The Benefits of Using Silicon Nutrient on Strawberries Including Reducing Epidemics of Strawberry Powdery Mildew (Podosphaera aphanis)

Abstract

Powdery mildew of strawberry plants caused by the fungus Podosphaera aphanis is a significant fungal disease of protected strawberry crops in the UK, causing yield losses between 20 to 70% of crop potential. At 20% losses, this can contribute to an industry volume of 23,100 tonnes, estimated at a market value of £56.8 million. Although growers frequently limit the spread of strawberry powdery mildew by a weekly to fortnightly application of fungicides (April to October), it has become more prominent in recent years. This research aimed to investigate four key areas. Firstly, the effects of the silicon delivered through a fertigation system on the development of strawberry powdery mildew (Podosphaera aphanis) disease levels in different strawberry plant cultivars. Secondly, examine the amounts and pattern of distribution of silicon in leaves, leaf petioles and roots of strawberry plants growing in glasshouse and field experiments. Thirdly, evaluate °Brix levels of silicon-treated strawberry plant fruits and leaf petioles with the untreated control, and lastly, measure growth parameters of strawberry plants in the absence and presence of silicon in a glasshouse hydroponic experiment. In this thesis, silicon fertigation field experiments were set up on a commercial strawberry farm to evaluate the effects of silicon in reducing levels of disease of Podosphaera aphanis throughout the growing season. Results from this study (chapter three) revealed that the application of silicon as a nutrient reduced levels of strawberry powdery mildew in 2016. The lowest disease levels (P<0.05) occurred in Malling Centenary strawberry crops that received silicon twice-a-week with fungicides (AUDPC, 410) and without fungicides (AUDPC, 375) compared with the untreated control (AUDPC, 3423). Results from this experiment showed that the addition of silicon delayed the rise in disease levels by 29 days in the silicon twice-a-week treatment with and without fungicides compared with the untreated control. Disease level assessments carried out in 2017 and 2018 field experiments using the cultivar Amesti showed low levels of disease were only found in the untreated control plot compared to all other treatments in 2016. A silicon deposition experiment was conducted on strawberry plants in a glasshouse (chapter four) in 2017. The results revealed that high amounts of silicon were deposited in the upper and lower cuticle, epidermis, palisade layer, and vein of the leaf (fluorescence intensity,7.2cps) of silicon-treated plants compared with the untreated control (fluorescence intensity, 2.2cps) (P<0.05). In the leaf petiole, more silicon was found in the upper and lower cuticle, epidermis and xylem (fluorescence intensity, 7.7cps) of silicon-treated plants compared with the untreated control (fluorescence intensity, 1.9cps) (P<0.05). In the roots, more silicon was found deposited mainly in the xylem (fluorescence intensity, 11.6cps) of silicon-treated plants compared with the untreated (fluorescence intensity, 1.2cps) (P<0.05). Results from a fertigation field experiment in 2017 also found that more silicon was laid down regularly in the upper and lower cuticle, epidermis and palisade layer of the leaves (fluorescence intensity, 19.4cps) of silicon-treated plants compared with the untreated (fluorescence intensity, 7.9cps) (P<0.05). In the leaf petiole, more silicon was found in the xylem (fluorescence intensity, 16.7cps) of silicon-treated plants compared with the untreated (fluorescence intensity, 10.2cps) (P<0.05). In the roots, silicon was found in the xylem (fluorescence intensity, 8.4cps) of silicon-treated plants compared with the untreated (fluorescence intensity, 6.0cps) (P<0.05). The 2018 deposition field experiment showed that more silicon was laid down in the upper and lower cuticle, epidermis, and palisade layer of the leaves (fluorescence intensity, 4.98cps) of silicon-treated plants compared with the untreated (fluorescence intensity, 2.2cps) (P<0.05). In the leaf petiole, silicon was found mainly in the xylem (fluorescence intensity, 3.72cps) of both silicon-treated and untreated plants (fluorescence intensity, 1.98cps) (P>0.05). In the roots, silicon was also mainly found in the xylem (4.97cps) of silicon-treated and untreated plants (1.76cps) (P>0.05). The hypothesis for chapter four is that the silicon can enhance the passive defence pathway of strawberry plants and is absorbed regularly in this manner. Chapter five assessed strawberry plants grown hydroponically in Hoagland’s solution to measure growth parameters between silicon-treated and untreated plants. This experiment revealed that the plants treated with silicon had significantly increased (P<0.05) numbers of leaves, runners and fruits compared with the untreated control. No significant differences (P>0.05) were found in the experiment’s chlorophyll contents of strawberry leaves. These results suggested that silicon improved the quality of strawberry plants (treated with silicon) in a hydroponic glasshouse experiment by enhancing these growth parameters. This thesis demonstrates that the use of silicon via fertigation not only reduces the severity of strawberry powdery mildew (Podosphaera aphanis) and but has some additional benefits in strawberry production. Therefore, it is recommended that growers incorporate silicon nutrient to manage strawberry production, including strawberry powdery mildew disease control.