Waxflower (Chamelaucium spp. and hybrids) is the singlemost important Australian export cut-flower. The major problem in waxflower trading is flower abscission after harvest. While several factors are involved, ethylene production resulting from preharvest infection with the fungus Botrytis cinerea is the most important cause. The general objectives of this study were to investigate the biology of Botrytis infecting waxflower flowers and potential elicitation of host defence against this pathogen. Effects of anti-ethylene and S-carvone treatments on Botrytis-induced flower abscission were also evaluated. Infection of flowers by Botrytis was studied on two waxflower cvs. Mullering Brook and My Sweet Sixteen using light and electron microscopy. Conidial germination and protoappressorial formation occurred within 8 h post-inoculation (hpi). Infection of most floral organs, including petals, anthers and filaments, stigma, and hypanthium, was within 24 hpi. Infection cushions on stamen bases were formed at 36 hpi by saprophytic hyphae that originated from anthers. This infection route probably gives rise to the typical tan-coloured Botrytis symptoms that appear to radiate from this part of the flower. Subcuticular hyphae were present at very high density near stamen bases. They evidently resulted at multiple penetrations from single infection cushions. Flower abscission occurred at 72 hpi. At this time, floral tube tissues remained uninfected. This temporal pattern infers the possible transmission of a signal (e.g. ethylene) upon Botrytis infection (6–36 hpi) that intiates a defence response of shedding infected flowers (72 hpi). Susceptibility of waxflower before and after harvest to B. cinerea under various environmental conditions (laboratory, greenhouse, and field) was investigated. Flowers, either on plants or on cut stems showed similar susceptibility to B. cinerea and abscised under cool temperatures (~20 ºC) and high humidity (>95% RH) conditions following infection. Compared to cv. Mullering Brook, cv. My Sweet Sixteen was somewhat more resistant to B. cinerea infection under field conditions. Constitutive and inducible antifungal compounds in waxflower flower tissues were screened in cvs. CWA Pink, Stephan’s Delight, Mullering Brook and My Sweet Sixteen using thin layer chromatography bioassays with isolates of B. cinerea and Alternaria alternata (pathogenic) and Cladosporium cladosporioides (non-pathogenic). Common inhibition zone observed at Rf 0.28–0.38, 0.46–0.56 and 0.67–0.76 contained phenolic compounds. There were at least five (cv. Mullering Brook) and one (cv. My Sweet Sixteen) inducible antifungal phenolic compounds as judged by increases in inhibition area as a result of B. cinerea infection and methyl jasmonate treatment. The total areas of B. cinerea- and MeJA-induced inhibition zones were approximately 2.0- and 2.5-folds greater, respectively, than zones from control flowers. Preharvest sprays of three different known host plant defence elicitors, methyl jasmonate (MeJA), benzothiadiazole (BTH), and silicon (Si), were applied to waxflower cvs. Mullering Brook and My Sweet Sixteen plants. BTH or Si sprays generally had no significant effect on postharvest Botrytis severity on either cultivar. MeJA sprays did not reduce B. cinerea on cv. Mullering Brook. MeJA slightly suppressed B. cinerea on cv. My Sweet Sixteen at 500 and 750 µM. Overall, field applications of these host plant defence elicitor chemicals as spray treatments had little effect on vase life, water uptake and relative fresh weight of the cut sprigs. Moreover, they did not appreciably suppress B. cinerea or associated postharvest floral abscission. The efficacy of combined elicitor treatments and combined pre- and postharvest MeJA treatments were assessed. Preharvest foliar applications of MeJA (1000 µM; 2 or 4 times), MeJA (1000 µM) combined with BTH (150 mg/L), and MeJA combined with Si (1500 mg SiO2/L) generally did not suppress postharvest B. cinerea development and flower abscission from harvested sprigs. A pre- plus post-harvest 1000 µM MeJA spray treatment consistently but only slightly suppressed B. cinerea infection on flowers from both pot- and field-grown plants. Pre- and post-harvest MeJA treatments reduced B. cinerea development, but increased flower abscission. Combined MeJA and anti-ethylene treatments were then screened for potential to suppress B. cinerea while preventing flower abscission. However, the combined MeJA and 1-MCP treatment reduced neither Botrytis disease nor flower abscission on sprigs from pot- and field-grown plants. The combined MeJA and STS treatment reduced disease severity for up to 6 days on sprigs harvested from pot-grown plants but tended to increase Botrytis severity on sprigs from field-grown plants 6 days after inoculation. Antifungal effects of the essential oil S-carvone against B. cinerea germination and mycelial growth were demonstrated in vitro. Inhibition increased with increasing S-carvone concentrations from 0.64 mM to 5.08 mM. However, in planta, S-carvone concentrations in this range did not affect either Botrytis disease levels or flower abscission on cut waxflower flowers.
| Identifer | oai:union.ndltd.org:ADTP/286021 |
| Creators | Son-Quang Dinh |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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