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- ItemDisentangling the relationships among abundance, invasiveness and invasibility in trait space(Springer Nature, 2023-06-09) Hui, Cang; Pysek, Petr; Richardson, David M.Identifying conditions and traits that allow an introduced species to grow and spread, from being initially rare to becoming abundant (defined as invasiveness), is the crux of invasion ecology. Invasiveness and abundance are related but not the same, and we need to differentiate these concepts. Predicting both species abundance and invasiveness and their relationship in an invaded community is highly contextual, being contingent on the community trait profile and its invasibility. We operationalised a three-pronged invasion framework that considers traits, environmental context, and propagule pressure. Specifically, we measure the invasiveness of an alien species by combining three components (performance reflecting environmental suitability, product of species richness and the covariance between interaction strength and species abundance, and community-level interaction pressure); the expected population growth rate of alien species simply reflects the total effect of propagule pressure and the product of their population size and invasiveness. The invasibility of a community reflects the size of opportunity niches (the integral of positive invasiveness in the trait space) under the given abiotic conditions of the environment. Both species abundance and the surface of invasiveness over the trait space can be dynamic and variable. Whether an introduced species with functional traits similar to those of an abundant species in the community exhibits high or low invasiveness depends largely on the kernel functions of performance and interaction strength with respect to traits and environmental conditions. Knowledge of the covariance between interaction strength and species abundance and these kernel functions, thus, holds the key to accurate prediction of invasion dynamics.
- ItemEtiology and management of black rot on 'Forelle' pears in the Western Cape(2023-03) Richard, Martin-John; Meitz-Hopkins, J.C.; Lennox, C.L.; Plant PathologyA recent increase in anecdotal black rot symptoms in stored pear fruit at Western Cape packhouses resulted in the initiation of the current study. An orchard survey for black rot decay confirmed the causal pathogen, Diplodia seriata, to be present in all orchards from all major pear production regions in the Western Cape. Black rot was better managed postharvest using a controlled atmosphere storage regime compared to regular atmosphere storage conditions. From isolations from black rot symptoms, Diplodia seriata was confirmed as the main causal organism for black rot with 86.21% of isolates representing these fungal species. Other Botryosphaeriaceae detected include Diplodia malorum, Neofusicoccum australe and Dothiorella sarmentorum. ‘Forelle’ pears are highly susceptible to black rot infection and develop severe lesions between 40.52 - 60.20 mm 6 days after wound infection. Fludioxonil and pyrimethanil are the currently relied-on postharvest fungicides. Diplodia seriata inoculated fruit were tested through a postharvest drench setup to determine the efficacy of a fungicide drench with these two products. The drench results showed fludioxonil is the most effective fungicide for black rot management and that D. seriata is non-sensitive towards pyrimethanil in vivo. In vitro trials were also performed to determine if the in vivo results persist, and this was in the form of a baseline sensitivity trial and a screening trial. For the baseline sensitivity, an EC50 value of 0.0055 mg/L and a discriminatory dose of 0.0500 mg/L were determined for fludioxonil. All D. seriata isolates obtained in this study were screened and killed at the discriminatory dose. For the screening with pyrimethanil, no isolates were reduced to 50% relative growth using a published EC50 value for D. seriata. From this study, there are several factors highlighted that could contribute to the increased incidence of black rot in the Western Cape. From management practices, the non-sensitivity of D. seriata towards pyrimethanil in vivo and in vitro experiments could prove a shift of resistance of these pathogens towards pyrimethanil. Other factors could be the increased stress on pome fruit trees after the 5-year drought that the Western Cape experienced. Drought stress weakens pome fruit trees which leads to increase canker infection and development. Another possible reason is the miss identification of black rot that could have occurred over the years. Black rot symptoms can easily be confused with symptoms of grey-mould (Botrytis cinerea) or Alternaria rot (Alternaria spp.) infections. The work done in this study is an initial step towards understanding black rot in the Western Cape. Key future work should investigate alternative management strategies for the control of black rot postharvest since industry currently relies on one fungicide active (fludioxonil). Aerial inoculum investigations should also be conducted to determine when a high inoculum load is present to optimise the timing of cultural and chemical canker management within the pome fruit orchards.