C.H. Dickinson

Colony interactions and hyphal interference between Septoria nodorum and phylloplane fungi

Interactions between dual cultures of Septoria nodorum and several other cereal phylloplane fungi were studied in vitro to determine their antagonistic ability and their tolerance of antagonism. No fungi tested were outstandingly successful antagonists and interference reactions between opposed hyphae rarely occurred. Septoria caused the hyphae of some test fungi to fail to plasmolyse and others to burst. In experiments designed to examine volatile and non-volatile metabolites growth of Septoria was rarely affected by colonies of the test fungi.

Effect of fungicides on the microflora of potato leaves

The phylloplane flora of potato leaves was examined from the time of shoot emergence through to leaf senescence. Studies on untreated leaves have been combined with others in which the plants were sprayed using one of two fungicides currently in use for late blight control. Saprophytic fungi were in general found to be more susceptible to captafol than to maneb plus fentin acetate. Bacterial numbers were apparently unaltered by the spray treatments.

Effects of ethirimol and zineb on phylloplane microflora of barley

The phylloplane flora of barley leaves included several of the cosmopolitan fungi associated with leaf surfaces of temperate plants. The fungal population increased as the leaves matured and then approached senescence. Ethirimol seed treatment resulted in good mildew control but had no obvious effects on other phylloplane fungi. The leaf surface flora was, however, considerably reduced on plants grown from ethirimol-treated seed which were also regularly sprayed with zineb. The possible significance of spraying leaves with wide-spectrum fungicides is considered.

Pathogenicity of Alternaria and Cladosporium isolates on Phaseolus.

Phaseolus leaves were inoculated with conidia of Alternaria alternata, Cladosporium cladosporioides and C. herbarum. Plants were incubated in growth rooms at high humidity. Using light microscopy and cultural techniques it was shown that several of the isolates of all three species were able to penetrate into leaves via stomata. With many isolates most infections remained localized in the substomatal cavity. In other isolates, especially of Alternaria, a more extensive, intercellular mycelium developed. A few isolates also caused host mesophyll cells to become necrotic. These parasitic activities were correlated with an accelerated loss of chlorophyll from infected leaves and an increase in their levels of ribonuclease. These data suggest that some strains of these common filamentous phylloplane fungi are able to behave as weak parasites on herbaceous tissues. The use of fungicides which restrict their activity may result in yield increases over and above those attributed to the control of other, established pathogens.

Host range and taxonomy of Peronospora on crucifers

Isolates of Peronospora from Brassica and Raphanus were examined to determine their host ranges and the effect of the host on sporangium morphology. The behaviour of both isolates was examined on a wide range of crucifers and the Brassica isolate was also inoculated onto a number of Brassica oleracea cultivars. The results obtained are discussed in relation to the problem of determining the exact host ranges for pathogens which behave differently in different host-pathogen combinations. The data obtained support the current taxonomic approach and the identity of both isolates, as P. parasitica, was confirmed.

Germination and growth of Alternaria and Cladosporium in relation to their activity in the phylloplane

The effect of humidity, temperature and nutrient-poor conditions on conidial germination and hyphal growth of Alternaria alternata, Cladosporium cladosporioides and C. herbarum were studied in vitro. Alternaria conidia germinated quickly at high humidity, and in nutrient-free water its germ-tubes grew to form extensive hyphal networks. This development was accompanied by translocation of materials towards the hyphal apices, which resulted in a progressive increase in necrotic, ghost-like, hyphal compartments. Cladosporium conidia germinated irregularly and those of C. cladosporioides tended to form microcyclic conidia. Germ-tubes grew more slowly than those of Alternaria and ghost-like compartments were seen only after longer periods of incubation. Several physiological characteristics of the Cladosporium isolates, and their conidial dimensions, supported the morphological evidence which suggested that they represent two distinct species.

Phylloplane and litter fungi of Ilex aquifolium

Leaf impression peels, scanning microscopy, and several cultural techniques were used to investigate the fungi inhabiting attached, green leaves and newly fallen, litter leaves of holly. Green leaves remain physiologically active for several years but there was no evidence for a progressive build up of the phylloplane mycoflora. There were marked seasonal fluctuations in populations on green leaves and when numbers increased the older leaves supported larger and more varied assemblages than newly formed leaves at shoot apices. Several connexions were found between the mycoflora of the green leaves and that on the litter. In particular litter appeared to provide a refuge for phylloplane yeasts during the winter. Acremonium strictum and Phoma sp. were amongst the most common fungi colonizing litter, but both initially became established on older green leaves prior to their abscission.

Growth of fungi in cattle dung

The vegetative growth of fungal hyphae in cow dung was studied using two direct examination methods. Sporulation on dung was followed in the field and under controlled conditions in the laboratory. The results obtained confirm that fungi are active coprophilous organisms, but their growth and sporulation on this substrate is not always as profuse as has been inferred from studies of fruit body development under ideal conditions. Fungal activity in dung appears to be adversely affected by water content. The succession of fungi sporulating on dung frequently departs from the idealized pattern and it may be drastically altered if the dung is mechanically disturbed during decomposition.

Interactions between germinating spores of Septoria nodorum and phylloplane fungi

Interactions between Septoria nodorum and several phylloplane fungi from cereal leaves were investigated using cellophane films or detached barley leaves. In several combinations, conidium germination and germ-tube growth on cellophane were restricted, especially when the spores of the test fungus were added 24 or 48 h after those of a potential competitor. On leaves these inhibitions were less marked. Several common phylloplane fungi caused only slight inhibition by comparison with Botrytis , which totally inhibited germination of Septoria spores adjacent to its hyphae.

Behaviour of phylloplane fungi on Phaseolus leaves

The behaviour of Cladosporium cladosporioides, Alternaria alternata and Sporobolomyces roseus on Phaseolus vulgaris leaves was studied by direct examination. On plants in growth rooms germination and growth of the fungi was best under conditions of very high humidity. The physiological age of the leaves, leaf senescence, washing leaves prior to inoculation or adding sucrose with the inoculum all had relatively slight effects on growth and germination at low humidity. The results of a field experiment may be interpreted as supporting the suggestion that humidity is a prime factor in determining the extent of leaf colonization.