Planted forests play a fundamental role in Brazil's economy and in protecting the environment. Due to the edaphoclimatic conditions and the technological level adopted, the Brazilian forestry sector is one of the most competitive in the world.

The occurrence of diseases is a limiting factor for the sector's production. The reduction in forest productivity attributed to plant diseases must be considered from the point of view of quantity (reduction in production in number, weight, volume) and quality (of products, higher cost with reagents, etc.), which generates large losses, (losses in reais) both for producers and for society. It is estimated that plant diseases are responsible for annual damage of 15 to 20%, in some cases up to 100%.

The appearance and development of a disease result from the interaction of three factors: susceptible plant, pathogenic agent and favorable environmental factors, forming the disease triangle. The environment is a relevant component in this interaction, and may even prevent the occurrence of the disease even in the presence of the susceptible host and the pathogen.

In the holistic view of pathogen-host-environment relationships and human action, the focus is on the disease and the main factors related to its development. In the outermost segments, there are important elements (society, economy and ecosystem), which are influenced by the disease. Man is in the intermediate segment, as a modifying agent of the system.

The word epidemiology has a Greek origin, where epi means “about”, demos means “peoples”; and logos means “study”. So, epidemiology would be a science of populations. In plant pathology, the same term is used in a broader sense. Important populations for the epidemiology of plant diseases are those of the host, on the one hand, and the pathogen, on the other. The contact of these two populations leads to a third population, that of lesions. The environment interferes with the development of the three populations. Finally, man increasingly interacts with all these populations and, consequently, suffers their effects (rapid growth of lesions).

If all these interactions occur in a coordinated manner, the population of lesions can develop very quickly, and the disease can damage crops. This growth can occur both over time and spatially in the area. When this occurs, we say that there has been an epidemic of the disease in question. Epidemic is defined, then, as an increase of the disease in a population of plants in intensity and/ or extension, that is, an increase in the incidence or severity of the disease and/or an increase in the geographical area occupied by the disease.

Epidemiology has an academic aspect in which one seeks to understand the behavior of the disease in time and space, identifying, for example, what are the favorable and unfavorable times for the occurrence of a certain disease, what is the origin of a certain disease and how it is disseminated in the area, among other information. From that point on, we have the applied side, based on problem solving, in which the information obtained by the academic side will be used to optimize the management of these diseases.

As an example of famous epidemics in the forestry sector, we have the rubber tree leaf blight (Pseudocercospora ulei). This disease was described at the beginning of the 20th century on leaves collected from native rubber trees around Belém in Pará. Symptoms were few, causing no defoliation or other damage to the plants, as susceptible rubber trees naturally grow at low density in forests, 3 to 4 trees per hectare.

However, the devastating potential of this disease was detected in the first attempts to domesticate the species and establish commercial plantations in the Guianas and Brazil. In Brazil, the plantations of the Ford Motor Company became very famous, not for the rubber produced, but for the leaf blight epidemics , which decimated the company's plantations in Fordlândia and Belterra , in 1927 and 1940, respectively.

Today, management measures are available to ensure a minimum risk of epidemics for the rubber tree crop in several regions of Brazil. Among these measures is the planting of resistant plants associated with evasion (choice of areas favorable to the cultivation of rubber trees and unfavorable to the pathogen). These actions reduced the damage caused by this disease and, consequently, favored large-scale production in Brazil and in other countries that aim at the self-sufficient production of rubber, a strategic raw material.

Maps of risk or favorable zones, coupled to simulation models, can be useful to indicate geographic areas or even times of the year that are more favorable to the occurrence of epidemics. For eucalyptus rust in Brazil, we carried out studies based on maps of average temperature and duration of leaf wetness in hours, in order to understand the temporal distribution of the disease.

The season of the year with the highest favorable area was autumn, with 92.90% of areas with a climate favorability index greater than 70%; the seasons summer, spring and winter presented, respectively, 90.1%; 75.1% and 71.3% of areas with a climate favorable index greater than 70%. As for the spatial distribution of eucalyptus rust in Brazil, the North, Midwest and Southeast regions had large areas with a climate favorable index above 70%, throughout the year.

In the Northeast, there is a larger area with a favorable climate index below 60%, mainly corresponding to the northeastern sertão. The South region, in the autumn and winter periods, has a large part of its territory with a climate favorable index of 0 to 60%. The main producing states had areas with a climate favorable index above 70% throughout the year. With the increase in planted areas, consisting mainly of homogeneous plantations and with restriction of the genetic base, the occurrence of epidemics is a present reality in the daily life of the forestry sector.

In this way, knowledge of disease epidemics in forest species is of paramount importance so that phytosanitary management is carried out in a rational way, using different control methods (physical, cultural, biological, genetic and chemical), with the aim of reducing the intensity of the disease in the field, avoid damages and losses and preserve the environment.

For this, we have as a basis a plan to prevent and/or reduce the disease in the area, based on the correct diagnosis and monitoring the occurrence of diseases by monitoring, based on periodic sampling. Based on this monitoring, decisions are made and the ideal control method to be used is chosen. We make ourselves available for the development of research in partnerships, in order to contribute to the understanding of disease epidemics in forest species, in order to propose solutions for optimizing the management of these diseases in the forestry sector.