Gene-for-gene relationships between strawberry and the causal agent of red stele root rot, Phytophthora fragariae var. fragariae

Red stele (red core) root rot is the major soil-borne disease of strawberries (Fragaria spp.) in many areas with cool, moist soil conditions. It is caused by the soil-borne fungus Phytophthora fragariae var. fragariae. Red stele is a quarantine disease in Europe with a zero tolerance for commercial stock plants. Any lot of such plants with even just a trace of the disease has to be destroyed. The plot on which the plants were harvested has to be abandoned for commercial strawberry propagation forever. Consequently, this disease is not only a menace to growers, but also to nurseries. Red stele has been successfully combatted by resistant cultivars in the USA and Canada. In Western Europe, no cultivars with an effective resistance are available. The Centre for Plant Breeding and Reproduction Research (CPRO-DLO) in The Netherlands initiated the breeding for resistance in 1968 and a number of advanced resistant selections has been produced. Their horticultural value remained behind that of the successful CPRO-DLO cultivar Elsanta, so none of them was as yet released as cultivar. The breeding for resistance has thus far been complicated by a general lack of insight in the genetics of resistance as well as the absence of a reliable resistance test. The research described in this thesis aimed to increase opportunities to create elite cultivars with red stele resistance by overcoming these complications.Tests for resistance . Resistance tests were developed for use under controlled environmental conditions. Their main innovation is their systematic account for incomplete as well as for complete resistance. This is achieved by comparing the level of disease of the tested genotype with that of a universally susceptible reference cultivar. The tested genotype is supposed to possess resistance when it is significantly less diseased than the reference cultivar. This approach thus includes the introduction of a flexible disease threshold to distinguish resistance from susceptibility. An other innovation deals with the assessment of segregation ratios for resistance in inheritance studies. The percentage of genetically resistant descendants is estimated by adjusting the proportion of relatively healthy seedlings for susceptible descendants which escaped from (severe) infection. These two innovations led to the identification of some individual resistance genes, including one for moderate resistance of the cultivar Cambridge Favourite.Genetics of resistance . A genetic model was developed. It explains the resistance of strawberry cultivars to isolates of P.fragariae by the interaction of five resistance and five avirulence factors. This so called gene-for-gene (GFG) model is similar to the one developed by Flor for the flax - Melampsora l ini relationship. Inheritance studies on two of the five resistance factors revealed that each of them was based on a single gene, which were designated Rpf1 and Rpf2. This study thus showed that race specific resistance in strawberry for P . fragariae is single-gene based. The model is the first GFG-model for a soil-borne fungus, while its genes are the first identified resistance genes in strawberry.The establishment of a gene-for-gene relationship is a major step in the elucidation of the genetics of resistance to P . fragariae. The proposed GFG-model can improve the efficiency of breeding programs since it allows the combining of consciously chosen resistance genes into cultivars. The model makes it also possible to develop a universally applicable differential series of strawberry genotypes, which series is essential for the identification of fungal races.The tests for resistance and the genetic model as proposed in this thesis can be a positive contribution in the breeding of red stele resistant cultivars, due to which they are fully incorporated into the strawberry breeding program at CPRO-DLO. The results of this research can thereby ultimately lead to a more reliable, paying, and environmentally acceptable culture of strawberry in red stele infested areas.

Bibliographic Details

Main Author:	van de Weg, W.E.
Other Authors:	Parlevliet, J.E.
Format:	Doctoral thesis biblioteca
Language:	English
Published:	Landbouwuniversiteit Wageningen
Subjects:	disease resistance, fragaria, pest resistance, phytophthora, plant breeding, plant pathogenic fungi, strawberries, aardbeien, plaagresistentie, plantenveredeling, plantenziekteverwekkende schimmels, ziekteresistentie,
Online Access:	https://research.wur.nl/en/publications/gene-for-gene-relationships-between-strawberry-and-the-causal-age
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