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202009 Fresh Quarterly Issue 10 04 Forewarned Is Forearmed
Issue TenSeptember 2020

Forewarned is forearmed

New research projects will help growers plan for the future. By Anna Mouton.

We could all use a crystal ball. Imagine if we could know today whether Saturday a month hence will be braai weather? Or whether or not we should spray rest-breaking agents next spring? Or when the next drought will hit? And for how long?

There was a time when producers could expect growing conditions to remain relatively stable throughout their lives. Now nobody knows what next year will bring. Luckily we can turn to scientists for help in taking the horror out of our horoscopes.

Hortgro Pome and Hortgro Stone recognise that producers need answers to three broad questions: What will growing conditions look like in the coming decades? What is the likely impact on tree phenology and performance? How do cultivars differ in phenology?

Read on about new and ongoing research projects that aim to generate this information so that producers can make appropriate planting choices.

Mapping climate change

Prof. Stephanie Midgley of the Department of Horticultural Science at Stellenbosch University has been leading a project to model the impact of climate change on pome- and stone-fruit production areas.

The study focuses on ten regions: Elgin-Grabouw-Vyeboom-Villiersdorp; Riviersonderend; Somerset West; Koue and Warm Bokkeveld; Piketberg; Breede River Valley; Stellenbosch and Berg River; Wolseley-Tulbagh; Klein Karoo; and upper-Langkloof (Avontuur). A request has also been submitted to include the entire Langkloof.

The research team is using historical data and five different global climate models to forecast conditions for the 2030s and 2050s. Their results will be presented as a series of maps. Key parameters under investigation include mean, minimum and maximum temperature; number of days above 35 ┬░Celsius; chill and heat unit accumulation; potential evaporation; risk of dry and wet spells; groundwater recharge; and stream flow.

This information will allow growers to assess the risk of climate change in the main pome- and stone-fruit production areas.

Assessing the impact on trees

The project led by Midgley goes further than modelling factors such as temperature and rainfall. The scientists also aim to paint a picture of the direct impact on trees.

One aspect is predicting sunburn and red colour development. Maps will show which areas are currently at risk of sunburn compared to which areas will be at risk in the 2030s and 2050s, respectively. The models also show the climatic potential for red colour development in March and April in apples in different areas and how that can be expected to change. Growers can use these results to make decisions about mitigation measures such as nets as well as to inform cultivar selection.

Another aspect is figuring out the response of pests and diseases. The project includes forecasts on the number of life cycles per year of codling moth and Oriental fruit moth with recommendations on crop protection measures.

Midgley has another project in the pipeline which specifically investigates the risks associated with changes in early-season temperatures. High temperatures in spring can wreak havoc with flowering and fruit set. Heat waves can also permanently impair the ability of young leaves to photosynthesize. Maps that show the risk of future extreme temperatures and erratic weather changes for various areas are essential for forward planning.

Knowing what to plant

Climate modelling is only useful to growers when the response of different cultivars to various climatic conditions is known. Sunburn risk can be predicted because we know that sunburn browning or necrosis occur after a certain number of hours or days above a critical temperature. The impact of rainfall and evaporation can be assessed because we have a feel for soil moisture and irrigation requirements.

But many other characteristics are uncharted. This is why a new project led by Louisa Blomerus, senior researcher at the Agricultural Research Council, will develop a protocol to determine the effective pollination period of plum cultivars.

Cultivars with a short effective pollination period are more likely to yield poor harvests when conditions for pollination and fruit set are suboptimal. Growers would do well to either plant cultivars with long effective pollination periods or take measures to improve pollination and set in cultivars with short pollination periods. The problem is that the effective pollination period of most plums is not known.

Blomerus and her team will record environmental parameters and the different stages of flower development in the orchard. They will then hand-pollinate blossoms at various times using pollen of known fertility. Fruit set will be monitored to determine which flowers were successfully pollinated and fertilised.

The long-term goal of this study is to assess the effective pollination periods of both existing and new plum cultivars. This assessment should form part of the evaluation process for all new cultivars so that growers can have the full picture of the risks associated with each cultivar. Ideally the sensitivity of effective pollination period to temperature in different cultivars should also be clarified.

We all know that change is inevitable. Knowing what tomorrow holds is essential for the grower who is thinking of planting an orchard today. These new projects will assist producers by discovering how trees are likely to respond to a warming world and by giving the industry a glimpse of what that future looks like.

The images below show the number of days combined in March and in April that climatic criteria are met for red colouring of mid- to late-season blushed and bi-colour apples under historical climatic conditions (figure 1) and into the intermediate future climates of the 2050s (figure 2), with the difference in optimal climatic conditions shown in figure 3. The latter two are derived from the CMIP3 ECH GCM (global climate model).

Images supplied by Stephanie Midgley | Stellenbosch University.




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