Phytophthora collar rot of apple: influence of the rootstock on scion variety resistance

In field experiments with young trees great differences were found in the resistance to Phytophthora cactorum of Cox's Orange Pippin apple scions grafted on different clonal rootstocks. The rootstock effect on scion resistance was inversely related to the effect on tree vigour: the rootstocks inducing high resistance were dwarfing (M. 9) or semi-dwarfing (M. 7, M. 26, MM. 106), and those inducing low resistance were vigorous or very vigorous (M. 25, MM. 104, MM. 109). Mean lesion lengths in Cox on MM. 104 were five to eight times greater than those in Cox on M. 9. The rootstock influence on scion resistance was associated primarily with effects on the rate of lesion extension: during the early stages only of host colonization there appeared to be threshold extension rates below which host resistance factors effectively suppressed a large proportion of infections. The influence of the root-stock on scion resistance was apparently unrelated to inherent rootstock resistance. On all rootstocks Cox showed diminished resistance to infection during the period from the swelling of buds to the early stages of shoot growth. Although most susceptible during the ‘mouse-ear’ and ‘pink bud’ stages of development, suscpetibility was not associated with flowering per se. Rootstock type did not affect the resistance of Cox scions to P. syringae, for which the period of susceptibility to infection occurred in the dormant season.     

Tree litter and the lower limits of subalpine herbs and grasses in the Brindabella Range,ACT

Abstract Herbs and grasses form a dense understorey in subalpine woodlands dominated by Eucalyptus pauciflora but are absent from stands of Eucalyptus dives immediately down slope. We hypothesized that a relatively high rate of litter accumulation beneath E. dives was responsible for the downward limit of subalpine plants. We tested this hypothesis by removing litter and by adding E. dives litter to plots in both forest types during two growing seasons. Litter accumulation rates were 50% higher under E. dives than E. pauciflora, but understorey bio-mass in control plots was eight times higher under E. pauciflora than E. dives. After the first growing season, seedling densities in both forest types were significantly higher in plots from which litter had been removed but, after the second growing season, understorey biomass was not higher in litter removal plots. The results indicate that litter has the potential to limit germination in both forest types, but that environmental filters other than litter subsequently limit recruitment and are ultimately responsible for the lower distributional limits of subalpine herbaceous species.     

Chilling-Induced Ethylene Production in Relation to Chill-Sensitivity in Phaseolus spp.

Guye, M. G, Vigh, L. and Wilson, J. M. 1987. Chilling-inducedethylene production in relation to chill-sensitivity in Phaseolusspp.—J. exp. Bot. 38: 680–690. Ethylene production from the primary leaves of six bean (Phaseolusspp.) cultivars known to differ in chill-sensitivity, was monitoredat 23 ?C following chilling of whole plants at 5 ?C for 24 h.The more chill-tolerant cultivars produced greater amounts ofchilling-induced ethylene than the chill-sensitive cultivars.The onset of maximum ethylene production rates and the followingdecline in rates was more rapid in chill-tolerant cultivars.This pattern of ethylene production was also similar when chill-tolerancewas chemically enhanced by choline treatment. The low levelsof ethylene production in chill-sensitive genotypes was alsoreflected by their poor ability to convert the exogenously appliedethylene precursor, 1-aminocyclopropane-l-carboxylic acid (ACC),to ethylene. Moderate levels of leaf water deficit induced by chilling chill-tolerantcultivars and choline treated plants appeared to stimulate chilling-inducedethylene production. High levels of leaf wilt, shown by morechill-sensitive cultivars, reduced this stimulatory effect.Ethylene production was slightly greater when warming was carriedout in the light rather than in the dark. Key words: Ethylene, ACC, choline, chill-sensitivity, Phaseolus     

The role of Thielaviopsis basicola in the specific replant disorders of cherry and plum

The hypothesis that Thielaviopsis basicola might be the causal agent of the specific replant disorders of cherry and plum was tested in experiments with pot-grown trees and by observations on the distribution of the pathogen. The results obtained with certain strains of T. basicola simulated and were consistent with all features known to characterize the specific replant disorders, viz. inter- and intra-generic specificity of effect; host symptoms; establishment, immobility and persistence of the causal agent in soil; normal growth of affected trees after transfer to ‘non-replant’ soil; limited influence of soil type on the incidence of the disorder. T. basicola was therefore considered to be the probable causal agent of the specific replant diseases of certain Prunus species. The etiology of these diseases is discussed. T. basicola was neither pathogenic to apple nor associated with the growth of apple. The apparently closely similar specific replant disorder of Malus is therefore considered to be causally distinct from those of cherry and plum.     

Host plant associations, diversity and species—area relationships of mesophyll-feeding leafhoppers of trees and shrubs in Britain

Abstract. 1. Sixty-two British species of Typhlocybine leafhoppers are known to feed on the leaf-mesophyll tissue of trees and shrubs. British host records for fifty-five of these are given.
2. The leafhopper faunas of thirty-six species of native and introduced trees and shrubs are described.
3. The Shannon-Wiener equation was used to calculate species diversity for adult samples collected from twenty different species at sixteen different localities in Wales, southern England and northern Scotland.
4. Sørensen's coefficients were calculated for rearing data from Britain generally, and subjected to cluster analysis. Most trees have low similarities with respect to leafhopper faunas and are quite distinct. Taxonomic relationships of trees appear to be relatively unimportant in determining the similarities of their leaf-hopper faunas.
5. Using the same data, species—area relationships were calculated for thirty-four different tree and shrub species and their associated leaf-hoppers. A significant regression was obtained, but it explained only 16% of the variation. It is thus suggested that host plant range is relatively unimportant in determining the numbers of these species associated with different trees in Britain.
6. Some introduced species of trees, particularly the recently planted Nothofagus , have acquired large leafhopper faunas.