Abstract
Percentage in-vitro conidial germination of inaequalis was inversely related to the log conidial concentration. The filtrate from a conidial suspension of inaequalis was shown to inhibit conidial germination. Ferrous iron was shown to reduce conidial germination. Anthranillic acid reversed the inhibitory action of ferrous iron and enhanced germination of conidial suspensions not containing additional iron. Continuous rolling or ''pre-treatment" of conidial suspensions of V. inaequalis used for inoculum increased levels of infection. It was suggested that this improvement may be a result of higher numbers of germinated conidia present on the leaf surface. Pre-treatment of inocula was adopted as a standard inoculation technique. Symptom expression was studied under two categories :- i) Effect on seedling growth (stunting/promotion) ii) Foliar symptoms (total disease expression score - TDE) Stunting and TDE values were correlated to lesion number and positively correlated to inoculum concentration. High inoculum concentrations caused stunting and low ones resulted in growth promotion. Similar responses were found with all apple varieties tested. Experimental evidence was presented to support the hypotheses :- 1) Conidia only produce stunting. 2) The filtrate from a pre-treated conidial suspension of V. inaequalis contained a factor which induced growth promotion of apple seedlings. Washing pre-treated conidia with GDW immediately before inoculation enhanced stunting of apple seedlings. Foliar sprays of the pre-treatment filtrate promoted seedling growth, concentrating it further enhanced promotion. The factor responsible was resistant to boiling, suggesting it to be non-protein but after standing for 24 h lost its ability to promote growth. Bioassays of the filtrate for IAA and cytokinins were negative. A dock leaf disk test was positive for gibberellins. Analysis of the filtrate using mass spectrometry indicated the presence of high molecular weight ions but it was not possible to identify any particular fragment or conclude whether these could be responsible for growth enhancement. It is suggested that the filtrate acts by increasing cell elongation rather than cellular proliferation. Use of the fungicides dodine and captan produced a host response to infections by inaequalis ranging from stunting to growth promotion. The potential of pre-treatment and symptom assessment for use as a fungicide screening technique was demonstrated. It was suggested that summation of the ranked scores for TDE and phytotoxic stunting provided an accurate assessment of a chemical's suitability for further testing.