Benzothiadiazole (BTH) was found to be highly effective in increasing resistance of two poinsettia cultivars -'Coco White' and 'Malibu Red', moderately susceptible to the fungus Botrytis cinerea. BTH applied at a concentration of 0.3 mM on the discs cut out from the leaves of these poinsettia cultivars reduced disease symptoms by more than 60 % in comparison to the control discs treated with water and exposed to infection.It was also observed that the applied inducer at a concentration of 0.03 and 0.3 mM had a favourable influence on the increase of poinsettia systemic resistance of SAR type (systemic acquired resistance). The effectiveness of BTH was much less when disease development was examined on detached leaves (a 20 % reduction of lesion area) in comparison with a pronounced inhibition of grey mould development on intact leaves of previously induced plants (a 80 % protection of intact plants).Benzothiadiazole in the concentration range from 0.03 to 1.4 mM added to in vitro agar medium was not found to have an inhibitory influence on Bot~tis cinerea mycelium growth and sporulation.
Development of Botrytis cinerea Pers. ex Fr. on leaves of common poinsettia (Euphorbia pulcherrima Willd.) (Otrzymano: 06.04.2005) S u m m a r y The development of Botrytis cinerea was assessed on six cultivars of common poinsettia, differing in the colour of bracts, and being in great demand among buyers of these ornamental plants. Resistance to this pathogen differed in the investigated poinsettias. Cultivar 'Malibu Red' (red bracts) turned out to be most susceptible, while cv. 'Marblestar' (cream-pink) and cv. 'Coco White' (white) -relatively resistant to this fungus.After application of various inoculation methods (leaf discs, cut off leaves, whole plants) the differences in resistance to B. cinerea were confirmed for two extreme cultivars -susceptible ('Malibu Red') and resistant ('Coco White'), which indicated genetic background of this polymorphism.The rate of disease development on poinsettia leaves was affected by the amount of spores used for inoculation (optimum density of 3.5·10 5 B. cinerea conidia / ml suspension) and the addition of stimulants (0.1 M glucose with 0.05 M KH 2 PO 4 ), which facilitated germination and infection of the host tissue.The inoculated poinsettia leaves showed high stability of plasma membranes. In the susceptible cultivar, in spite of the development of necrotic spots, a significant increase in the membrane damage index (by 13%) was found only on day 7 of the disease development.
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