Evaluation of plum rootstocks for South African Industry: Robertson planting

The main objective is the evaluation of high potential imported as well as local bred plum rootstocks on different soil types and under different climatic conditions to be able to make rootstock recommendations that can complement yield and fruit quality for plum production. This trial is specifically targeted at a semi-arid region with hot dry summers and low humidity and little rainfall during the ripening/harvest period (i.e., the Klein Karoo).

In 2015, eight rootstocks were planted in a randomised complete-block trial lay-out, using 10 replications included as part of a commercial orchard in Robertson. The new ARC bred rootstock selections OSK99-6 and 8B-35-10 were included. These selections were recommended for advanced evaluation by the Rootstock Evaluation Committee. Other rootstocks included in this trial were: GF677, Garnem, Atlas, Guardian, Viking, and Marianna. The rootstocks were evaluated across an eight-year period assessing tree mortality, tree vigour, productivity, yield efficiency, fruit size and quality along with sensory and visual evaluation to provide a consumer aspect.

Atlas, Garnem, Guardian, Marianna, and OSK 99-6 recorded no tree deaths after eight years. Viking, 8B-35-10, and GF 677 showed tree mortality. Garnem, Atlas and GF 677 were the most vigorous trees and a positive correlation (73%) was found between the trunk circumference and yield suggesting that more vigorous rootstocks lead to higher yields. During the evaluation process Atlas, GF 677 and Marianna showed the most impressive trees in the Palmette system. GF 677 was the best performing rootstock in terms of cumulative yield. It had accumulated an estimated 111.7 tons/ha more than the second highest producing rootstock for accumulated yield. GF 677 was followed by Garnem and Atlas which ranked second and third respectively. In field conditions, we observed that Atlas, Garnem and Viking tended to increase fruit weight, thus these rootstocks should be considered when a larger plum size is desired. Based on the laboratory evaluated fruit samples, all rootstocks (except Marianna) produced fruit in the AA size category of the EU size chart for plums. In terms of sensory and visual evaluation, there was no superior rootstock.

Overall, this study showed that GF 677, Atlas and Garnem were the superior rootstocks in Robertson: GF 667 had exceptional yield with average fruit size and paired with strong tree vigour. GF 677 had a concerning tree mortality as nearly a third of trees had died. Atlas displayed excellent tree health with high tree vigour and successfully established itself in the training system. Atlas ranked third for cumulative yield. Atlas also showed improved fruit size in field and laboratory conditions with a tendency for higher TSS thus the fruit of Atlas could be more profitable. Garnem had no tree deaths and ranked second in cumulative yield. Garnem trees were successful in the Palmette system and produced the largest trunk circumferences therefore the trees were high in vigour. The fruit from Garnem were among the largest in field and laboratory conditions.

In line with the main objectives of this trial, local and imported rootstocks were evaluated and GF 677, Garnem and Atlas can be recommended to producers with similar planting conditions. Each of these rootstocks were improvements over Marianna and each had their strengths and weaknesses. GF 677 provided evidence as a high yielding rootstock with impressive tree vigour. Atlas has a durable ability to survive and establish high tree vigour, highly productive with near 50 tons/ha with improved fruit size. Garnem has a good ability to tolerate environmental conditions and produce high levels of yield with over 50 tons/ha.  The trees of Garnem were highly vigorous and fruit was among the largest.

The performance of the certain clonal rootstocks outperformed Marianna in Robertson, the current plum rootstock in South Africa. The study justifies why producers should buy into new clonal rootstocks to increase survival rate, yield, fruit size and fruit quality. The evaluation results of the trial provided realistic expectations of expected yield, fruit size, fruit quality and consumer perspective of each rootstock included in the trial. Therefore, the trial has provided insight and allows producers to make informed decisions.