Incomplete resistance to coffee leaf rust (Hemileia vastatrix)

Incomplete resistance to coffee leaf rust ( <u>Hemileia vastatrix</u> ) may be of value in obtaining durable resistance, which is of great importance for the perennial coffee crop. Methods were developed to assess incomplete resistance to coffee leaf rust by using illustrated scales ranging from 0 to 9 (Chapter 1). A laboratory screening method, which uses leaf disks, has been standardized. The method is satisfactory for the assessment of complete and incomplete resistance (Chapter 2). Resistance to coffee leaf rust appeared to be affected by light intensity and leaf age (Chapters 3 and 4). In general, coffee leaves were more resistant under low than under high light intensities. The effect of leaf age varied with the coffee genotype. In general, however, old leaves were more susceptible than adult or young leaves.From 1976 to 1980, seven new races of <u>H. vastatrix</u> were found in breeding plots in Campinas (Chapter 5). Four races overcome combinations of known resistance genes in <u>Coffea arabica</u> and three races overcome yet unidentified resistances from <u>C. canephora</u> . Three other rust genotypes were found with intermediate virulence to certain resistance genes. Possibilities for obtaining durable resistance based on major genes are discussed.In Chapters 6, 7 and 8 a characterization is given of incomplete resistance in <u>C. arabica</u> , <u>C. canephora</u> cv. 'Kouillou' and in the 'Icatu' population. Icatu derives from a cross between <u>C. canephora</u> and <u>C. arabica</u> . In <u>C. arabica</u> , the period between the onset of sporulation till leaf abscission (leaf retention period) appeared to be an important determinant of the disease level in the field. Significant variation for latency period and lesion density was observed, but results were fairly inconsistent. Disease level in the field was positively correlated with productivity. In 'Kouillou' large variation for incomplete resistance was found. Genotypes were detected with high resistance expressed by a low lesion density and a quite high reaction type. These genotypes may have value for obtaining durable resistance to coffee leaf rust. Incomplete resistance in Icatu was mainly expressed by heterogeneous reaction types. Components of resistance were related to reaction type. Incomplete resistance of varying degrees was race-specific.In the general discussion the relevance of the findings for coffee breeding and the links to resistance theories are pointed out