Development of an integrated strategy to manage Sclerotinia sclerotiorum : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Dassanayaka, Madhavi Prasadani
Fields of Research
ANZSRC::310805 Plant pathology , ANZSRC::310799 Microbiology not elsewhere classified , ANZSRC::300804 Horticultural crop protection (incl. pests, diseases and weeds)
Sclerotinia sclerotiorum is a ubiquitous, necrotic fungus with a broad host range and as a plant pathogen infects a wide range of plants in New Zealand, including many economically important crops. Managing S. sclerotiorum by a single management practice is challenging due to the naturally high pathogenicity and the production of survival structures (sclerotia) in the soil. Hence, the aim of this study was to investigate the potential of combined applications of biofumigation, Perlka® and the biocontrol agent, Coniothyrium minitans, for controlling S. sclerotiorum. In the in vitro experiments done to test the effect of medium and cold preconditioning on carpogenic germination of sclerotia and development of apothecia, it was observed that carpogenic germination of sclerotia and the total number of apothecia per Petri plate was greater in water agar compared to potting mix and sand. Comparatively, the differentiation of apothecia was greater in potting mix and faster in sand. The isolates which originated from the Auckland region (LUPP2650, LUPP2651 and LUPP2662) showed greater carpogenic germination and development of apothecia compared to those originating from Canterbury (LUPP469, LUPP480, LUPP483, LUPP462) under the in vitro conditions used in this study. A few isolates (LUPP479, LUPP480, LUPP2660) showed a positive response for cold preconditioning (at 4°C) in terms of germination of sclerotia (%) and/or time to develop the first apothecium. The in vitro assessment showed that Brassica juncea ‘Caliente 199’ has greater potential to inhibit the mycelial growth of 10 isolates of S. sclerotium (60%) compared to the other tested biofumigant crops, Eruca sativa ‘Nemat’ (rocket) (29%), Brassica juncea ‘Unknown’ (brown mustard) (10%) and Sinapis alba ‘SKU 4295’ (white mustard) (7%). Plant tissue of ‘Caliente 199’ harvested at 50% or 100% flowering and adjusted to 80% (w/w) moisture resulted in greater mycelial inhibition compared with the vegetative tissue. High quantities of plant tissue of ‘Caliente 199’ of 5 g and 10 g resulted in greater mycelial inhibition than lower quantities. The EC50 of the S. sclerotium isolates LUPP475 and LUPP2650 was 3.76 g and 3.23 g, respectively. Using whole plant tissue (shoots + roots) and only shoot tissue resulted in a similar inhibition of the mycelial growth in all four isolates tested. The S. sclerotiorum isolate LUPP475 showed more tolerance to biofumigation with ‘Caliente 199’ compared to LUPP2650 in terms of mycelial inhibition. Using the plant tissue of ‘Caliente 199’ at 50% or 100% flowering and adjusting to 80% (w/w) moisture at the higher quantities (10 g) resulted in the maximum inhibition of mycelial germination of sclerotia of four S. sclerotiorum isolates. Whole plant tissues showed better inhibition of mycelial germination of sclerotia than only shoots. S. sclerotiorum isolates LUPP475 and LUPP2650 indicated a fungistatic effect on mycelial germination of sclerotia by ‘Caliente 199’ but isolates LUPP458 and LUPP463 germinated rapidly even after exposure to 10 g of whole plant tissues. Using ‘Caliente 199’ equivalent to the field rates of 75 t/ha and 100 t/ha inhibited the carpogenic germination of sclerotia by ~46% compared to the control but failed to reduce the sclerotial viability. In the box bioassays, using Perlka® alone, equivalent to the field rate of 400 kg/ha, resulted in complete inhibition of carpogenic germination although sclerotial viability was not reduced. Applying C. minitans resulted in complete inhibition of carpogenic germination and significant sclerotial mortality (95.0%). Applying C. minitans in combined application with Perlka® also resulted in 100% suppression of carpogenic germination and greater sclerotial mortality (97.5%). A synergistic effect of the combined application on carpogenic germination of S. sclerotiorum sclerotia was not observed, and sclerotial mortality was indicated to be predominantly due to the antagonistic effect of C. minitans. Under glasshouse conditions, applying C. minitans in the combined treatment with biofumigation resulted in significant suppression of apothecial development and Sclerotinia disease of lettuce. Applying Perlka® also provided a significant reduction of apothecial development and effective control of Sclerotinia disease of lettuce. Biofumigation alone failed to provide substantial control of either apothecial development or Sclerotinia disease of lettuce. The biological control of S. sclerotiorum using C. minitans can be recommended as a vital component in an integrated pest management (IPM) programme. Also, Perlka® provided effective control of S. sclerotiorum and proved to be a compatible component in an IPM approach. It is recommended to allow at least 6-8 weeks between C. minitans treatment and planting biofumigant plants because of the greater sensitivity of C. minitans to the volatile bioactive compounds and to allow the antagonist to colonise the sclerotia. Leaving 7-14 days between the Perlka® pretreatment and sowing biofumigant seeds (Brassica spp.) is also recommended to avoid the possible phototoxic effect of Perlka®. The results of this study will aid in the development of an IPM strategy to achieve sustainable control of S. sclerotiorum. Field experiments testing the selected treatments or treatment combinations are needed utilizing different crops and seasons within New Zealand.
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