View Phenotypic Characterization
The term pathogenicity is generally used to define the ability of a microorganism to produce a disease. Studies of Flor (1942) in the plant pathogenic fungus Melampsora lini introduced the concept of pathogenicity as a quantifiable character. Quantify precisely the relative pathogenicity of different isolates of the same microorganism resulted in the use of the term virulence. In fact, Flor called the strains with higher levels of pathogenic as more virulent. The virulence in this sense, can be assessed through those phenotypic characteristics evaluated in controlled experimental conditions, that allow a quantitative comparison between variants of the pathogen. This basic knowledge of the phenotypic variability of the pathogen, however, is only part of the phenomenon, since the attributes that allow this measurement does not necessarily explain the relative success of certain variants of the pathogen in nature. Thus, for quantitative assessment of pathogenicity under field conditions has been used the term aggressiveness (Green and Campbell, 1979) defined as the ability of the pathogen to cause a reaction compatible with the host under natural conditions. Although there are several uses for virulence and aggressiveness in terms of literature, in this investigation, we use these in the above sense.
According to the Gen-Gen theory (Flor, 1974), the virulence of a pathogen is given by the presence of avirulence genes (Avr) and resistance genes (R ) recognized one by one. Given that the phenotype is the result of genotype-environment interaction is important to study how environmental variables influence the virulence of the pathogen and in its aggression (the extent of disease in the field). Among the many environmental variables that can be evaluated for their effect on virulence and aggressiveness of H. vastratix, we start our study evaluating the effect of different Temperatures over the development of the disease.
Effect of temperature on urediniospore germination of Colombian coffee rust isolations
Successful germination of rust spores has been highly related in the literature with its virulence. The germination process occurs in a few hours after the spores come in contact with the epidermis of the coffee leaf, generally dispersed due to the rain. Spore germination is the first critical factor, intrinsic to the pathogen nature, associated with subsequent differences observed in the damage levels caused by the disease. For this reason, germination (usually measured as a percentage) has been used as virulence variable in studies focused on identifying differences between breeds, strains, pathotypes or isolations of fungal pathogens.
From the point of view of geographical distribution of the disease, studies of the effect of abiotic factors on germination and therefore, on the pathogen ability to initiate their infective process become relevant. The temperature within the abiotic factors, is the most consistently related to the potential germination of H. vastatrix urediniospores. Most of studies have found a suitable range for germination between 16 and 28 ° C (with an optimum between 22 and 24 ° C) out of which, there is a significant decreases in the number of germinated spores or even, the process inhibition. Recent changes in environmental condition show the relevance of updating not only the knowledge about of the coffee rust variants in our country but also evaluate the potential effect of deviations of the temperature range (today a reality in the field) on its germination.