Showing papers in "Annals of Applied Biology in 1980"

Pepino mosaic virus, a new potexvirus from pepino (Solanum muricatum)

Abstract: SUMMARY Pepino mosaic virus (PepMV), a previously undescribed virus, was found in fields of pepino (Solanum muricatum) in the Canete valley in coastal Peru. PepMV was transmitted by inoculation of sap to 32 species from three families out of 47 species from nine families tested. It caused a yellow mosaic in young leaves of pepino and either a mild mosaic or symptomless infection in 12 wild potato species, five potato cultivars and potato clone USDA 41956 but S. stoloniferum and potato cultivars Merpata and Revolucion reacted with severe systemic necrotic symptoms. The virus was transmitted by plant contact but not by Myzus persicae. It was best propagated and assayed in Nicotiana glutinosa. Sap from infected N. glutinosa was infective after dilution to 10-1 but not 10-6, after 10 min at 65°C but not 70°C and after 3 months at 20°C. PepMV had filamentous particles with a normal length of 508 nm; the ends of some seemed damaged. Ultra-thin sections of infected leaves of N. glutinosa revealed many inclusions containing arrays of virus-like particles some of which were banded or whorled; small aggregates of virus-like particles were also common. The virus was purified by extracting sap from infected leaves in a solution containing 0·065 M disodium tetraborate, 0·435 M boric acid, 0·2% ascorbic acid and 0·2% sodium sulphite at pH 7·8, adding silver nitrate solution to the extract, and precipitating the virus with polyethylene glycol followed by two cycles of differential centrifugation. Particles of PepMV normally yielded two proteins with molecular weights of 26 600 and 23 200, but virus obtained from infective sap aged overnight yielded only the smaller protein suggesting that it was a product of degradation of the larger one. The virus is serologically related to two potexviruses, narcissus mosaic and cactus X and its properties are typical of the potexvirus group.

Properties of a resistance-breaking strain of potato virus X.

Abstract: SUMMARY During indexing of a potato germplasm collection from Bolivia, a strain of potato virus X (PVX), XHB, which failed to cause local lesions in inoculated leaves of Gomphrena globosa was found in 7% of the clones. XHB was transmitted by inoculation of sap to 56 species from 11 families out of 64 species from 12 families tested. It was best propagated in Nicotiana glutinosa or N. debneyi; Montia perfolia and Petunia hybrida were useful as local lesion hosts. Inoculated leaves of G. globosa plants kept at 10°, 14°, 18°, 22°, or 26 °C after inoculation were always infected symptomlessly. XHB caused a mild mosaic, systemic chlorotic blotching or symptomless infection in 16 wild potato species and eight Andean potato cultivars, systemic necrotic symptoms in clone A6 and cultivar Mi Peru, and bright yellow leaf markings in cultivar Renacimiento. It caused necrotic local lesions in inoculated leaves of British potato cultivars with the PVX hypersensitivity gene Nb but then invaded the plants systemically without causing further necrosis; with gene Nx systemic invasion occurred but no necrotic symptoms developed. These reactions resemble those of PVX strain group four. XHB differed from other known strains of PVX in readily infecting PVX-immune clones 44/1016/10, G. 4298.69 and USDA 41956, cultivars Saphir and Saco, and Solanum acaule PI 230554. XHB had slightly flexuous filamentous particles with a normal length of 516 nm. It was transmitted readily by plant contact and it partially protected G. globosa leaves from infection with XCP, a group two strain of PVX. Sap from infected N. glutinosa was infective after dilution to 10--6 but not 10--7 after 10 min at 75° but not 80 °C and after 1 yr at 20 °C. XHB was readily purified from infected N. debneyi leaves by precipitation with polyethylene glycol followed by differential centrifugation. Microprecipitin tests showed that XHB and XCP are closely related serologically.

The biology of the ground beetle Harpalus rufipes in a strawberry field in Northumberland

Abstract: SUMMARY Harpalus rufipes (Degeer) was studied in a strawberry plot in Northumberland from 1973 to 1978 by pitfall trapping, and in the laboratory. Adults were active from April until November. Overwintered male beetles predominated at the beginning of each season until May, followed by overwintered females in June and July. Newly emerged, mainly female, beetles were active from August onwards. Overwintered females matured during early summer and laid eggs in August with a fecundity of 10–15 eggs/female. In the laboratory about 30% of beetles survived from one breeding season to the next. First- and second-instar larvae were caught in pitfall traps in autumn; in the laboratory they made approximately vertical burrows in which they stored seeds taken from the soil surface. Third-instar larvae fed on these seeds and were not active on the surface. Preferred seeds were those of grasses and Chenopodium album L. Larvae were usually aggregated in the soil at densities of 3–20/m2.

Seed survival and periodicity of seedling emergence in some species of Atriplex, Chenopodium, Polygonum and Rumex

Abstract: SUMMARY Seeds or fruits of 10 weed species were collected in each of 3 years and mixed with the top 7·5 cm of sterilised soil which was confined in cylinders sunk in the ground outdoors and cultivated three times yearly. The numbers of seedlings emerging were recorded for 5 yr and the numbers of viable seeds remaining then determined. Few, if any, seedlings of Atriplex, Chenopodium or Polygonum spp. emerged in the autumn of sowing; the main emergence was consistently in March–May (A. hastata, A. patula), April–May (P. lapathifolium, P. persicaria), April–June (C. polyspermum), May–June (C. rubrum) or June–August (C. hybridum). When sown before November, some seedlings of Rumex crispus, R. obtusifolius and R. sanguineus appeared immediately. Most emerged in April in subsequent years. Seedling emergence was usually greatest in the first year; thereafter, with all species, seedling numbers decreased approximately exponentially from year to year. The mean numbers of seeds found to be viable after 5 yr ranged from 2·5% (R. sanguineus) to 16·6% (C. rubrum) of those sown.

Studies on the seed-borne phases of dark leaf spot Alternaria brassicicola and grey leaf spot Alternaria brassicaeof brassicas

Abstract: SUMMARY Tests in Britain on samples of basic and commercial Brassica oleracea seed between 1976 and 1978 showed that many lots were infected with Alternaria brassicicola. A. brassicae was uncommon in basic seed in these years and in commercial seed harvested in 1976 and 1977 but was frequent in seed harvested in 1978. Most affected seeds were contaminated by surface-borne spores and mycelium of A. brassicicola but many were internally infected by the fungus situated within the seed-coat and in some seeds in the embryo tissues. Superficial contamination by the fungus declined rapidly after 2 yr in cabbage seeds stored at 10 °C, 50% r.h. but internal infection persisted for up to 12 yr. In some samples, internal infection was commonly associated with small shrivelled seeds. Surface contaminated and internally infected seeds transmitted the disease but seedling infection was more closely correlated with the latter.

Physiological responses of apple fruits to oxygen concentrations

Abstract: SUMMARY The effects of oxygen concentration on respiration and ripening processes in apple fruits at storage temperatures have been investigated. The response of carbon dioxide production to a change in O2 concentration occurred after a delay of 1–4 days, depending upon the time and direction of change. When apples were stored in flowing streams of oxygen/nitrogen mixtures the time of onset of ethylene production was inversely related to oxygen concentration and the maximum rate of production was directly related to oxygen. However when apples were stored in product-generated atmospheres low ventilation caused rapid accumulation of ethylene. The retardation of ripening processes by low oxygen concentrations observed under these conditions was not, therefore, a result of inhibition of ethylene synthesis. When permanganate was present in the storage container the build up of ethylene was delayed, especially at low oxygen concentrations. In these conditions there was sometimes a further retardation of softening and soluble polyuronide formation. Rates of chlorophyll degradation in peel and cortex, flesh softening and soluble polyuronide formation were all half maximal at 2.5–4.0% O2.

Factors affecting the detection of potato leafroll virus in potato foliage by enzyme‐linked immunosorbent assay

Abstract: SUMMARY Using antiserum globulins that reacted only weakly with plant materials, potato leafroll virus (PLRV) at 10 ng/ml was detected consistently by enzyme-linked immunosorbent assay (ELISA). The reaction with PLRV particles was slightly impaired in potato leaf extracts that were diluted less than 10-1 but not at greater dilutions. Antiserum globulins that reacted more strongly with plant materials could be used satisfactorily for coating microtitre plates but were unsuitable for conjugating with enzyme. The detection end-point of PLRV, in leaf sap of potato cv. Cara plants grown from infected tubers in the glasshouse, was about 10-2 and the virus was reliably detected in extracts of composite samples of one infected and 15 virus-free leaves. PLRV concentration was much less in extracts of roots or stolons than in leaf extracts. The virus was detected in infected leaves of all 27 cultivars tested. PLRV was readily detectable 2 wk before symptoms of secondary infection developed in field-grown plants of cv. Cara and Maris Piper and remained so for at least 5 wk. Its concentration was slightly greater in old than in young leaves and was similar to that in glasshouse-grown plants. In field-grown plants of cv. Maris Piper with primary infection, PLRV was detected in tip leaves 21–42 days after lower leaves were inoculated by aphids; in some shoots it later reached a concentration, in tip leaves, similar to that in leaves with secondary infection. Symptoms of primary infection developed in the young leaves of some infected shoots but were inconspicuous and were not observed until at least a week after PLRV was detected by ELISA.

Screening for resistance to potato cyst nematode using closed containers

Abstract: SUMMARY A screening method is described whereby potato plants are grown in soil in closed plastic containers in the dark, producing root systems upon which potato cyst nematodes can reproduce Tubers are planted in John Innes No 2 compost at 30% moisture and inoculated with an egg suspension The canisters are held at 20°C for 7 wk after which, visible developing females are counted This method of screening is more rapid, economical and produces data as reproducible as those from conventional pot tests The value of the test for detecting resistance early in a potato breeding programme is discussed

The effects of temperature on leaf growth of sugar beet varieties

Abstract: SUMMARY Leaf growth of nine varieties of sugar beet (Beta vulgaris L.) was studied at constant temperatures of 7, 11, 15 and 20·C, using generalised logistic curves fitted to the data to estimate the parameters of growth. The rate of leaf appearance increased linearly with temperature and was the same in all varieties. There were differences between varieties in the weighted mean rates of expansion of leaf area per plant (Ā), the temperature coefficient of Ā and the leaf area duration (D); these differences were caused more by differences in rates of expansion and final sizes of individual leaves than by differences in rates of leaf production. The growth of the first six leaves produced by each plant was examined in detail. The greater size of successive leaves of plants and genotypic differences between comparable leaves were more attributable to differences in the rate than differences in the duration of leaf expansion. Increasing temperatures increased leaf size because they accelerated the rate of expansion more than they shortened the duration of the expansion phase. It is inferred that all effects arose through differences in the initial sizes of leaves before they unrolled from the shoot apex. Dry matter production was proportional to D but was partitioned more to the storage root at the colder temperatures. This may have been related to the differential effects of temperature on cell division and expansion and the relative contribution of these two processes to the final sizes of the leaves and storage root.

Water stress in field-grown perennial ryegrass..: I. Its effect on growth, canopy photosynthesis and transpiration

Abstract: SUMMARY Two sets of irrigated and water-stressed swards of perennial ryegrass (Lolium perenne) growing in the field were compared. One set of swards was grown normally (field swards) and the other was established in boxes of low water-holding capacity (simulated swards). Rain covers were used to exclude rain from half of both sets of swards: the others were kept within 20 mm of field capacity. Daily minimum values of leaf water potential fell to —12 bars (—1.2 MPa) in the irrigated swards, —16 bars in the stressed field swards and —20 bars in the stressed simulated swards. Dry-matter production was reduced in the stressed field swards and leaf extension declined markedly in both the field and simulated swards. Canopy photosynthesis at saturating light intensity was reduced by about half in the stressed field swards and by more than 80% in the stressed simulated swards. In the former case a proportion of this reduction was due to the lower leaf area but the mean rate of leaf photosynthesis at saturating light intensity (Pmax/LAI) was reduced by about 40% and this was attributable to increased stomatal resistance calculated from canopy transpiration rates or measured with a porometer. The more marked decline in photosynthesis in the stressed simulated swards was not only the consequence of more complete stomatal closure but also a decrease in the quantum efficiency of photosynthesis. When water stress occurs naturally in the field it appears that changes in the leaves take place as stress is developing and these changes result in a less severe effect of water stress on leaf photosynthesis. When stress is imposed with unnatural rapidity, as in the simulated swards, there is no opportunity for these changes in the leaf to occur and stress leads to a rapid and severe decline in leaf photosynthesis. The importance of these findings for the grass sward under water stress is discussed.

Studies on Entomophthora in populations of Aphis fabae on field beans

Abstract: SUMMARY The population of Aphis fabae on field beans at a site in Highfield, Rothamsted in 1973 reached its peak 1 wk earlier than that at an equivalent site in Mill Dam Close, Woburn, 29 km NW of Rothamsted. Epizootics of Entomophthora caused weekly maximum mortalities of adult apterae of 71% at Highfield and 67% at Mill Dam Close. These epizootics and the periodicity of Entomophthora conidia in the air closely paralleled the development of the aphid population. There was little evidence of a consistent relationship between Entomophthora infection and any of the weather factors considered. At both sites most mortality was caused by E. planchoniana though many aphids were killed by E. aphidis and E. obscura. E. fresenii and E. virulenta killed very few aphids. Most conidia in the air were of the E. aphidis-type. Up to 44% of alatae emigrating from bean crops were infected with Entomophthora, confirming that aphid migration is an important means of distributing the fungi. Aphid numbers rose to more than 1600/plant at both sites, in spite of the action of Entomophthora, and would probably have been less had the fungi been more abundant earlier in the season.

Field control of take‐all of wheat by avirulent fungi

Abstract: SUMMARY In field experiments in New South Wales, wheat inoculated with Gaeumannomyces graminis var. tritici had less take-all and yielded significantly more grain (P≤ 0·05) when simultaneously inoculated with G. graminis var. graminis and a lobed hypho-podiate Phialophora sp. synonymous with Phialophora radicicola Cain var. radicicola sensu Deacon than when unprotected with these fungi. An isolate of P. radicicola var. graminicola did not confer any protection. The yields of cross-protected wheat approached those of healthy wheat where take-all incidence was slight to moderate while control was less economically significant where disease was severe. If this form of control proved similarly effective against natural inoculum it might be best employed at the beginning of a wheat monoculture after a break crop and serve to delay the onset of severe take-all. A preliminary survey of 28 wheat fields in New South Wales using a bait technique and examining for characteristic fungal structures showed that G. g. graminis was rarely present in the soil and the lobed hyphopodiate Phialophora sp. and P. r. graminicola were absent. Significant populations of G. g. graminis only occurred in first wheat crops after a long period of native pasture. In New South Wales, at least, these avirulent fungi would have to be introduced to wheat fields for take-all control.

Studies on the taro leaf blight fungus Phytophthora colocasiae in Solomon Islands: control by fungicides and spacing

Abstract: SUMMARY With mist blower application, copper oxychloride at 2.25 kg/ha was effective in controlling PHYTOPHTHORA colocasiae but mancozeb at 3.6 kg/ha was not. Phytotoxicity from captafol at 1.8 and 3.6 kg/ha nullified any potential gain in yield from control of blight. Leaf removal from healthy plants to maintain four leaves per plant for 90 days, to simulate roguing of leaves for disease control, was shown to cause no loss in yield. However, regular roguing of diseased leaves over the same period in plots affected by a severe epiphytotic did not eradicate the pathogen. When roguing ceased at 90 days disease increased rapidly to epidemic proportions that seriously affected final corm yield. An attempt to reduce the effect of blight by wider than traditional spacing was unsuccessful. Under continuous epiphytotic conditions disease had the maximum possible impact regardless of spacing. Whereas plants free from competition normally bear six to seven leaves, this number was reduced by severe disease to three or four, the same number as was borne by plants under the competitive conditions of closer-than-traditional spacing. Instead of the traditional 20–30 000 plantdha, it was shown that density can be doubled under conditions of high leaf blight hazard and increased yields still be obtained.

Field and laboratory observations on the substrates of the mushroom fungus gnat Lycoriella auripila (Diptera: Sciaridae)

Abstract: SUMMARY Observations in mushroom houses showed that the number of larvae of L. auripila in the casing was higher than in the compost throughout the post-casing phase of culture. The intestines of larvae from casing contained both peat and fungal material. Choice of oviposition site was accordingly non-specific. Females from cropping houses appeared to consist of a mixture of monogenic (predominantly unsexual) and digenic (bisexual) strains. Males matured c. 2 days before females at 20 °C; such protandry appeared to be related to sexual differences in the extent of migration undertaken by the dimorphic sexes. Sciarids were reared from egg to adult equally effectively on (a) detached sporophores or on portions of (b) pileus, (c) stipe or (d) gills, all placed on moist peat, or on detached sporophores placed on a variety of inert substrates. Development was completed where sporophore material was the only food source and also occurred on peat to which only protein, particularly egg albumin, had been added. Hence, assuming no protein metabolism by a third organism, for development in the casing layer to occur, the mushroom need contribute only such proteins. However, their replacement by amino acids was not successful in limited tests. Addition of increasing amounts of the fungal metabolites mannitol and calcium oxalate to compost delayed development, and the accumulation of these materials during fungal growth may be responsible for the antagonism between mycelial and sciarid development which results in subsequent migration of emerging adults.

Insect feeding on different sorghum cultivars in relation to cyanide and phenolic acid content

Abstract: SUMMARY Insect feeding on sorghum was studied in the field and in the laboratory. Quantitative differences in cyanogenesis and phenolic acid content were also measured on the same or simillar plants. High concentrations of cyanide were correlated with a reduction in feeding by grasshoppers and by first-instar larvae of Chilo partellus; high concentrations of phenolic acids were correlated with reduced feeding by various grasshoppers and by the planthopper Peregrinus maidis. Mythimna separata larvae, and adults and nymphs of Rhopalosiphum maidis, were apparently unaffected by these chemicals. Some sorghum cvs which were relatively unpalatable to grasshoppers and to P. maidis had low levels of the test chemicals, so that here some other factor or factors must be involved.

Application of enzyme-linked immunosorbent assay to the detection of potato leafroll virus in potato tubers

Abstract: SUMMARY Factors affecting the detection of potato leafroll virus (PLRV) by enzyme-linked immunosorbent assay (ELISA) in tubers of field-grown potato plants with primary or secondary infection were studied. The reactions of extracts of virus-free potato tubers were minimised by pre-incubating the extracts at room temperature and by careful choice of the dilution of enzyme-conjugated globulin. PLRV was reliably detected in tubers produced by secondarily infected plants of all six cultivars tested. PLRV concentration was greater in heel-end than in rose-end vascular tissue of recently harvested tubers but increased in rose-end tissue when tubers stored at 4°C for at least 5 months were placed at 15–24°C for 2 wk. PLRV occurred at greater concentration in tubers from plants of cv. Maris Piper with natural or experimentally induced primary infection than in tubers from secondarily infected plants; again PLRV concentration was greater in heel-end than in rose-end vascular tissue. Plants whose shoots were infected earliest in the growing season were invaded systemically and produced the greatest proportion of infected tubers; plants infected late in the season also produced infected tubers but PLRV was not detected in their shoot tops. PLRV concentration in tubers from the earliest-infected plants was less than in tubers from later-infected plants. PLRV was detected reliably by ELISA in tubers from progenies that were totally infected but was not detected in all infected tubers from partially infected progenies. ELISA is suitable as a routine method of indexing tubers for PLRV, although the virus will not be detected in all infected tubers produced by plants to which it is transmitted late in the growing season.

Detection of six nepoviruses in their nematode vectors by immunosorbent electron microscopy

Abstract: SUMMARY Particles of six nepoviruses were detected by immunosorbent electron microscopy (ISEM) in extracts of their respective vector nematodes. This technique was at least a thousand times more sensitive than conventional electron microscopy. It was also more rapid, reliable and sensitive than infectivity tests in which extracts of nematodes were inoculated to indicator plants. The viruses were detected in extracts of single nematodes, and in the roots and leaves of infector and bait plants.

Barley yellow dwarf virus infectivity of alate aphid vectors in west Wales

Abstract: SUMMARY Live trapping at 0.9 m of alate aphid vectors of barley yellow dwarf virus (BYDV) at Aberystwyth from 1970 to 1979 showed that ten species transmitted the virus to oat test plants. Conversion of percentage infective at 0.9 m to numbers infective based on continuous trapping at 1.2 m showed Rhopalosiphum padi and R. insertum to be the main vector species in most years, whilst Metopolophium dirhodum and Sitobion auenae were normally of minor importance. The data obtained suggest that epiphytotics of BYDV in autumn-sown cereals were caused by numerous infective vectors flying late in the year and transmitting severe strains of the virus. Evidence is presented that gynoparae and males of R. padi are involved in the autumn spread of BYDV and that three further aphid species, Anoecia corni, Metopolophium albidum and M. frisicum are BYDV vectors. The use of live and continuous trapping techniques in forecasting BYDV epiphytotics is discussed.

Effects of soil water level on the development of adult plant resistance to powdery mildew in barley

Abstract: SUMMARY Barley grown in dry soil developed greater adult plant resistance (APR) to powdery mildew (Erysiphe graminis DC. f. sp. hordei Merat) than barley grown in wet soil. Conidial germination and appressorium formation were less, and fungal development between formation of appressoria and elongating secondary hyphae on upper leaves was inhibited, when adult plants were grown in dry soil. Mildew colonies expanded more slowly on leaves of adult plants than on leaves of seedlings, especially if adult plants had grown in dry soil. APR was reduced if plants, previously grown in dry soil, were well watered more than 32 h before inoculation. Conidia originating from plants grown in dry soil had a lower solute potential and greater ability to infect plants grown in dry but not wet soil than conidia originating from plants grown in wet soil. APR could not be attributed simply to increased cell wall or cuticle thickness, nor to lowered leaf solute potentials, as has sometimes been suggested for powdery mildew diseases. Increasing plant age and water stress induced increases in cell wall and cuticle thickness, but these changes did not always coincide with changes in disease resistance. Increasing plant age and water stress also lowered leaf solute potentials but fungal solute potentials were lower than leaf solute potentials and, more importantly, were lower than leaf water potentials. Thus, fungal growth was not limited by the availability of water from the host during penetration and hyphal establishment. It is suggested that resistance levels may be determined not by the thickness of epidermal structures, nor by lowering of solute potential per se, but by specific substances harmful to the fungus which accumulate in either cell wall, cuticle or sap, and whose concentration is dependent on the age and water stress of leaves.

Behaviour of the systemic nematicide oxamyl in plants in relation to control of invasion and development of Meloidogyne incognita

Abstract: SUMMARY A single foliar application of oxamyl (12.5 μg) in acetone significantly reduced invasion of cucumber seedlings by Meloidogyne incognita juveniles for at least 21 days but did not affect the early stages of development of juveniles which had already invaded the roots. In contrast, application of oxamyl to the roots significantly reduced both invasion and development of juveniles. Concurrent studies using radiolabelled oxamyl showed that the amount of toxicant in the roots after 3 days was 13 times greater following root application than after foliar treatment. It is probable that oxamyl concentrates at the sites of nematode attack as an overall concentration of only 3 ng oxamyl g-1 root was sufficient to prevent invasion. Much greater concentrations than this were required to affect the nematode in vitro. Oxamyl appeared to be lost from the roots into the soil principally in the form of its non-toxic oxime and it is suggested that the site of action following foliar application is at the root surface or outer cortex. Studies on the invasion behaviour of M. incognita juveniles on agar showed that the action of oxamyl had a sensory component.

Studies on double strain occupancy of nodules and the competitive ability of Rhizobium trifolii on red and white clover grown in soil and agar

Abstract: SUMMARY An investigation concerning double strain occupancy of nodules and the competitive ability for nodulation of Rhizobium trifolii strains 7A strr and 32 spcr was carried out using red and white clover varieties grown in agar and soil. Only two nodules of the 498 typed from plants grown in soil contained both inoculated strains and none of those analysed by the replica plating technique were formed by both 32 spcr and an indigenous strain. In contrast, plants grown on agar produced between 17% and 25% doubly infected nodules. Strain 32 spcr was more competitive for nodulation than 7A strr both in soil and agar. Indigenous rhizobia in soil were highly competitive for nodulation with the introduced strains; increasing the density of the inoculum resulted in an increase in the proportion of nodules occupied by the inoculated strains. The strains used as mixed inocula in soil were less successful in nodulation than the same strains used as pure inocula. The results also indicate that there was a host effect on the nodulation success of 7A strr.

Comparative physiology of field-grown tomatoes during ripening on the plant or retarded ripening in controlled atmospheres

Abstract: SUMMARY Tomato fruits of six cultivars were harvested at three different stages of maturity or were harvested when mature-green and then stored in a modified gas atmosphere (2·5-4% O2; 4% CO2) for 2 months. The fresh and stored fruits were analysed for their contents of sugars, organic acids and free amino acids, while proteins were separated by discontinuous electrophoresis on polyacrylamide gels. In general, the low molecular weight components decreased during storage. A comparison of mature-green fruits before and after storage showed that although total protein was not decreased, a different electrophoretic pattern was obtained following controlled atmosphere (CA) storage. Thus, although controlled atmosphere storage repressed the loss of chlorophyll and synthesis of lycopene, carotenoids and xanthophylls, the biochemical parameters measured showed a controlled change towards the conditions exemplified by ripe fruits. This was not so marked in some cultivars as it was in others.

Vector relationships and other characteristics of barley yellow striate mosaic virus (BYSMV)

Abstract: SUMMARY Barley yellow striate mosaic virus (BYSMV) was inoculated by its planthopper vector Laodelphax striatellus (Homoptera, Delphacidae) to 44 species of Gramineae, 26 of which in eight tribes were infected. The virus was not transmitted through wheat seed nor did it infect five dicotyledonous hosts of other rhabdoviruses. The most susceptible species were in the tribes Festuceae and Hordeae. Barley, Bromus spp., oats, Phalaris canariensis, Setaria italica, Sorghum spp., and sweet corn cv. Golden were diagnostic hosts. Electron microscopy of crude sap was also a sensitive diagnostic method. Properties of BYSMV were determined by injecting L. striatellus with crude sap from infected barley. Sap was infectious after 10 min at 50–55 °C but not after 10 min at 60 °C, when diluted with buffer to 10--2 but not to 10--3, when stored for 2 but not 4 days at 5 °C or when kept for 1 but not 2 days at 22 °C. The planthopper Javesella pellucida was an experimental vector of BYSMV but the virus was not transmitted by the leafhoppers Macrosteles sexnotatus or Psammotettix striatus (Homoptera, Cicadellidae). The latent period of BYSMV in L. striatellus was most commonly 15 or 16 days (minimum, 9 days; maximum, 29 days). The minimum acquisition access period for transmission was between 1 h and 5 h, and the minimum inoculation feeding time was 15 min. After 24 h and 8 day acquisition feeds, 30.4% and 42.8% respectively of L. striatellus transmitted BYSMV. When transferred daily, infective hoppers transmitted virus intermittently. The maximum retention of infectivity by L. striatellus was 36 days. Two of five infective females transmitted BYSMV transovarially. Larvae became infective in the second wk after hatching and transmitted for up to 3 wk.

Host range, purification and some properties of hop mosaic virus

Abstract: SUMMARY Hop mosaic virus (HMV) causes severe symptoms in sensitive hop (Humulus Zupulus) cultivars, but over 90% of hop plants grown in England belong to tolerant cultivars and most of these plants are infected. HMV infected 11 herbaceous species symptomlessly but infection was systemic in only three of these: Nicotiana clevelandii, N. debneyi and Urtica urens. HMV did not infect 33 other species in 13 families. The virus was transmitted in the non-persistent manner by the aphids Myzus persicae, Macrosiphum euphorbiae and Phorodon humuli. It was not seed-borne in N. clevelandii, U. wens or hop. HMV was purified by precipitation from N. clevelandii extracts with polyethylene glycol followed by centrifugation in sucrose density gradients. The preparations contained filamentous particles c. 14 × 650 nm composed of c. 6% single-stranded RNA of mol. wt c. 3.0 × 106 and a single protein species of mol. wt c. 34 000. HMV showed distant serological relationships to carnation latent, potato M, cowpea mild mosaic and hop latent viruses but it did not react with antisera to six other carlaviruses. The cryptogram for HMV is R/1: 3.0/(6): E/E: S/Ve/Ap.

Acid phosphatase isoenzymes of Brassica oleracea seedlings and their application to sib testing in F1 hybrids

Abstract: SUMMARY Crude extracts from cotyledons of 2-day-old seedlings were separated by electrophoresis on polyacrylamide gels and stained for acid phosphatase. At least 22 bands were recognised, distributed over seven zones. Analysis of zone 1 allowed estimation of sib frequencies in 12 out of 19 hybrid cultivars. By applying either seedling zone 1 or acp-1 seed acid phosphatase analysis, sib estimates could be made in 15 of these hybrids. It is suggested that this technique offers a practical and rapid means of estimating sib frequencies in commercial seed stocks.

Effect of inoculum density, stage of plant growth and dew period on the incidence of black point caused by Alternaria alternata in durum wheat.

Abstract: SUMMARY Glasshouse studies showed that the incidence of black point caused by Alternaria alternata in durum wheat was positively correlated with both the density of the inoculum and the growth stage of the wheat plants at the time of inoculation. A curvilinear relationship of the form Y=a + log X was found between inoculum density and disease incidence. The incidence of black point was linearly related to the stage of plant growth, between anthesis and the late milk stages of development, at the time of inoculation. A better relationship between growth stage and disease incidence was found when plant growth was expressed as days after anthesis than when the Romig scale was used (R2= 0.30 and 0.24 respectively). A threshold dew period of 3–6 h was required for black point symptoms to develop. The incidence of black point increased with increasing duration of the dew period until maximum disease incidence occurred after exposure to a dew period of 48 h.

Fireblight in Kent, England in relation to weather (1955-1976).

Abstract: SUMMARY A system for the assessment of potential for fireblight Erwinia amylovora activity (PFA), based on standard temperature and rainfall records, has previously been outlined Here some recent modifications are described and the system is discussed more fully and tested for its ability to explain outbreaks of fireblight in different hosts in Kent, south-east England for the years 1955–76 In most cases, there was a satisfactory match between PFA patterns and recorded field outbreaks and incidents and it is concluded that warnings based on the system could have lessened risk of disease in some hosts in past years In the short term (depending on the accuracy of weather forecasts) the system can be used predictively Its use in this way is discussed together with underlying principles of some of the criteria used and possible ways in which the precision of the system might be improved

Fireblight (Erwinia amylovora) and weather: a comparison of warning systems.

Abstract: SUMMARY Warning systems for fireblight Erwinia amylovora developed in New York, Illinois and California, USA, and in south-east England are compared. General principles which might be applicable in the different climates were sought. The consequences of applying threshold temperature values chosen for one area in a different climatic area were examined using Sacramento, California; Rochester, New York; Vlissingen, The Netherlands; Kent, England as examples. A graded system for assessing fireblight risks, derived from all the systems, is suggested. It takes into account both risks of infection and risks of high insect activity and it is best used in conjunction with Billing's incubation period assessment system.