Undergraduate Student Report 2021

The effects of water availability and citric acid on cucurbit powdery mildew.

Hazel Irving

Hazel Irving worked with Dr Liz Beal, Royal Horticultural Society, to investigate the effect of the environment on disease in plants with high susceptibility to powdery mildew.

HI 1.jpgCucurbitaceae is a plant family including many economically valuable crops, including cucumber (Cucumis sativus), and courgette, (Cucurbita pepo subsp. pepo). These crops are grown globally and are important constituents of the diets of many people. Powdery mildew disease (PMD) is an important fungal disease to affect cucurbits, causing significant crop and economic losses.

In many parts of the world the principal method of PMD management is the selection of powdery mildew resistant cultivars and repeated applications of fungicides in most cucurbit crops, however governments are moving away from traditional fungicides. Safer management methods include avoidance of stresses, planting resistant cultivars, improving water management, utilising biocontrol agents and the utilising resistance inducers. ‘Green chemicals’, such as citric acid, as resistance inducers, offer a cheaper, safer and more widely available alternative to traditional fungicides.

Over the Summer I carried out a combined experiment at RHS Wisley, investigating the effects of differing watering regimes as well as the effects of foliar applications of citric acid solutions against powdery mildew infections. Citric acid solution was applied preventatively, as a foliar mist, at concentrations of 2000ppm and 4000ppm and compared against Provanto Fungus Fighter Plus, a home gardener’s fungicide, suitable for ornamentals as well as cucumber, courgette and summer squash. 

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Figure 1: Conidia of Golovinomyces orontii visualised through 40X objective.

The results of my study revealed that overwatered courgettes were twice as likely (50%) as the control (25%) and underwatered group (25%) to develop PMD, whilst courgettes in both the fungicide group and low concentration citric acid treatment group did not develop PMD. 12.5% of courgettes in receipt of a higher concentration of citric acid developed PMD, indicating that a higher concentration of citric acid had a reduced resistance inducing effect. These findings show that citric acid shows promise as a potential resistance inducer for courgettes.

During my RHS Summer placement, I also learned other valuable skills alongside designing and carrying out my experiment, such as the visual identification of powdery mildews on a variety of plants including edibles and perennials, examining powdery mildew structures and identifying species using light microscopy, preparing slides and using stains, molecular identification using PMITS primers and Mcm7 primers, practising good horticulture, inoculation techniques and monitoring for pests and diseases.

The RHS Plant Health Summer Studentship has been an exciting first step into the world of plant health. I am very grateful to have had the opportunity to work at Wisley alongside the plant health team and to the BMS for funding this project. The skills that I have gained over the Summer will stand me in good stead as I continue my studies in plant science and progress into a career in plant health.