J.P. Blakeman

Germination of Botrytis cinerea conidia in vitro in relation to nutrient conditions on leaf surfaces

Conidia of different isolates of Botrytis cinerea Pers. ex Fr. varied widely in ability to germinate in water. Sugars and amino acids were added at concentrations similar to natural levels on leaf surfaces. To a limited extent simple sugars promoted germination. Fructose was most effective whilst sucrose was least effective. Amino acids stimulated germination with the exception of taurine and lysine. Lysine reduced the stimulatory effect of a mixture of amino acids. Germination of conidia was only increased by an inorganic nitrogen source, ammonium salts or by a mixture of mineral salts, when glucose was present. Growth factors, yeast extract, gibberellic acid and kinetin had no effect on germination but indole-acetic acid caused a small stimulation. Abscisic acid depressed germination. It is suggested that, like soil fungi, the germination of spores of phylloplane fungi may be nutrient-dependent, or independent with respect to exogenous nutrients. Poor in vivo germination of B. cinerea may be associated with competition.

Effect of substances associated with leaf surfaces on appressorium formation by Colletotrichum acutatum

In six species of Colletotrichum, appressoria developed more readily on leaves than on glass. Colletotrichum acutatum Simmonds germinated equally well on glass and leaves but produced very few appressoria on glass. A chloroform extract from the surface of sugar-beet leaves at all concentrations increased numbers of proto-appressoria on glass. At low concentrations the water-soluble component of the chloroform extract promoted formation of appressoria whilst at higher concentrations it inhibited germination. Formation of mature appressoria on chloroform extract was enhanced in the presence of a phylloplane bacterium, Pseudomonas sp. isolate 14. A steroid, which was the major single component of the chloroform extract, had no effect on formation of appressoria. A chloroform extract from lower leaf surfaces was more effective than that from upper surfaces in stimulating formation of appressoria when used in amounts corresponding to unit area of leaf. A similar extract from chrysanthemum leaves prevented germination of conidia but when diluted 5 times stimulated formation of appressoria. A chloroform extract from the surface of tomato leaves was divided into 14 fractions by column chromatography. The first fractions (containing mainly alkanes) had little effect on formation of appressoria; the middle fractions, which inhibited fungal growth at higher concentrations, enhanced formation of appressoria at lower concentrations. The last three fractions from the column greatly stimulated growth of germ-tubes but prevented development of appressoria. A pure sample of a 28 carbon alkane had no effect on formation of appressoria.

Microbial colonization and uptake of 14C label on leaves of sycamore

Low numbers of micro-organisms were found on sycamore leaves during early May and June when bacteria predominated. From July onwards numbers of micro-organisms on the leaves increased steadily, with either white yeasts or Aureobasidium pullulons being dominant. Low levels of amino acids and sugars were recorded on leaves early in the season but a sharp increase at the end of June coincided with the arrival of aphids on the leaves. Low amounts of 14 C-labelled amino acids and glucose were taken up early in the season but uptake could be markedly increased by prior wetting of the leaf for 24 or 48 h. This and leaf wetness recordings indicated that at this time the microflora was limited by availability of water rather than shortage of nutrients. During July and August competition for nitrogen compounds seemed to be a major factor limiting the size and composition of microbial populations.

The effect of ultraviolet and visible light on infection of host leaf tissue by four species of Ascochyta

The effect of white light and long- and short-wave ultraviolet light on spore germination, penetration and infection was determined for Ascochyta chrysanthemi on chrysanthemum, A. pisi and A. pinodes on pea and A. imperfecta on lucerne, by means of a detached leaf culture technique. White light and longwave ultraviolet light had no effect on these processes. Short periods of exposure to short-wave ultraviolet light resulted in increased infection, which was most marked in A. chrysanthemi and A. imperfecta , whilst longer exposures (up to 200 s) caused a progressive reduction in infection and spore viability. The increased infection by A. chrysanthemi was due to an increase in the susceptibility of the leaf tissue as a result of exposure to short-wave ultraviolet light. The response of A. chrysanthemi on irradiated leaf tissue of other plants was also altered; the susceptibility of lettuce was increased and the partial resistance of sunflower was destroyed. Spores of all four species survived the effects of short-wave ultraviolet irradiation better on leaf tissue than on agar, and only spores of A. chrysanthemi showed a stimulation in germination on leaf tissue after short periods of exposure to ultraviolet light. There was no alteration in the growth rate of colonies which had developed from spores on agar which had survived irradiation by any of the three light sources.

Germination of Botrytis cinerea spores on beetroot leaves treated with antibiotics

Multiplication of bacteria on beetroot leaves was reduced when dilute aqueous solutions of chloramphenicol or streptomycin, but not penicillin, were placed as droplets on the leaves. There was also a change in the composition of the bacterial flora. Streptomycin and penicillin at higher concentrations inhibited germination of B. cinerea spores in vitro. After removal of droplets containing antibiotics from leaves and addition of fresh B. cinerea spores in vitro there was improved germination with chloramphenicol and streptomycin, but germination remained poor with penicillin. Satisfactory germination of B. cinerea spores on beetroot leaves occurred only in the presence of 50 μ/ml streptomycin. Application of streptomycin to beetroot leaves caused increased leakage of amino acids, and to a lesser extent, of carbohydrates. A reduction in amounts of streptomycin and penicillin, but not of chloramphenicol, in droplets on beetroot leaves was recorded after 24 h.

Microflora associated with urediniospores of Uromyces viciae-fabae and effects on urediniospore germination

The associated microflora of urediniospores of Uromyces viciae-fabae on broad bean differed quantitatively rather than qualitatively from that of broad bean ( Vicia faba L.) leaves alone. Gram-negative bacteria predominated with urediniospores whilst Cryptococcus yeasts were the most abundant micro-organisms on leaves. Rust-infected broad bean leaves supported much higher numbers of micro-organisms than uninfected leaves. Growth of germ tubes of U. viciae-fabae urediniospores was appreciably reduced on previously-wetted broad bean leaves compared with previously dry leaves. Bacteria isolated from urediniospores reduced growth of germ tubes and one isolate prevented germination on leaves. Growth of urediniospore germ tubes was greatly stimulated in the presence of Cryptococcus and Sporobolomyces yeasts and by Aureobasidium pullulons . Stimulation was more marked on glass than leaves. Trichoderma viride and Penicillium sp. also stimulated growth of urediniospore germ-tubes but to a lesser extent than the yeasts.

Stimulation of Uromyces viciae-fabae in vitro and in vivo by the phylloplane yeast Cryptococcus

Abstract Germination, growth of germ tubes and infection by uredospores of Uromyces viciae-fabae (Pers.) Schroet. were increased by the addition of yeast cells of the genus Cryptococcus. Inoculation of broad bean ( Vicia faba L.) leaves with Cryptococcus cells 24 or more hours prior to addition of uredospores caused a greater stimulation of infection than simultaneous inoculation. Leachate from Cryptococcus cells also increased uredospore germination and infection although to a lesser extent than the presence of yeast cells. Germinating uredospores utilized only a small proportion of the carbon-containing compounds in a leachate from 14 C-labelled Cryptococcus cells. Addition of low concentrations of yeast extract solution increased uredospore germination and growth of germ tubes, but a mixture of glucose and amino acids at leaf surface concentrations and diluted nutrient broth were less effective.

The pattern of asexual sporulation in Mycosphaerella ligulicola.

The asexual sporulation pattern of Mycosphaerella ligulicola Baker, Dimock & Davis was followed on agar culture. At 23°C pycnidial initials were laid down only in mycelium which was 10–30 h old. Spore production commenced in mycelium which was 18 h old and continued for at least 20 days. Where septate spores were produced, evidence suggested that septa were formed after abstriction of the spores from the sporogenous cells of the pycnidium. On agar medium and host leaf disks the spore number per unit area of culture increased with increase in temperature but spore size and the proportion of septate spores decreased. The increase in spore number was due primarily to a higher productivity per pycnidium. Total volume of the spore mass remained virtually unchanged over a wide temperature range. Within limits determined by temperature, illumination caused a reduction in spore size which was associated with a reduction in total volume of the spore mass.

Asexual sporulation of Mycosphaerella ligulicola in relation to nutrition

The number of pycnidia and spores formed by Mycosphaerella ligulicola Baker, Dimock & Davis per unit area of culture was influenced by the type of medium and the nutrient level, whereas the number of spores produced per pycnidium was less affected. Spores were smaller on media of low nutrient status and there were large differences in total spore volume per pycnidium. Large, diffuse pycnidia lacking defined ostioles and invested with vegetative hyphae were formed on medium with excess of the carbon source (or low nitrogen). The continuous provision of nutrients extended the period during which pycnidia were produced, increased the total number of pycnidia and spores, and caused changes in hyphal morphology. Removal of spores from cultures by washing did not influence the sporulation pattern.

Apparent silver accumulation by germinating conidia of Botrytis cinerea in autoradiographs

Germ-tubes of Botrytis cinerea cause movement of silver ions from photographic emulsion, separated from the germ-tubes by a membrane filter, to the germ-tubes where a partial reduction of the cation to metallic silver is effected.