The mode of action of sodium bicarbonate / additive mixtures against cucumber powdery mildew disease : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Applied Science in Plant Pathology, Massey University
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Date
1997
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Massey University
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Abstract
The host-parasite-complex of cucumber (Cucumis sativus L.) and Sphaerotheca fulginea (Schlechtendal ex Fr.) Pollacci, the causal agent of cucumber powdery mildew, was studied to determine whether stimulation of host defences was a mode of
action of sodium bicarbonate/additive mixtures. Additives included shellspray mineral oil and sunlight dishwashing liquid. The roles in cucumber deface of phenolic compounds, enzymes: peroxidase (PO) and phenylalanine ammonia lyase (PAL), and induced structural barriers, papillae, were investigated. Comparisons were made between the effects of bicarbonate/'additive mixtures and the plant extract Milsana on these host defences, as related to three levels (cultivars) of host resistance to disease: susceptible, disease-tolerant and resistant. Long and short duration glasshouse studies found disease incidence highest on less resistant cultivars and lowest on the most resistant cultivar, 'Slice King' ("SK'). Applied alone at 2 g/litre, sodium bicarbonate failed as a protectant. When combined with either additive, the fungicidal activity of bicarbonate was enhanced. Shellspray oil and bicarbonate-oil mixtures were the most effective treatments for less resistant cultivars, 'Lebanese' ('Leb') and 'Slicemaster' ('SM'). These treatments and Milsana provided comparable control against disease affecting 'SK'. The role of phenolic compounds was investigated in thin-layer chromatogram bioassays for antifungal phenolics in leaf extracts. This revealed much variability in presence and absence of disease. Glycosidically-bound phenolics hydrolysed into their free form, aglycones, were more abundant than free phenolics. Because of their frequency and arbitrary concentrations, it was unclear whether stimulation of aglycone production had occurred in infected leaves, as a result of any specific treatment or level of host resistance. The presence of these fungitoxic substances could not solely account tor differences in disease severity. Hence, data did not support the concept of stimulation of host materials by either mildew infection or the treatments applied. The responses of PO and PAL to treatment and infection were assessed by spectrophotometric measurements of activity in leaf samples. Higher enzymic activities in healthy leaves were coupled with increasing levels of host resistance to disease. During the 14 days post-treatment, significant increases were detected in healthy and inoculated leaves treated with bicarbonate and Milsana, levels varied between cultivars. General enzymic-activity was higher in inoculated than noninoculated 'Leb' and 'SM', and of a comparable level in 'SK' controls and leaves treated with materials other than shellspray oil and bicarbonate-oil mixtures. For these apparent "normal" increases in plants treated with other materials occurred during this time. Fluorescence and bright-field microscopy revealed similar numbers of conidia germinated 24 h after inoculation, irrespective of treatment and cultivar, although significantly fewer conidia germinated on leaves treated with oil and bicarbonate-oil mixtures. With increasing host resistance the number of multiple germ tubes produced 72 h post-infection was less, papillae deposited in epidermal cells had increased, and the number of haustoria less, 120 h post-infection. Shellspray oil and bicarbonate-oil mixtures provided the most significant reduction. in multiple germ tubes and haustoria and the least apparent stimulation of papillae production.