No abstract

Selfed and cross-pollinated progenies of Icatu coffee plants and derivatives of Hbrido de Timor, with susceptibility or moderate resistance to coffee rust in the field, were tested with Hemileia vastatrix race II and isolate 2 (Is . 2) in the greenhouse or laboratory . Progenies from plants with a susceptible reaction type in the field (scores 8 and 9) all showed homogeneous levels of susceptibility similar to that of the control cultivars Mundo Novo and Catuaf of Coffea arabica . Variation for incomplete resistance was mainly expressed by low or heterogeneous reaction types and also by longer latency periods . Incomplete resistance to race II of some Icatu and Catimor plants, which is overcome by Is . 2, appeared to depend on a partially dominant gene . The expression of this gene may vary also according to the genetic background and some residual resistance to Is .2 was observed . A few derivatives of Hbrido de Timor were more resistant to Is . 2 than to race II, suggesting the presence of a resistant factor to Is . 2 in this germplasm . Segregation in progenies from other plants with incomplete resistance to race II suggested presence of one or a few genes only, which in homozygous condition or in combination seem to confer near-complete or complete resistance . The results suggest that selection for incomplete resistance, expressed by intermediate reaction types, in Icatu and derivatives of Hbrido de Timor may not provide durable resistance to coffee rust . On the other hand, due to apparent additive gene action, phenotypic selection of plants with a highly resistant reaction type could facilitate the accumulation of several resistance alleles in one genotype, which could provide an efficient barrier against new race formation of the pathogen .