Phenotypic expression of resistance to coffee leaf rust and its possible relationship with durability

A review is made on different types of complete and incomplete resistance of coffee against Hemileia vastatrix, as observed through visual observations carried out after inoculation of different host genotypes, including segregating populations and different races of the fungus. Firstly, methods for resistance testing are revised. Incomplete resistance can be effectively evaluated by using 0-9 assessment scales for reaction type (RT) and for lesion density (LD). Important factors that affect the level of resistance are light intensity, leaf age and yield. These influence the disease level in the field and need to be considered to evaluate resistance, or virulence, correctly. Variation for apparently race non-specific incomplete resistance in different coffee populations is analysed. Genetic variation for this type of resistance in C. arabica appears insufficient to protect cultivars under modern cultivation techniques. In C. canephora cv. Kouillou, large genetic variation for "adult leaf resistance" was identified, but may be difficult to transfer into C. arabica. Incomplete race-specific resistance has been identified in many populations. Certain 'major' resistance genes have shown to vary in level of expression depending on test conditions (yield, leaf age, light intensity), gene dosage, and probably also on the genetic background. Incomplete race-specific resistance is most pronounced in young leaves, with older leaves becoming susceptible more or less rapidly. Race-specific genes conferring incomplete resistance do not seem to be very useful in obtaining durable resistance, certainly when utilized alone. Cultivated C. canephora varieties of the Congolese type (called also "Robusta") in West-Africa and in Indonesia are apparently effectively and durably protected by a combined action of several major and minor resistance genes (gene pyramid). This contrasts with rapid breakdown of major genes for resistance in C. arabica, used either singly or in combination, and with the appearance of races with complex virulence with regard to Catimor in India and Indonesia. Genetic analyses of segregating populations have shown a certain degree of additive gene action among resistance genes from C. canephora and C. liberica (SH3). Genotypes with extremely low RT might contain a higher number of resistance alleles or a higher gene dose than genotypes with higher RT segregating in the same population. This feature could be used to try to accumulate many resistance genes and alleles into one genotype, aiming at obtaining complex and durable resistance against the most virulent races known. Besides the continuous need to follow up the appearance of new races in breeding trials and in cultivated resistant varieties, more studies on the bases of durable resistance displayed by C. canephora cultivars in Africa, Brazil, India and Indonesia would be desirable to enlarge our insight in durable resistance to coffee leaf rust present in this species. International collaboration is required to select more host differentials in interspecific hybrid populations and in diploid species, especially in C. canephora.

Bibliographic Details

Main Author:	Eskes, Albertus
Format:	book_section biblioteca
Language:	eng
Published:	Universidade Federal de Viçosa
Subjects:	F30 - Génétique et amélioration des plantes, H20 - Maladies des plantes, Coffea, Hemileia vastatrix, maladie fongique, résistance aux maladies, pouvoir pathogène, durabilité, sélection, http://aims.fao.org/aos/agrovoc/c_1720, http://aims.fao.org/aos/agrovoc/c_32323, http://aims.fao.org/aos/agrovoc/c_11042, http://aims.fao.org/aos/agrovoc/c_2328, http://aims.fao.org/aos/agrovoc/c_5629, http://aims.fao.org/aos/agrovoc/c_33560, http://aims.fao.org/aos/agrovoc/c_6951,
Online Access:	http://agritrop.cirad.fr/528582/
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