Quantitative relationships between infection by the hop downy mildew pathogen, Pseudoperonospora hutnuli, and weather and inoculum factors

Experiments are described in which successive groups of healthy, susceptible, potted hop-plants were exposed, each 24 h, to the weather and to sources of Pseudoperonospora kumuli inoculum 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. R² values 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 downy mildew.     

The biology of Peronospora viciae on pea: laboratory experiments on the effects of temperature, relative humidity and light on the production, germination and infectivity of sporangia

Germination of Peronospora viciae sporangia washed off infected leaves varied from 20% to 60%. Sporangia shaken off in the dry state gave 11–19% germination. Most sporangia lost viability within 3 days after being shed, though a few survived at least 5 days. Infected leaves could produce sporangia up to 6 weeks after infection, and sporulating lesions carried viable sporangia for 3 weeks. Sporangia germinated over the range 1–24 °C, with an optimum between 4 and 8 °C. Light and no effct. The temperature limits for infection were the same as for germination, but with an optimum between 12 and 20 °C. A minimum leaf-wetness period of 4h was required, and was independent of temperature over the range 4–24 °C. Maximum infectivity occurred after 6h leaf wetness at temperatures between 8 and 20 °C. Infection occurred equally in continuous light or in darkness. After an incubation period of 6–10 days sporangia were produced on infected leaves at temperatures between 4 and 24 °C, with an optimum of 12–20 °C. Exposure to temperatures of 20–24 °C for 10 days reduced subsequent sporulation. Sporangia produced at suboptimal temperatures were larger, and at 20 °C. smaller, than those produce at 12–16 °C. Viability was also reduced. No sporangia were produced in continuous light, or at relative humidities below 91%. For maximum sporulaiton an r.h. of 100% was required, following a lower r.h. during incubation. Oospores wre commonly formed in sporulating lesions, and also where conditons limited or prevented sporulation. The results are discussed briefly in relaiton to disease development under field conditions.     

Overwintering of Sphaerotheca mors-uvae on black currant and gooseberry

The ability of Sphaerotheca mors-uvae to perennate as cleistocarps, and as mycelium in buds was examined during the winters of 1965-6, 1966-7 and 1967-8 in relation to its two principal hosts, gooseberry and black currant. Cleistocarps on black currant leaves were examined from August 1965 to April 1966 and from July 1966 to March 1967. In 1965 cleistocarps were first observed on the leaves on 5 August; in 1966 on 11 July. These continued to develop through August and September and by October approximately 70% contained well-defined ascospores. The ascospore content remained generally at this level until February 1966 and November 1966; then the numbers of cleistocarps with ascospores fell and by April 1966 and March 1967 few such cleistocarps remained. From 21 March 1966 and 15 February 1967, but not otherwise, discharge of ascospores from the overwintered cleistocarps was readily obtained in laboratory tests. The viability and infectivity of the ascospores was demonstrated by allowing them to discharge on to leaf discs of black currant in the laboratory and also on to leaf discs and plants in the field. Sporulating colonies of S. mors-uvae developed within 8 days. Cleistocarps from shoots of black currant were examined from 4 August 1966 to 9 March 1967, and from 27 July 1967 to 1 January 1968. They developed in a similar manner to those on black currant leaves and by September in both 1966 and 1967 over 60% contained ascospores. This level was not maintained; the number of cleistocarps with ascospores fell gradually and by 8 December 1966 and 1 January 1968 few remained. Only in one laboratory test (21 November 1967) were ascospores discharged from a sample of these cleistocarps. Cleistocarps from shoots of gooseberry were examined from July 1966 to March 1967, and from August 1967 to January 1968. The pattern of ascospore development and subsequent decline in number of cleistocarps with ascospores was similar to that observed for black currant shoots. No discharge of ascospores could be demonstrated in laboratory tests. Evidence that S. mors-uvae perennates in buds of gooseberry was obtained by dissecting buds and by inducing buds on surface-sterilized shoots to burst under conditions which precluded chance infection. Field observations also suggested that bud infection occurred on gooseberry. Similar experiments failed to demonstrate the fungus in buds of black currant, and there was no indication of bud infection of this host in the field.     

Factors affecting the development of spraing in potato tubers infected with potato mop-top virus

A larger proportion of tubers of Arran Pilot potato growing at the surface of soil infested with potato mop-top virus (PMTV) showed spraing symptoms (brown rings) at harvest than of tubers from below the surface. Infected tubers with or without spraing developed a spraing ring when stored in darkness, first for 1–2 wk at 18 d?C and then for 1–2 wk at any constant temperature between 5 and 13 d?C. Only a faint surface ring developed when either of these periods was decreased to 1 day; 4-day periods were needed to induce distinct symptoms. Internal tuber symptoms developed more slowly than surface symptoms, and their formation was favoured by cutting the tubers in half. Additional pigmented surface rings were produced outside the first ring by successive cycles of treatment at 18 and 9d?. Spraing did not develop when the first stage of treatment was at 22–25d?, when the tubers were kept first at 10d? and then at 5d?, when the treatment at 5–13d? preceded that at 18d?, or when the tubers were kept at constant temperatures ranging from 5 to 25d?. When tubers of six potato varieties were grown in PMTV-infested soil and then stored at temperatures designed to induce symptoms, the varieties known to be the most susceptible in the field were those which had the greatest tendency to develop spraing during storage. When infected tubers were exposed to light, typical spraing symptoms were not induced, but greening of the tuber surface was much delayed in localized ring-shaped areas, so that pale weals appeared. Spraing symptoms were produced, in favourable conditions, by the reaction of cells at the periphery of the PMTV-invaded zone. Internal spraing did not prevent PMTV invading tissue outside the brown arcs; its rate of spread was about 10 μm/h at 14–18d?.     

Epidemiology and control of powdery mildew (Sphaerotheca pannosa) on roses

In examinations between September 1966 and December 1968 of 741 specimens of rose species and cultivars, cleistocarps of Sphaerotheca pannosa were found on thirty-two cultivars, mostly ramblers and old shrub roses, and on nine of these they were found in two or three successive seasons. On stem pieces placed on soil in each of the winters 1966-7, 1967-8 and 1968-9 the number of cleistocarps with asci and ascospores decreased during November and December and rose slightly in January, but none showed dehiscence. Cleistocarps on rose bushes examined during the winters of 1967-8 and 1968-9 showed a progressive degeneration of ascospores, and by December none was found. Perennation of S. pannosa in buds was demonstrated by field observations, by inducing bursting of dormant buds on surface-sterilized shoots and by dissection of dormant apical buds. In field studies in 1968 of the development of mildew, infected buds were noted on 22 March but secondary infections did not appear until 17 April, though viable conidia and susceptible leaf tissue were present during this period. Low temperatures appeared to be partly responsible for this lag. On detached leaves in the laboratory the fungus developed from germination to sporulation in 4 days at 20°, 7 days at 15°, 11 days at 10° and 28 days at 3°. Keeping inoculated detached leaves at 0° for 10 days apparently did not affect the viability of the conidia. In both 1967 and 1968 there were two host growth periods, each culminating in flowering, between May and September; mildew did not develop on the shoots until the second growth phase, then the disease increased logarithmically on shoots and blooms during August and September. The disease on the shoots was effectively controlled in the field during 1968 by applications of ‘Benlate’ (benomyl) or dinocap, but not by methy-rimol; these fungicides were less effective in controlling mildew on pedicels. Laboratory tests showed that ‘Benlate’ inhibited sporulation of S. pannosa by deforming the conidiophores.     

Observations on Nothia aphylla Lyon ex Høeg

Naked branched axes associated with sporangia, originally described by Kidston and Lang (1920) as the probable fertile region of Asteroxylon mackiei, were later shown by Lyon (1964) to belong to an unknown plant which he named Nothia aphylla. The name was validated by Høeg in 1967. Nothia aphylla is described here in greater detail from new material collected from the Rhynie locality. The sporangia are lateral in terminal branched spikes. Their arrangement ranges from spiral to semiverticillate. The fertile axes terminate in clusters of 2–5 sporangia. The stalked sporangia are rather reniform with a distal transversely-extended dehiscence slit. Systematically Nothia has similarities to both Zosterophyllophytina and Rhyniophytina.     

The biology of Peronospora viciae on pea: the development of local and systemic infections and their effect on vining yield

The development of infection by Peronospora viciae (Berk.) Casp. was studied in field and plot experiments during 1967, 1968 and 1969. Numbers of primary systemically infected plants appeared shortly after emergence and reached a maximum by the end of May. Such plants acted as primary infectors and did not survive more than 3 weeks. Maximal sporangial release occurred between 06.00 and 08.00 h BST with a peak at 07.00 h, after 1–2 h insolation. Inoculation of leaves at 10.30 h resulted in greater infection than inoculation at 20.00 h. The result suggested a possible maturation requirement of sporangia following release from the sporangiophore. Initially the spread of foliar (local lesion) infection in the crop was sporadic and localized in cvs. Jade, Gregory, Dark Skinned Perfection (D.S.P.) and Frühe Kleine Pfälzerin (F.K.P.) during the 3 years. The number of plants of all cvs. with foliar infection reached 100% by mid-June. Lesions rarely exceeded 15% of the leaf and stipular area of the mature plant. No effect of foliar lesions, at the highest level of infection, in haulm or seed fresh weight could be detected in cvs. F.K.P., Jade and D.S.P. Secondary systemic infection occurred sporadically in field crops usually with dense haulm growth. Plants of Jade with systemic infection were rated according to the number of nodes affected. There was a small but significant increase in haulm and seed fresh weight in plants with up to two nodes affected. But plants with three to five nodes showed a progressive reduction in the yield of green seed (resulting from a reduction in the number of podding nodes) and in pod size and haulm fresh weight. Total losses in Jade during 1968 and 1969 with all grades of infection did not exceed 4 % of the green seed crop. Systemic infection in some plants resulted in the death of the apex and the production of healthy, normally podding tillers. Such plants had no commercial value due to delayed maturation. Pod infection in Jade occurred largely independently of systemic symptoms: the overall level during 1969 did not exceed 16 % and only 2 % were severely infected. Total losses (from both systemic and pod infection) in Jade during 1968 and 1969 represented only a negligible part of the crop.     

Infection Efficiency of Four Phytophthora infestans Clonal Lineages and DNA-Based Quantification of Sporangia

The presence and abundance of pathogen inoculum is with host resistance and environmental conditions a key factor in epidemic development. Therefore, several spore-sampling devices have been proposed to monitor pathogen inoculum above fields. However, to make spore sampling more reliable as a management tool and to facilitate its adoption, information on infection efficiency and molecular tools for estimating airborne sporangia concentration are needed. Experiments were thus undertaken in a growth chamber to study the infection efficiency of four clonal lineages of P. infestans (US-8, US-11, US-23, and US-24) by measuring the airborne sporangia concentration and resulting disease intensity. The relationship between the airborne sporangia concentration and the number of lesions per leaf was exponential. For the same concentration, the sporangia of US-23 caused significantly more lesions than the sporangia of the other clonal lineages did. Under optimal conditions, an airborne sporangia concentration of 10 sporangia m⁻³ for US-23 was sufficient to cause one lesion per leaf, whereas for the other clonal lineages, it took 15 to 25 sporangia m⁻³ to reach the same disease intensity. However, in terms of diseased leaf area, there was no difference between clonal lineages US-8, US-23 and US-24. Also, a sensitive quantitative real-time polymerase chain reaction (qPCR) tool was developed to quantify P. infestans airborne sporangia with detection sensitivity of one sporangium. The specificity of the qPCR assay was rigorously tested for airborne inoculum and was either similar to, or an improvement on, other published PCR assays. This assay allows rapid and reliable detection and quantification of P. infestans airborne sporangia and thereby, facilitates the implementation of spores-sampling network.     

Temperature and Leaf Wetness Effects on Infection of Sugarcane by Puccinia melanocephala

Brown rust epidemics in sugarcane, caused by Puccinia melanocephala, vary in severity between seasons. To improve the understanding of disease epidemiology, the effects of leaf wetness, temperature and their interaction on infection of sugarcane by the pathogen were studied under controlled conditions. Disease severity was low at 15 and 31°C regardless of leaf wetness duration. No infection occurred with a 4‐h leaf wetness period. Increasing leaf wetness duration from 7 to 13 h lowered the temperature required for disease onset from 21 to 17°C. More infection occurred with 13 compared to 10 h of leaf wetness at 17°C, and severity decreased for all leaf wetness periods at 29 compared to 27°C. Postinfection suboptimal low and high temperatures increased the time required for lesion development and high temperatures decreased maximum disease severity. The observed effects of leaf wetness and temperature on infection by P. melanocephala could help explain the initiation, rate of increase and decline of brown rust epidemics in the field.     

Fireblight epidemic among Kentish apple orchards in 1969

Evidence from field observations and orchard plans indicates that the fireblight epidemic among apples in 1969 followed a critical combination of weather conditions. There is little doubt that diseased hawthorns, situated entirely or mainly to the north-west of the main lanes of infection, were the source of inoculum. Initial infection was confined to shoots. No blossom infection was seen at any time. Weather data from a nearby meteorological station point to distribution of inoculum having taken place on one of the days 6–10 July when strong winds, rainfall and hailstorms occurred. These days were followed immediately by temperatures above 18d?C which are considered suitable for infection and for progress of the bacterium within the shoot tissues. The control measures advised were the removal and destruction of diseased parts, the disinfection of pruning implements, the protection of wounded surfaces against infection and the destruction of nearby diseased hawthorns.     

Influence of Temperature, Wetness Duration, and Leaf Type on the Quantification of Monocyclic Parameters of Bean Rust

Monocyclic parameters of bean rust (Uromyces phaseoli var. typical) were quantified in growth chambers, on rwo bean cultivars for three temperatures (17, 21, and 25 °C), two types of leaves (unifoliolate and trifoiiolate leaves), and nine leaf wetness periods (0, 4, 7, 10, 13, 16, 19, 22, and 25 hrs). The expression of disease was greatly influenced by past-inoculation temperatures. The incubation and latent periods were shortest at 21 °C for both cultivars and leaf types. For both cultivars, trifoiiolate leaves were more susceptible than unifoliolate leaves. A wetness period of at least four hours was required for disease to occur. The maximum disease efficiency for both cultivars occurred with 22 hrs of leaf wetness at 17 °C. The disease efficiencies for temperatures of 17–29 °C and leaf wetness periods of 0–25 hrs were adequately described by a response-surface model. Because of the great influence of temperature and leaf wetness on infection, bean rust is unlikely to occur at high temperatures (> 25°C) and short leaf wetness periods (< 7 hrs).     

Glasshouse experiments on apple scab I. Foliage infection in relation to wet and dry periods

A technique of investigating apple scab infection periods using MM 109 rootstocks in the glasshouse is fully described. Inoculation by ascospores in aqueous suspension was less reliable than that by sedimenting the spores direct from source leaves on to the host plants, but fresh conidia in aqueous suspension consistently gave high levels of infection under optimum conditions.
Ascospores required a shorter period of continuous wetness (6 hr.) than conidia (7–9 hr.) for infection at near-optimum temperatures. Maximum infection from both sources was reached after about 18 hr. continuous wetness; much longer periods were sometimes inimical. With discontinuous wetness, most ascospores tolerated a dry interval of 24 hr. on the host leaves, although infection was somewhat reduced if the dry period began when the spores were starting to germinate. Conidia were more inhibited than ascospores by 24 hr. drying during minimal infection periods, but many survived and produced lesions.
Some ascospores survived dry periods of at least 96 hr., but mature leaves had acquired resistance during the interval and thus infection was reduced. The reduction was partly offset by greater infection of the youngest leaves, which meantime had expanded and were thus easier to wet. No infection resulted, however, when the dry interval was extended to 10 days.
The implications of the results are discussed in relation to the interpretation of infection periods in the field.     

In vitro tetraploid induction and generation of tetraploids from mixoploids in hop (Humulus lupulus L.)

This paper describes an efficient colchicine-mediated technique for the in vitro induction of hop tetraploids and its confirmation by flow cytometry. A window of conditions generated a high percentage (>20%) of tetraploid induction, with the highest induction (25.6%) achieved with 0.05% colchicine for 48 h. Colchicine-induced tetraploids remained stable after 6 months in soil. Leaf characteristics of diploid and tetraploid hops were compared, and it was determined that stomatal length and width are suitable parameters for identifying putative hop tetraploids. As well as generating tetraploids, this technique generates mixoploid hops. Calli, derived from mixoploid leaves, were induced to form shoot buds and shoots. Individual shoots were classed as diploid, mixoploid or tetraploid after screening by flow cytometry. This callus-based technique can be employed when a genome-doubling agent generates mixoploids but fails to generate tetraploids.     

In vitro micropropagation of Basella rubra L. through proliferation of axillary shoots

In vitro micropropagation protocol for Basella rubra regeneration was tried through proliferation of axillary shoots of the potted mature plant. The improved seed germination (70%) was recorded upon 2% urea treatment. The nodal shoot segments from matured potted plant were used to initiate the multiple shoot proliferation. The shoot segments exhibited 70% shoot initiation when cultured on Murashige and Skoog (MS) medium supplemented with Indole-3-acetic acid (IAA)?+?N₆ – Benzylaminopurine (BAP) (0.25?+?2.0 mg/L) and BAP?+?Kinetin (Kin) (2.0?+?0.5 mg/L) respectively. Multiple shoots (5–6) were obtained on MS medium supplemented with BAP?+?Kin and IAA?+?BAP respectively. When compared with silver nitrate (AgNO₃) (2–40 µM) and activated charcoal (AC) (0.1–1.0%), the MS medium devoid of any plant growth regulator showed good number of shoots (5.48?±?2.42), elongation (15.64?±?2.42 cm) and root length (14.52?±?2.78 cm). Upon transferring of regenerated microshoots to MS medium, simultaneous elongation of shoots with more shoot number, shoot length and rooting was achieved during four subcultures that carried out at 6 weeks’ interval. The regenerated in vitro shoots showed 100% rooting in MS medium and also in MS medium supplemented with 0.1–1.0% AC. Hundred percent survival of micropropagated shoots well rooted was established successfully under greenhouse condition and the plants were subsequently acclimatized and transferred to the field conditions wherein 90% success rate was noted.

     

Cadmium uptake kinetics and plants factors of shoot Cd concentration

Background and aims

Accumulation of Cd in the shoots of plants grown on Cd contaminated soils shows considerable variation. A previous preliminary experiment established that one major reason for this variation was the rate of Cd influx into the roots (mol Cd cm^?2 root s^?1). However, this experiment did not distinguish between solubilization of soil Cd on the one hand and difference in Cd uptake kinetics on the other. The main objectives of the present study were thus to characterize Cd uptake kinetics of plants continuously exposed to Cd concentrations similar to those encountered in soils. Furthermore we determined the factors responsible for differences in shoot Cd concentration such as net Cd influx, root area-shoot dry weight ratio, shoot growth rate and proportion of Cd translocated to the shoot.

Materials and methods

Maize, sunflower, flax and spinach were grown in nutrient solution with five constant Cd concentrations varying from 0 to 1.0 μmol?L^?1. Root and shoot parameters as well as Cd uptake were determined at two harvest dates and from these data Cd net influx and shoot growth rates were calculated.

Results and conclusions

Cadmium uptake kinetics, i.e. the net Cd influx vs. Cd solution concentration followed a straight line. Its slope is the root absorbing power, α, $ \left( {\alpha ={{{\mathrm{Cd}\;\mathrm{net}\;\mathrm{influx}}} \left/ {{\mathrm{Cd}\;\mathrm{solution}\;\mathrm{concentration}}} \right.}} \right) $ . The α values of spinach and flax were about double that of maize and sunflower (5?×?10^?6?cm?s^?1 vs. 2.5?×?10^?6?cm?s^?1). Spinach and flax had a 3–5 times higher shoot Cd concentration than maize and sunflower. The difference in shoot Cd concentration was partly due to the higher Cd influx but also to a higher translocation of Cd from root to shoot and also to a slower shoot growth rate.     

Pustules on wood as sources of inoculum in apple scab and their response to chemical treatments

The green stems and leaves of potted Cox's Orange Pippin trees were susceptible to infection by Venturia inaequalis from June to August. Some isolates of the fungus showed conspicuous ability to attack the wood. In the orchard, many conidia were released from pustules in early March (2—3 wk before bud-burst) but in the 3 years examined (1967—9) most were liberated in April and May. In 1969 it was shown that a decline in numbers in June was accompanied by a decrease in viability of conidia. Infections near the tips of leading shoots were partially controlled by a single treatment with 5% urea (aqueous solution) in October 1966 or 2% urea or 0·01% phenylmercuric chloride (PMC) in March 1967. The subsequent reduction in inoculum was correlated with a reduction in the numbers of primary infections developing on leaves. Basal infections on leading shoots were not susceptible to treatment in October 1967 or March 1968, but in 1969 similar infections were controlled with PMC applied mid-April when the pustules had burst and were actively sporulating.     

Genetic transformation of hop (Humulus lupulus L. cv. Tettnanger) by particle bombardment and plant regeneration using a temporary immersion system

The efficiency of micropropagation of double-node shoots of hop (Humulus lupulus L. cv. Tettnanger) was evaluated using semi-solid and liquid culture medium in RITA® temporary immersion bioreactors. The highest fresh and dry weight of shoots, average number of shoots, and multiplication rate were obtained using the RITA® system, whereas the longest shoots were obtained on semi-solid medium. Moreover, shoot length was affected significantly by the inoculum density of double-node shoots in RITA® vessels. In addition, the RITA® bioreactors were suitable for shoot induction from organogenic calli. The percentage of shoot induction and the shoot fresh and dry weights were significantly higher in the RITA® system than in semi-solid medium. The age of organogenic calli and inoculum density significantly affected the induction of shoots from organogenic calli. The optimum conditions for DNA delivery into hop organogenic calli using the biolistic particle delivery system were also determined. Organogenic calli were bombarded with the plasmid pSR5-2 (gusA and nptII) varying helium pressure (900, 1,100, or 1,350 psi) and target distance (6, 9, or 12 cm). The highest gusA transient activity was obtained using a pressure of 900 psi and a target distance of 6 cm. For stable genetic transformation, 3-wk-old organogenic calli were bombarded with the plasmid pCAMBIA1303 (gusA, mgfp5, and hpt) using these optimum conditions. Stable gusA expression was observed in organogenic calli and shoots after 4 wk of culture on selection medium containing 2.5 mg l^?1 hygromycin. The presence of the mgfp5 gene in the hop genome was confirmed by PCR.     

Studies on plant growth-regulating substances: Interactions between auxins and inactive analogues

Applications of streptomycin sulphate or protectant fungicides reduced numbers of secondary infections in severely infected hop plantings. When primary infected shoots treated with streptomycin were left on plants for 3 weeks no significant increase in secondary disease resulted. Streptomycin, which was absorbed rapidly by hop plants, could not be detected in expressed shoot sap by early June and had no effect on crop yield or quality.     

Effects of vesicular-arbuscular mycorrhizal infection and phosphate on Plantago major ssp. pleiosperma in relation to internal cytokinin concentrations

Relations between cytokinin concentrations and effects of P and vesicular-arbuscular mycorrhizal (VAM) infection were investigated in Plantago major L. ssp. pleiosperma Pilger. Both mycorrhizal infection by Glomus fasciculatum (Thaxt. sensu Gerdemann) Gerdemann and Trappe and P addition increased the shoot to root ratio, specific leaf area (SLA), and P concentrations of shoot and roots, and decreased the percentage of dry matter in the shoot during the experiment. In general, P concentration in the shoot and roots of each treatment correlated positively with the shoot to root ratio and specific leaf area, and negatively with the percentage of dry matter in the shoot. Cytokinin concentrations in the tissue of shoots and roots were determined using an enzyme-linked immunosorbent assay. Concentrations of zeatin and zeatin-ribosides in the free base and nucleotide fractions had increased more after P addition than in the case of mycorrhizal infection in both shoot and roots, whereas the P concentrations had increased less. It is suggested that zeatin and zeatin-ribosides are not the primary growth-substances involved in mediating VAM effects.     

Lesion and epidemic development of Alternaria longipes (Ell. & Ev.) Mason on tobacco

Lesion development of A. longipes was studied in the glasshouse and in the field. The incubation period on glasshouse plants varied from 3 to 8 days. Some pinpoint lesions failed to develop, and lesion development on the upper leaves of pot plants was less than on the lower ones. The maximum estimated lesion multiplication rate was 2.1 daughter lesions/mother lesion/day, but was generally ≤ 0.6. Epidemic development was studied in four seasons (1966 to 1969) by weekly assessment of infection levels on alternate leaves. Leaf area, airborne inoculum concentrations, air, leaf and soil temperature, soil moisture, saturation deficit, rainfall, duration of leaf wetness, and wind speed were also measured. Relative infection rates varied from 0.17-0.26 units/day. Slow disease development in 1968 (r = 0.17 units/day) could be related to the adverse effects of climate on host development and on infection processes. The relative rate of infection varied widely at different stalk position (0.16-0.29 units/day in 1966) and at different times at the same stalk position (0.17=0.48 units/day at position 25 in 1969). There was no consistent relationship between the stalk position of leaves and the rate at which they were infected. Similarly there was no consistent relationship between the rate of infection of whole plants or of leaves at a particular stalk position and climatic or inoculum factors, although during limited periods, changes in one or other of these factors did coincide with an alteration in the relative rate of infection. The interpretation of rates of infection and the theoretical results of greater field hygiene are discussed.