Management of cercosporiosis, an extremely aggressive disease in coffee crops

The coffee tree stands out on the national scene, being the Brazil the largest producer and exporter in the world, with planted area (species Arabica e Conilon) of 2.282.619 hectares, contributing to both the social and economic scenario of several Brazilian municipalities. Despite this importance, the coffee tree suffers numerous losses, caused mainly by diseases, pests, water deficit and nutritional deficiencies. Among the diseases present in coffee farming is cercosporiosis, caused by the fungus Cercospora coffeicola, one of the main and oldest known diseases of coffee plants. It can occur both in the nursery and in the field.

In the seedling nursery, the disease is favored by daily watering, excess K fertilization and lack of Ca, due to à absence of liming on the substrate. The seedlings show intense defoliation, becoming stunted and unsuitable for planting. On the leaves, the symptoms are circular spots of light to dark brown color, with a grayish-white center, almost always surrounded by a yellowish halo. 

In addition to the symptoms on the leaves, in production plants the disease can occur on the fruits, the symptoms of which are small, necrotic and depressed spots, brown to black in color. When the disease occurs in the fruits, it causes premature fall, maturation is accelerated, the number of dead grains increases and the pulp adheres to the parchment, making pulping difficult and causing losses to the farmer, in addition to reducing the quality of the drink.

The intensity of the symptoms of the disease, as well as the physiological variables of the plant, it can be influenced by factors such as temperature, leaf wetness, light intensity, offer of water, soil fertility and nutritional imbalance. Mineral nutrients exert specific functions in plant metabolism, thus affecting their growth and production. Furthermore, mineral nutrition can promote morphological or chemical changes that increase or reduce plant resistance. to diseases, such as, changes in the walls of epidermal cells, making them thicker, with greater accumulation of cellulose and lignin and even more resistant, as well as changing the chemical composition and synthesis of phenolic compounds. To express resistance, it is necessary not only to have material or seeds from resistant varieties, but also to provide environmental conditions necessary for the formation of barriers against the pathogen, for example, water and nutrients in balance.

Often, control of cercosporiosis is based on the use of protective and systemic fungicides, however, an alternative practice or one capable of maximizing resistance, reducing environmental impact and increasing financial sustainability is to manage mineral nutrition to constitute and increase barriers of resistance to the disease. When nutrition is balanced, lignified epidermal cells with a thick cuticle, are formed, serving as a physical barrier, preventing the penetration of hyphae. There is better control over the permeability of the cytoplasmic membrane, preventing the exit of sugars and amino acids into the intercellular spaces, capable of nourishing pathogens. In addition to the formation of phenolic compounds, with distinct fungistatic properties, the benefits of balanced mineral nutrition are many. Therefore, mineral nutrients, macro and micronutrients, have a strong influence on the incidence and severity of diseases and this influence depends on the functions that each element plays in the metabolism and natural defense mechanisms of plants. In nurseries, the use of substrates poor in organic matter, with nutrient imbalance, may predispose seedlings to blind spot. Low doses of nitrogen and high doses of potassium, without the presence of calcium, increase the severity of blind spot and consequently greater defoliation of the coffee tree. In other words, knowledge of the effects of mineral nutrients on the susceptibility of coffee plants to cercosporiosis can help in the development of management strategies.

Nitrogen (N) is the most abundant mineral nutrient in plant dry matter and is responsible for promoting vigorous plant growth, being essential for the production of amino acids, proteins, growth hormones, phytoalexins and phenols. In deficiency conditions, it can make the plant weak, leading to slower growth. However, its use in excessive doses, results in the production of young and succulent tissues, in addition to prolonging the vegetative stage and/, ou, delay plant maturity. Therefore, excessive or deficient doses create conditions favorable to pathogen attack. In the case of coffee leaf spot, increasing N doses up to the recommended maximum, contrary to what occurs with other diseases, reduces its intensity. In addition, the moment of application of the fertilizer, its largest installment, the dose used, the source of N used, the type of soil and the interaction of such factors, can influence the increase or reduction of the disease. One strategy would be to schedule balanced fertilization with sufficient quantity for adequate plant growth, avoiding periods of excess, loss or environmental conditions favorable to the pathogen.

Potassium (K) is one of the most important nutrients for coffee crops, being the second most required element by the crop. Plants deficient in K become more susceptible to disease due to a à metabolic function performed by this element. Potassium activates enzymes, which act in photosynthesis and respiration, assist in the formation of amino acids and sugars, act in cellular absorption, regulate the opening and o closure of the stomata, in addition to being involved in the transport of carbohydrates to the fruits and other organs of the coffee plant, being able to contribute to the plants' resistance to diseases, pests and dry spells. In general, adequate potassium nutrition results in lower disease incidence due to increased resistance to pathogen penetration and colonization. Once the optimal supply of potassium for plant growth is reached, there is no further increase in resistance with increasing doses. Above the equilibrium dose, in work with nutrient solution and in the field, an increase in blind spots was observed, probably due to the competition of K+ with Ca2+ and Mg2+ cations. This competition for the absorption of cations caused nutritional disorder, reduced the thickness of the cuticle and cell wall and favored the penetration of C. coffeicola. Some potassium fertilization management practices, such as the use of cultivars efficient in absorbing potassium, a use of appropriate application methodss, with a greater number of installments, always according to soil and leaf analysis, Ain addition to the adequacy of soil moisture, whether through irrigation or mulching, and also the supply of Ca and Mg through liming, can help control cercosporiosis. Both in the field and in the nursery, it is often common to find excess K without proper liming., EThe latter, essential both to increase pH and to supply Ca and Mg ions, fundamentals in resistance processes and also for photosynthesis and respiration, it can also reduce aluminum toxicity and increase the CEC (t) of the soil.

Calcium (Ca) is an essential constituent of the plant cell wall. The deficiency of Ca leads to a thin cell wall with less rigidity, making the plant vulnerable to the pathogen. Ca is used in the synthesis of new cell walls in the middle lamella as well as in cell division. Work carried out at the Federal University of Lavras showed that with increasing doses de calcium in nutrient solution and in the field, until the ideal and with balance in the soil solution, there was a reduction in the area under the progress curve of the incidence of blind spot. In the field, balanced doses must be applied so that calcium does not compete with potassium and o magnesium, e, also, unavailability of micronutrients due to à excessive liming.

Phosphorus (P) content in coffee plants and soil é relatively low, but excessive doses of phosphate fertilizers are usedas in the holes and in the crop planting furrows. The response of plants to phosphate nutrition in the initial stage of growth and development may be related to the role of P in the synthesis of proteins and enzymatic reactions, as it constitutes proteins necessary for cell division, acts in the process of ionic absorption, energy transfer, in addition to to have a great influence on the development of the root system. In soil, phosphorus can reduce offer of iron, manganese, zinc and calcium, elements that are involved in the plant resistance mechanism to diseases. Therefore, indirectly, excess phosphorus can affect the health of seedlings.

Another element, whose content in the plant is affected by diseases is magnesium (Mg). Plants absorb magnesium in the form of Mg2+ and the main function of this macronutrient is the activation of enzymes, an enzymatic cofactor and also as a component of the chlorophyll molecule. Therefore, magnesium can affect the size, structure and function of chloroplasts, being, therefore, essential in the processes of photosynthesis, respiration and synthesis of organic compounds.

The fungus attack Cercospora coffeicola is related to the mineral nutrition of plants, i.e., weaker plants,, due to lack of nutrition or excess production, they are the biggest targets of the disease. For this reason, it is extremely important that coffee crops are well formed and managed, in addition to being fertilized in accordance with the recommendations of soil and foliar analysis, their pending load, offer of water and following the characteristics of the soil. Most phytosanitary treatment programs for coffee crops have already includedi preventive control of the disease. Consequently, the applications of agricultural pesticides, the environmental impact and increase the sustainability of the crop.

This article was published in issue 184 of Cultivar Grandes Culturas magazine. Click here to read the edition.