Soil treatment options for a replanted Leucospermum orchard

This project aimed to investigate soil treatment options for an old Leucospermum orchard, in order to replant with the same genus. The soil-borne diseases and parasitic nematodes associated with fynbos production make it very difficult to reuse soil without any intervention to rid the soil of these pests.

Ten treatments were included: untreated control (1), pre-plant fumigation with 1,3-dichloropropene + chloropicrin (2), pre-plant fumigation and seasonal potassium phosphonate applications (3), pre-plant fumigation and seasonal Microbial Solutions product applications (4), pre-plant soil solarisation (5), pre-plant soil solarisation followed by seasonal potassium phosphonate applications (6), pre-plant soil solarisation followed by seasonal Microbial Solutions product applications (7), pre-plant furfural soil drench (8), soil drench followed by seasonal potassium phosphonate applications (9), and soil drench followed by seasonal Microbial Solutions product applications (10).

The trial was done in an old commercial Leucospermum ‘Succession’ orchard on a farm in the Bottelary Hills near Stellenbosch. Soil was prepared, an irrigation system was installed, and pre-plant treatments were applied. After planting, the post-plant treatments were applied. The general maintenance, which included irrigation, fertilisation, pruning, and weed control, was done by the producer.

Plant survival was recorded, and plant growth was assessed, using a specially developed score card. Plant volume was determined, and nematode analyses were done in 2017 and 2020. In May 2020, yield was assessed in terms of marketable flowering stems (stems longer than 35cm).

A survival rate of 75% was obtained in the first season after planting (2017). Dead plants were replaced over the following two seasons. In 2017 and 2018, the highest plant survival rate was recorded in the control, 1,3-dichloropropene + chloropicrin soil fumigation and furfural soil drench treatments. The death of established plants was found to be mainly caused by Phytophthora-root rot and Botryosphaeria stem-canker.

Plant growth showed no statistically significant differences between any of the treatments over four seasons, but the soil fumigation and soil drench treatments (with no post-planting treatments) showed the best growth over four seasons. The solarisation plus potassium phosphonate, furfural plus potassium phosphonate and furfural plus biologicals treatments all resulted in lower growth than the control in 2017 and they were still the worst performing treatments in 2020.

Root-knot nematode analysis showed that nematode numbers decreased from 2017 to 2020 in some treatments, while numbers increased in other treatments. Numbers increased markedly in the potassium phosphonate treatments. This could have affected the performance of the plants negatively. It appears that the furfural soil drench treatments had the longest-lasting effect on suppressing nematode numbers.

Plant volume showed no statistical differences between any of the treatments. In May 2020, there were no statistically significant differences between the treatments concerning the number of marketable flowering stems (>35 cm) or the average number of marketable flower spikes per treatment.

The drought and extreme heat experienced during the first two seasons after planting affected the establishment and growth of the plants negatively. Although additional water was obtained, it did not help to ease stress on the plants. The death of established plants was mainly caused by Phytophthora root rot and Botryosphaeria stem-canker.

The statistical layout of the field trial was problematic, due to only two replications/blocks. With the huge variation between the two replications, the differences between treatments were not statistically significant. The results, although not statistically significant, indicate that the best plant survival and plant growth were achieved by soil fumigation with 1,3-dichloropropene + chloropicrin and the application of furfural to the soil before planting. Solarisation of the soil did not seem to be as effective. The addition of potassium phosphonate or biological products to these treatments did not improve their performance. Root-knot nematode analysis showed that nematode numbers increased markedly in the potassium phosphonate treatments from 2017 to 2021. This could have affected the performance of the plants that received this treatment negatively. The real test will be to see how long the plants remain in production, which will indicate if any of the treatments affected the productive life span of the plants.

Plant volume also showed no statistical differences between any of the treatments. However, when walking through the field, it appears that the fumigation treatments are doing better because plants appear visually bigger. Yield did not differ statistically between plants in the different treatments either.

In the future, it is recommended that field trials with pre-plant treatments and post-plant treatments be conducted separately, as the results showed that pre-plant treatments had a bigger influence on the growth of the plants than the post-plant treatments. In trials with post-plant treatments, the pre-plant treatment should be the same for the entire trial.

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