Research rundown

A summary of past and present Hortgro-funded Forelle research not covered elsewhere in this issue. Hortgro-funded work on other pear cultivars is not discussed here – consult the research inventory in the PDF for a complete list of pear-related projects. By Anna Mouton.

Dormancy and rest-breaking

Insufficient winter chill leads to protracted bud break in many South African pome-fruit growing areas. Producers manage the problem by spraying rest-breaking agents but do not always achieve consistent results.

Laura Allderman of Hortgro tested rest-breaking agents on Forelle in the laboratory over two seasons. She applied different concentrations to cut shoots from two one-year-old commercial orchards in Elgin and then forced them at different temperature regimes in growth chambers. Data were collected on the number of shoots showing bud break, percentage bud break, and time taken to start and complete bud break.

Allderman tested three concentrations each of oil, hydrogen cyanamide, a combination of oil and hydrogen cyanamide, and thidiazuron in an oil base. The controls were untreated shoots that had received field chill and untreated shoots that had been refrigerated to ensure sufficient chill. Shoots were forced at four different temperatures in the first year and two in the second year.

During the first year, the shoots received sufficient field chill before collection and rest- breaking agents either did not improve or were detrimental to bud break for most treatment-temperature combinations. The exception was improved bud break in shoots forced at 15 ºC from the orchard that received 610 chill units suggesting that higher field chill is needed when bud-break conditions are mild.

Rest-breaking agents increased bud-break percentage but did not condense it in shoots forced at 20 ºC in the second year.

Allderman concluded that rest-breaking agents improved bud break in field-chilled shoots, but the effect varied depending on the type and concentration of the rest-breaking agent, and the forcing temperature. However, the best bud break was seen in untreated refrigerated shoots that had received sufficient chill.

Fruit set and thinning

Alternate bearing and excessive fruit drop due to overly vigorous growth can be a problem in South African Forelle orchards. Prof. Karen Theron of Stellenbosch University tested the effects of gibberellic acid, gibberellins⁴⁺⁷, 6-benzyladenine, aminoethoxyvinylglycine, and prohexadione-calcium on fruit set in Forelle over two seasons in Grabouw. Treatments were applied during bloom to girdled and ungirdled trees.

All the growth regulators improved fruit set, but gibberellic acid and prohexadione-calcium reduced return bloom and aminoethoxyvinylglycine reduced fruit size. Theron concluded that gibberellins⁴⁺⁷ can potentially be used to increase fruit set without decreasing fruit size or return bloom.

Theron also tested the effects of plant growth regulators applied during the cell-division and cell-enlargement stages on fruit size. Trials were conducted over two seasons in a Forelle orchard in Grabouw.

She found that 6-benzyladenine applied 30 and 50 days after full bloom did not improve fruit size but did increase return bloom. Gibberellins⁴⁺⁷ in combination with prohexadione- calcium applied 65 and 80 days after full bloom slightly increased fruit diameter in the first season but reduced return bloom the following season.

Controlling crop load by thinning is necessary to ensure fruit quality and return bloom. Effective early chemical thinning reduces labour-intensive manual thinning and improves fruit size. Dr Xolani Siboza of Hortgro tested different chemical thinning regimes in a Forelle orchard in Wolseley over three seasons.

Siboza conducted trials with 1-naphthylacetic acid, 6-benzyladenine, and metamitron. The following thinning treatments were compared to a manually thinned control:

•  1-naphthylacetic acid at petal drop and 6-benzyladenine 7 days later
•  6-benzyladenine at 8-12-millimetre fruit size
•  Metamitron at 8-12-millimetre fruit size
•  Metamitron plus 6-benzyladenine at 8-12-millimetre fruit size
•  1-naphthylacetic acid plus 6-benzyladenine at 8-12-millimetre fruit size
•  1-naphthylacetic acid at petal drop and 6-benzyladenine 7 days later
•  Metamitron at 4-millimetre fruit size
•  Metamitron at 4- and 12-millimetre fruit size
•  Metamitron at 4-millimetre and 6-benzyladenine at 12-millimetre fruit size.

The results showed that the treatments were ineffective in thinning Forelle. This was in line with earlier research by Theron and Dr Tinashe Chibikwa.

More recently, S-abscisic acid showed promise when applied to Forelle pears at the 8-12- millimetre fruitlet stage. An increase in fruit size was observed in addition to thinning. S- abscisic acid is already registered in South Africa for other applications, which should expedite registration as a thinning agent.

Theron also tested mechanical thinning as a more environmentally friendly and less weather-dependent alternative to chemical thinning. She evaluated a tractor-driven mechanical string thinner during full bloom over two seasons in a palmette-trained Forelle orchard in Grabouw.

Mechanical thinning was compared to manual flower removal, manual thinning, and chemical thinning with 1-naphthylacetic acid and 6-benzyladenine. The mechanical thinner was assessed at four different tractor power-take-off speeds.

The tractor-driven mechanical thinner gave inconsistent results as the orchard proved ill- suited to mechanisation.

Climate adaptation

Agriculture worldwide is feeling the effects of climate change. Dr Stephanie Midgley of Stellenbosch University led a study assessing the impacts of climate change on South African pear production. Midgley has since joined the Western Cape Department of Agriculture in the position of Climate Change and Risk Scientist.

The report, published in 2008, stated that pears are vulnerable to rising temperatures in the already relatively warm Western Cape region. The researchers identified several potential impacts, including reduced winter chill, increased sunburn damage, poor red colour development, and greater drought stress.

In 2021, Midgley and collaborators completed a scientific and practical guide to climate change and pome- and stone-fruit production in South Africa. The guide contains maps illustrating likely immediate and medium-term changes in many climate-related variables, including temperatures, evapotranspiration, chill units, precipitation, and stream flows.

According to the guide, loss of winter chill is the main threat to pome-fruit production in the southwestern coastal regions, whereas high temperatures are the primary challenge in the northwestern elevated regions and the Langkloof. Autumn conditions in all regions will likely become less favourable for red colour development.

Hortgro has also co-funded the development of TerraClim – a climate and terrain tool – by Dr Tara Southey of Stellenbosch University. The tool merges climate data from a network of weather stations with elevation models to provide high-resolution temperature maps. Growers and researchers can access the maps at the online TerraClim platform.

TerraClim allows users to view temperature, terrain, and bioclimatic data – for example, long-term mean monthly temperatures, elevation, slope, daily-positive chill units, and Utah chill units – for specific farms and blocks. This can inform decisions about establishing new and managing existing orchards.

Identifying appropriate cultivars for different regions is essential for coping with climate change. Hortgro is funding long-term adaptability trials of different pome- and stone-fruit cultivars. As of 2022, the trials include pears. Ten different pears with varying chill requirements will be planted at three sites to learn more about their performance under different climatic conditions and to assess which chill models best predict their phenology.

Rootstock evaluation

The BP – Bien Donné pear – series of rootstocks was locally selected and remains popular in South Africa but may be less appropriate than quince rootstocks for high-density pear plantings. Less vigorous and more precocious rootstocks can potentially improve yields and colour in blushed cultivars such as Forelle.

Trials conducted by Dr Nigel Cook two decades ago showed that Forelle on BP 1, BP 3, and Quince A rootstocks produced similar yields up to the fourth leaf in Vyeboom and the Warm and Koue Bokkeveld. The trees were spaced at 4 x 1 metres.

The Agricultural Research Council subsequently evaluated Forelle on eight pear and three quince rootstocks over four seasons at three sites with different soil types and training systems. Not all rootstocks were planted on all sites.

The results suggested that OHxF 217 is the best alternative to BP 1. Cumulative yields on OHxF 40 and OHxF 217 were as good as on BP 1 but OHxF 40 is more vigorous. In general, quince rootstocks did not perform as well as pear rootstocks in this trial, but the quince rootstocks were planted at an inappropriate spacing.

Further evaluation of rootstocks for Forelle under Dr Xolani Siboza of Hortgro started in Wolseley in 2016 with planting on BP 1, OHxF 40, OHxF 217, Quince C51, and Pyriam, as well as BA 29, Quince A, and Quince C51 with Beurre Hardy interstems. Tree spacing is 4.5 x 1.5 metres. These trials are set to run until 2023.

A new project under the management of independent cultivar-evaluation company Provar commenced in 2022. Forelle on various rootstocks will be planted in four main production regions to examine their performance under different environmental conditions.

Phytosanitary treatments

Post-harvest phytosanitary treatments are essential for maintaining market access. Dr Shelley Johnson of Hortgro tested whether CATTS – controlled-atmosphere temperature treatment system – could be used to eliminate phytosanitary pests from Forelle pears.

She exposed fruit to two heating rates – 12 and 24 ºC per hour – combined with either regular or controlled-atmosphere conditions. Fruit quality was assessed after treatment, cold storage, and shelf life. The CATTS treatment proved unsuitable as it caused scalding and shrivelling of the Forelle.

Johnson also investigated ethyl-formate fumigation for use against grain chinch bugs.

Forelle pears and grain chinch bugs were exposed to ethyl formate for 1 hour at three concentrations: 50, 100, and 150 grams per cubic metre. Total mortality of grain chinch bugs was obtained at all concentrations. Internal and external fruit quality was not affected, and a panel could discern no difference in taste between the fumigated pears and the controls.

Johnson is currently working on scaling up ethyl formate for commercial use.