摘要:

SUMMARYInspection of naturally or artificially infectedHevearoots showed thatForms lignosuscan penetrate undamaged roots directly, but does so more readily through wounds or natural openings like lenticels, or through the bases of lateral roots and bark scales. Therefore, Pomes‐infected trees should be identified by leaf symptoms rather than by uncovering and inspecting roots, as this generally leads to root injury, which facilitates fungal penetration.Initial fungal entry into host tissue appears to be by mechanical pressure alone, but deeper penetration is through the action of extracellular enzymes. The fungus remains intercellular in the cortex but is intracellular in the woody tissue. Ray cell walls are penetrated mechanically, but the xylem through pits. The time taken for various stages of infection to occur is assessed.The amount of damage done by the fungus to roots and the blocking of xylem vessels by tyloses suggest that yellowing, curling and buckling of leaves on infected trees are drought symptoms and not a reaction to fungal toxins. The host reacts to the invasion of the cortex by forming a cork cambium and to the invasion of the woody tissue by blocking individual cells with phenols and resins, which could be important when breeding disease resistantHevearoot stock

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1973.tb01302.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SUMMARYEpidemiological studies, especially of obligate pathogens, often require healthy test plants and the incubation of infections in isolation. Ventilation of glasshouses with spore‐free air is helpful but costly, and provides only a few discrete environments. The apparatus described is cheap and simple and enables many separate covered plant pots to be supplied with filtered air. It was designed to help measure the incubation time and frequency of viable conidia ofErysiphe graminis, but may also be useful for studying other pathogens and, with some modification, for plant breeding or quarantin

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1973.tb01303.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SUMMARYIncubation time ofErysiphe graminisf.sp.hordeivaried seasonally, but between early May and late September it was always between 5 and 8 days. This is approximately the time of year when the 30‐year weekly mean air temperature exceeds 10oC. Amount of infection during summer was decreased by hot, dry weather. In winter, incubation times were more variable and long incubation times were usually associated with few infections. Variation in incubation time during the summer is unlikely to be of practical importance in determining the rate of epidemic developmen

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1973.tb01304.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SUMMARYExperiments are described in which successive groups of healthy, susceptible, potted hop‐plants were exposed, each 24 h, to the weather and to sources ofPseudoperonospora kumuliinoculum in an unsprayed hop garden for 48 h periods during May and June in 1969, 1970 and 1971. The infection which arose after standardized incubation in isolation was measured and then related, both by inspection and by multiple regression analysis, to the conditions during exposure.Severe infection could be associated with certain sequences of events in which rain contributed substantially to plant wetness; relatively light or no infection developed when dew provided the wetness of a period. Infection was markedly correlated with the occurrence of infection periods, as defined from earlier growth‐room studies, especially when dew‐wetness was omitted.The regression analysis showed that, in all years, infection was highly correlated with variables reflecting wet conditions. It was inconsistently correlated with vapour pressure deficit (VPD) and with sunshine, not at all with temperature, and with airborne spore concentration only in the combined years. The spore catches of a funnel trap, used in 1971, which depended upon rain varied closely with infection.R2values for multiple regression equations indicated that, in the years separately and together, well over 70% of the variation in infection could be explained by the environmental variables and, in 1971, almost 90%. Variables (temperature, VPD, airborne spores) which singly were poorly correlated with infection often significantly improved regression equations which were based on variables expressing wetness. When prediction of each year's infection levels was attempted using selected regression equations calculated from data of the other 2 years there was, in general, very good agreement between measured and estimated values. The best predictions were given by an equation utilizing rain‐wetness duration, rainfall amount and airborne spore concentration while an equation based solely on relative humidity and rainfall was only slightly inferior.The results are discussed in relation to the possible value of the regression models for short‐term forecasting of hop dow

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1973.tb01305.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SUMMARYInvestigations have been made into the possibility of a chemical basis for resistance to coffee berry disease in certainCoffea arabicavarieties. Chloroform extracts of the cuticular wax of berries, purified by fractionation and chromatography, proved toxic to a range of fungi includingColletotrichum coffeanum, the causal organism of coffee berry disease. Leaf cuticular wax also contained this fungitoxic principle, though at a lower level. The possible importance of this antifungal material in determining resistance to coffee berry disease is discussed.

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1973.tb01306.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SUMMARYTwo virus diseases, vein yellows and mosaic, that commonly infect pear trees, reduced the growth and crop of four cultivars (Beurré Hardy, Doyenne du Cornice, Conference and Williams' bon Chrétien) in an experiment continued for 13 years. Reduction in yield was mainly due to the smaller size of infected trees. These produced about 30% less fruit than healthy trees when infected with vein yellows and 40 % less fruit when also infected with mosaic. Fruit size was decreased in only one year. The fertility of Cornice flowers was impaired.The greater vigour of healthy trees did not delay cropping; they produced over 30% more fruit than infected trees in the first three harvest

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1973.tb01307.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SUMMARYArabis mosaic virus and narcissus degeneration virus were eliminated fromNarcissus tazettaL. cv. Grand Soleil d'Or by meristem‐tip culture. Cultures grew equally well in two liquid nutrient media, but tubes 25times75 mm size and temperaturec.12oC were optimal for growth and survival. Virus‐free bulbs grew more vigorously than infected bulbs, and gave more and larger flowers with a better colour. To increase the normal multiplication rate, either sector cuts were made in the basal plate, or bulbs were divided into chips(c.1 g), before planting. Multiplication was greater by chip‐cutting. Five virus‐free bulbs were propagated to over 2000 in 8 years; theoretically, 20000000 bulbs should be produced in a further 5–6 years, sufficient to replant the Isles of Scilly with virus

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1973.tb01308.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SUMMARYAt a site in eastern Scotland, nine common species of arable weeds were infected with tobacco rattle virus (TRV), and some of these, notablyViola arvensisandStellaria media, comprised an overwintering reservoir of the virus. TRV was seed‐borne both in naturally and in experimentally infectedV. arvensis(2–10%), and occasionally in other weed species. In the glasshouse at 20oC a naturally infective population of vector nematodes(Tricho‐dorusspp.) kept in soil free of plants retained its infectivity for 20 wk, although fewTrichodorussurvived for this period. In the field, the incidence of TRV infection in potato (spraing disease) in plots kept free of weeds for 1–5 years was 3–4 times that in weed‐infested plots butTrichodorusnumbers did not differ appreciably between the two treatments. Presumably the virus is retained for long periods in its vectors and these feed on potato more frequently when other hosts are not available. Weeds are probably important in the long term as hosts of both TRV and its vectors, but in the short term weed control seems unlikely to prevent potato spraing because of the long persistence of TRV in vector populations.In the field,Trichodorusaccumulated near the interface between topsoil and subsoil, and the incidence of spraing was greatest where the topsoil was shallowest. When cucumber seedlings were exposed to virus‐carryingTrichodorus, TRV reached a greater concentration in roots at 20oC than at 24oC, and the virus was not detected in roots at 29oC. In a sandy soil, TRV was transmitted only when the water content exceeded 15%, and at least 30 % water was needed for maximum transmission. Annual records of rainfall and spraing disease suggest that spraing is most prevalent when the summer is wettest.TRV is not confined to cultivated land. Stabilized sand dunes supporting a pure stand ofAmmophila armariawere colonized byTrichodorus pachyder‐mus, but TRV was detected only where the plant community had enlarged to includeV. arvensisand other dicotyledons. In such situations, TRV may be introduced in the seed ofV. arvensis, and the movement of soil by wind probably contributes to the dispersa

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1973.tb01309.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SUMMARYPure pathotype A populations ofHeterodera rostochiensisproduce a few females on exandigenahybrids with theH1gene for resistance. As the proportion of larvae able to become female on exandigenahybrids was not increased by reproducing the nematodes on such hybrids for 3 years, these females seem not to be genetically different from the rest of the population. The proportion increased rapidly when the initial population contained a few pathotype (species) E nematodes but again no increase in the proportion of pathotype (species) A larvae able to become female on exandigenawas detected and pathotype E replaced pathotype A.

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1973.tb01310.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SUMMARYA late (SR 52) and an early (N × K) variety were grown, with irrigation, on well fertilized soil at three sites, Chiredzi (altitude 420 m), Henderson (1260 m) and Grasslands (1620 m). Mean incoming radiation was similar at all sites, while mean temperature decreased as altitude increased.Final total and grain dry weights were greatest at Henderson, and those of SR 52 exceeded those of N × K, although not greatly at Chiredzi. Initially, leaf area index(L)decreased with increase in altitude, but plants flowered later as altitude increased, andLat the time of flowering was greatest at Henderson.Lat flowering was greater in SR 52 than in N × K, though only slightly so at Chiredzi. Leaves withered sooner after flowering at Chiredzi and Grasslands than at Henderson, but leaves of the two varieties withered at about the same time after flowering at each site.During most of the vegetative phase the efficiency of the leaves and crop growth rate (C) increased with decrease in altitude, and were greater in N × K than in SR 52. More dry matter was accumulated after flowering at Henderson than at the other sites, and more by SR 52 than by N × K at Henderson and Grasslands, but not at Chiredzi. Leaf area duration(D)after flowering was greater at Henderson than elsewhere, and greater in SR 52 than in N × K, except at Chiredzi. After flowering leaf efficiency (dry weight increase ÷D)was least at Henderson and greatest at Grasslands, but differed little between varieties at each site. The fraction in the grain of the dry matter accumulated after flowering decreased with increase in altitude and was greater in SR 52 than in

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1973.tb01311.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY