Abstract: Some secondary metabolites of plants function as antimicrobial products against phytopathogens and constitute an increasingly important class of pesticides. In the present study, the essential oil of Asarum heterotropoides var. mandshuricum was analyzed by GC/MS and its antimicrobial activity was evaluated against five phytopathogenic fungi. Major components of the oil were methyleugenol (59.42%), eucarvone (24.10%), 5-allyl-1,2,3-trimethoxybenzene (5.72%), and 3,7,7-trimethylbicyclo(4.1.0)hept-3-ene (4.93%). The essential oil and the most abundant component, methyleugenol, were separately assayed for inhibition of 5 pathogens: Alternaria humicola, Colletotrichum gloeosporioides, Rhizoctonia solani, Phytophthora cactorum and Fusarium solani. Both the oil and methyleugenol strongly inhibited the growth of the test pathogens (IC50 values <0.42 μg ml−1) except F. solani, with the best activity against P. cactorum (IC50 values = 0.073 and 0.052 μg ml−1, respectively). It is concluded that the essential oil of A. heterotropoides var. mandshuricum has a broad antiphytopathogenic spectrum, and that methyleugenol is largely responsible for the bioactivity of the oil. The mode of action of methyleugenol against P. cactorum is discussed based on changes in the mycelial ultrastructure.
Abstract: Stem canker and Phytophthora pod rot (PPR) or black pod caused by Phytophthora palmivora are serious diseases of cocoa (Theobroma cacao L.) in Sulawesi, Indonesia, causing high yield losses for smallholders, possibly exceeded only by losses due to the cocoa pod borer (CPB), Conopomorpha cramerella. Potassium phosphonate (phosphite) applied by trunk injection has been demonstrated to effectively control canker and PPR in Papua New Guinea. The method was tested in a field trial in south-east Sulawesi. Fifty trees were injected with phosphonate, 50 with water and 50 were left untreated. Phosphonate was applied at a rate of ~16 g active ingredient per tree per year, depending on the size of each tree. Trees were evaluated each month for canker severity, for PPR incidence and for CPB incidence and severity. From 4 months after the initial injection, trees treated with phosphonate had negligible levels of canker. Over a 2.5-year period, phosphonate significantly decreased PPR incidence. Cycles of PPR infection occurred in the wet season with PPR incidence fluctuating from less than 30% to greater than 75%. These fluctuations might have been due to variations in rainfall causing natural cycles of sporulation and infection. CPB incidence did not differ significantly between treatments. Since trunk injection of phosphonate effectively controls stem canker and decreases PPR in the long term it provides a valuable option for the management of these diseases for cocoa smallholders.
Abstract: Volatiles from potato plants infected with Phytophthora infestans (Mont.) de Bary were monitored by in situ headspace sampling. The sampling was done in four periods i.e. 28–42, 52–66, 76–90, and 100–114 h after inoculation (HAI). The headspace samples were analyzed by a gas chromatography–flame ionization detector (GC–FID) to assess the differences in volatile fingerprints between the infected-plant group and control groups, i.e. non-inoculated-plant and empty-vessel groups. The samples were subsequently analyzed by gas chromatography–mass spectrometry to identify specific peaks observed by GC–FID. Spore germination, infection, symptom development and sporulation were also monitored to ascertain the disease developmental stage when marker volatiles were first generated. The first symptoms of infection were visible after two days. Three marker volatiles i.e. (E)-2-hexenal, 5-ethyl-2(5H)-furanone and benzene-ethanol were found in the third and fourth trapping periods (3–4 days after inoculation) when sporangiophores were already formed. The volatile metabolites from blighted plants could be applied for sensor development to detect the occurrence of the disease in the field as well as for investigation of volatile production in relation to plant responses to infection.
Abstract: The combination of thymol, a monoterpene phenol compound originating from thyme, and acibenzolar-S-methyl (ASM; Actigard 50 WG), a systemic acquired resistance (SAR) inducer, was applied to tomato plants in field conditions to evaluate the effectiveness of both chemicals to control bacterial wilt. Thymol was applied as a soil fumigant at 9.43 kg per ha 24 h after soil infestation and seven days before transplanting. ASM was applied as a foliar spray at 3.59–8.98 ml per ha, once in the greenhouse and five times in the field. The field was inoculated by applying 50 ml of pathogen suspension (107 cfu/ml) into each transplanting hole eight days prior to transplanting. The experiment was performed in 2006 and repeated in 2008 at the North Florida Research and Education Center in Quincy, FL. In 2006, the combination of ASM and thymol significantly reduced disease in the bacterial wilt tolerant genotype 7514 compared to thymol alone. In 2008, the combination of ASM and thymol significantly reduced disease and increased yield compared to the control, whereas ASM or thymol alone did not significantly reduce disease or increase yield compared to the control. This is the first report of the use of both thymol and ASM to control bacterial wilt on moderately resistant tomato cultivars. Based on this study, control of the pathogen can be achieved by using both chemicals and moderately resistant cultivars.
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