Although exhibiting limited genetic polymorphism, the very large genome of H. vastatrix (c. 797 Mbp) conceals great pathological diversity, with more than 50 physiological races. Gene expression studies have revealed a very precocious activation of signalling pathways and production of putative effectors, suggesting that the plant-fungus dialogue starts as early as at the germ tube stage, and have provided clues for the identification of avr genes.

In Arabica coffee breeding, some of the most used sources of resistance to leaf rust (Hemileia vastatrix) are natural Coffea arabica x canephora hybrids ("Híbrido de Timor"). To decipher the cellular and molecular nature of that resistance, leaves of genotype HDT832/2, were challenged with H. vastatrix race II, and monitored using light microscopy and RT-qPCR expression analysis of genes involved in plant immunity mechanisms (receptor-like kinase, WRKY transcription factor 1, phenylalanine ammonia-lyase, chalcone synthase, 13-lipoxygenase, glycosyltransferase, pathogenesis related PR1b and PR10). These were compared to the nonhost resistance responses of HDT832/2 to the infection by the cowpea rust fungus (Uromyces vignae). H. vastatrix ceased growth more frequently after stomata penetration, forming few haustoria, inducing a hypersensitive-like response, phenol accumulation and haustorium encasement with callose. U. vignae could enter stomata but failed to form haustoria, while inducing hypersensitive-like responses and phenol accumulation. In host and nonhost interactions, activation of genes involved in signalling coincided with the differentiation of appressoria, and cellular responses (hypersensitive-like responses and accumulation of phenolic compounds) were recorded from the full appressorium or penetration hypha stages onwards. Similarly, a gene related to the JA pathway was first activated at the penetration hypha stage for both interactions, while genes related to the SA pathway were only activated in the host interaction, the latter being the single clear difference between host and nonhost interactions. The cellular and molecular resistance responses of HDT832/ 2 to these rust fungi suggest that common immunity components are shared between host and nonhost resistance, which may explain the longer durability of this resistance.

Coffee is one of the most consumed beverages in the world. It is also one of the most globally traded commodities. Coffee leaf rust (CLR), caused by the biotrophic fungus Hemileia vastatrix, is the most important disease affecting Arabica coffee growing worldwide, leading to significant yield losses if no control measures are applied. A deep understanding of the complex mechanisms involved in coffee-H. vastatrix interactions, such as the pathogen variability and the mechanisms governing plant resistance and susceptibility, is required to breed efficiently for durable resistance and design new approaches for crop protection. Here we summarize our current understanding across multiple areas related to pathogen infection, variability and candidate effectors, breeding for disease resistance, and the various components of the coffee immune system, by reviewing a comprehensive body of research on CLR and the advances recently made. We also update information about the defense responses activated by the application of plant resistance inducers, a promising alternative to fungicides in the control of CLR. Moreover, we identify and discuss future directions for further research.

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Colletotrichum kahawae is a specialized plant pathogen of arabica coffee in Africa, able to infect green berries. The economic impact of this pathogen means there is an urgent need to better understand its pathogenic lifestyle, in particular its aggressiveness. In this study, several quantitative traits including disease severity, latent period and incubation period were measured to concomitantly assess the aggressiveness of 26 C. kahawae isolates. The results show that the area under disease progression curve is the most informative variable, particularly when joined together with the index disease intensity 10 days after inoculation and latency period, while the incubation period is not a reliable trait to distinguish aggressiveness levels in C. kahawae. This study also confirms the suitability of hypocotyls and detached green berries to perform C. kahawae aggressiveness assays, revealing that hypocotyls are a more reproducible testing material. Based on isolate profiles, three aggressiveness classes were established (high, moderate and low). A cytological analysis of representative isolates from each class showed that aggressiveness can be related to the development of post‐penetration stages, rather than conidia germination and appressoria differentiation. This study provides, for the first time, the best metrics to evaluate C. kahawae aggressiveness, characterizing the profile of a broad range of isolates, and defining a set of parameters that can be used to classify new isolates. Furthermore, the collected information will contribute to the improvement of coffee breeding programmes, through the selection of tester isolates for prescreening of resistant coffee materials, and offers the opportunity to engage on future genotype–phenotype studies.