable depth, invading the xylem parenchyma, vessels and fibres.Spread in the peripheral tissues is checked to some extent by successive barriers laid down by wound phellogens, but the barriers are eventually passed by the mycelium. There appear to be two distinct ways in which these barriers are passed: there may be direct penetration which appears to be associated with an aggregation of the mycelium into blocks, and symptoms suggestive of mechanical rupture; or there is an alternative route in which the pathogen grows in the lumen of the fibres and emerges behind the barrier.In the xylem the spread of the pathogen is checked by tyloses and gumming in the vessels, and by gumming in the parenchymatous tissues, but in the fibres there appears to be no defence mechanism, and the spread beyond the gum barriers was usually recorded in the xylem fibres. It is suggested that the presence of the pathogen in the xylem fibres might provide the explanation of the formation of cankers on partially healed pruning cuts, and this type of canker is described in some detail.The development of the canker depends on the balance between the development of the pathogen and the resistance of the host, and there is some discussion on the relation of this balance to the control of the disease.

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1949.tb06944.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Samples of cured hay have been stored in closed containers at relative humidities of 0–95% at temperatures varying from 5 to 25° C. A curve relating equilibrium moisture content to relative humidity of the store has been constructed and the incidence of first mould growth noted. An explanation has been suggested to account for the apparent discrepancy between earlier published work concerning the onset of moulding and conditions normally prevailing on the farm. In laboratory storage experiments temperature conditions are relatively constant, whereas with hay stacked in the open air the high atmospheric humidity in winter is offset by a lowered temperature, and the higher summer temperature is accompanied by a lower moisture content in the h

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1949.tb06945.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Experiments on the hatching ofHeterodera rostochiensishave shown that the addition of ammonium carbonate to potato root water markedly inhibits hatching when the concentration of ammonia introduced is approximately 100 p.p.m. The strong acid salts of ammonia in equivalent amounts have no such inhibitory effect.These observations are linked with experiments on the effect of partial sterilization of soil on the hatching ofH. rostochiensis, and it is demonstrated that delay of hatching in such soils is only effective so long as the ammonia concentration within the soil is maintained at a sufficiently high level.

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1949.tb06946.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Mangold clamps in many districts of the British Isles were found to provide overwintering sites forMyzus persicae(Sulz.),Hyperomyzns staphyleae(Koch) andAulacorthum solani(Kalt.). After a severe winter, when other means of overwintering are few, clamps may be the most important source ofMyzus persicae.OnlyMyzus ascalonicusDoncaster was found in swede clamps.Factors affecting the infestation of clamped mangolds byM. persicaewere the number of aphids on the crop when lifted, the methods of topping and clamping the roots, and the temperature in the clamp.M. persicaewas introduced on the leaves, and close topping was often an efficient means of control. Close topping did not controlHyperomyzus staphyleae;normally, this aphid does not seem to be a root‐feeding species, but with artificially colonized mangolds it fed on both exposed roots and foliage. It is not known how this species enters the clamps. The temperature in clamps was influenced by that of the outside air and the type of cover, but changes were long‐term and did not reflect diurnal variations in external air temperature. Straw, covered with soil, was the best form of cover.In addition to harbouringMyzus persicae, mangold clamps are also important sources of sugar‐beet yellows

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1949.tb06947.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Colorado beetles can survive up to 10 days in sea water and still be capable of flying when the temperature reaches 80° F. (26·67° C.). The beetles that reached the Channel Islands in May 1947 and in May 1948 prove that they can travel across about 30 miles of sea, the approximate distance of Guernsey from the Cherbourg peninsula. The invasions occurred as a result of migrant beetles from the Cherbourg peninsula which dropped on the sea between the Channel Islands and the French coast and drifted asho

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1949.tb06948.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY