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Breeding for root rot resistance

Breeding for root rot resistance is a major objective of breeding programmes at the James Hutton Institute and elsewhere. Plant breeding however is a long slow process which involves crossing parents, known from breeders experience to be good candidates for the traits desired by end users. Rubus breeding however is not straight forward, being hampered by several genetic problems including polyploidy, pollen incompatibility and poor seedling germination. The highly heterozygous (genetically variable) nature of the germplasm requires evaluation of large seedling populations.

Breeding is based on a generation by generation improvement in breeding stock through selection and inter-mating of individuals showing promise of producing superior progeny. This translates to a strategy of crossing the best parents (best with the best) and evaluating a large number of the seedlings produced. Seedling numbers in excess of 12,000 plants are evaluated at an early stage in the glasshouse for characteristics that are easy to screen for example, spines and aphid resistance and planted in a seedling plot for further evaluation.

Five years later after general field evaluation the breeder is ready to select those seedlings with potential for further examination by which time the numbers have dropped dramatically to around 100 plants. These individual seedlings are propagated by root to produce five or more plants of each for further evaluation in five plant plots for a further two years. Once selections have been made from the five plant plots, numbers are usually down to single figures. At this stage they may be propagated and put into farm trials and also put into disease plots to assess their resistance, a procedure which in itself can take over four years. After this length of time few if any seedlings will contain all the traits necessary for a new variety (Graham and Jennings 2008).

This procedure is extremely time-consuming and costly in terms of field and glasshouse resources, breeder’s time and labour requirements. Breeding methods used in raspberry have changed very little over the last 40 years or so. What is required are methods for more accurately predicting at an early stage, what characteristics the seedlings have and especially those characteristics that are time consuming to determine before any screening and confirmation takes place. With Marker assisted breeding the time taken can be greatly reduced to approximately four years as can be compared in Figure 1, below.

Figure 1: Institute Raspberry Breeding Programme

Figure 1: Institute Raspberry Breeding Programme

Some novel germplasm has made its way into commercial cultivars. However, with the narrowing genetic base (Graham and McNicol 1995) coupled with the increasing demands from consumers, new breeding methods are required to meet demands of the market which dictates more rapid turnover of varieties.

References

Graham, J. and McNicol, R.J. 1995. An examination of the ability of RAPD markers to determine the relationships within and between Rubus species. Theoretical and Applied Genetics 90, 1128 - 1132.

Graham, J. and Jennings, S.N. 2008. Raspberry Breeding. In: Jain, S.M. and Priyadarshan, M. (eds.). Breeding Tree Crops. IBH and Science Publication Inc, Oxford, Breeding Plantation Tree Crops: Tropical Species. ISBN: 978-0-387-71199-7.

Research

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The James Hutton Research Institute is the result of the merger in April 2011 of MLURI and SCRI. This merger formed a new powerhouse for research into food, land use, and climate change.