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202206 Fresh Quarterly Issue 17 13 Rimpro
Issue SeventeenJune 2022

Can RIMpro work in South Africa?

Predictive models can help growers to better align their spray programmes with infection risk — but how well do they work in practice? By Engela Duvenage.

Predictive models are only as good as the data available for input. In addition, differences in climate might lead to a model developed in one country not performing as well in another, requiring some form of local calibration.

Since 2018, consultancy firm ProCrop and Stellenbosch University have been evaluating the Dutch RIMpro disease-forecasting model in South Africa, to see whether its predictions hold under local conditions. RIMpro has prediction models for apple scab, codling moth, powdery mildew, and fruit thinning, all of which are important to the South African apple industry.

RIMpro, like most other disease-prediction models, uses weather data, ontogenic resistance of fruit to scab, and parameters of pathogen or pest biology to simulate the development of specific diseases or pests, and to forecast when the most severe impact can be expected. Ontogenic resistance describes increasing resistance to scab as fruit matures.

RIMpro can use either virtual weather data or data from a weather station on a specific farm. ProCrop previously reported that the virtual weather data correlated very well with actual weather data.

A novel feature of RIMpro is the prediction of primary scab infections by using the maturation of ascospores — sexually produced spores — which determines the number of spores to be released during an infection event — as a parameter. This is thought to increase the accuracy of the forecast and provide better guidance on appropriate fungicide applications.

Since 2018, Prof. Adéle McLeod of the Department of Plant Pathology at Stellenbosch University has led research on the extent to which RIMpro apple-scab forecasts are affected by the South African climate, and whether ascospores are the main source of primary infections in spring. Previous research has shown that asexually produced spores surviving on fruit can also cause infections in spring.

Preliminary results suggest that, in addition to leaf litter, leaves that remain on trees during winter may be a reservoir for spores in local orchards.

According to Matthew Addison, crop-protection programme manager at Hortgro Science, this research will contribute to a more integrated disease-management approach that includes stringent management of the sources of infection alongside the use of chemicals. For more about the behaviour and control of apple scab, read our Apple scab 101 article.

Image: Engela Duvenage.

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