Influência da adubação com cálcio e potássio na resistência de eucalipto à Calonectria pteridis

Abstract

Calonectria pteridis is known to attack various hosts, causing diseases in seedlings and adult plants. Among them, leaf spot in eucalyptus is an important disease in the culture, because under ideal conditions to the pathogen, causes severe defoliation. The widely used control method for major crop diseases is the use of resistant genotypes. In order to combine cultural methods as an alternative to disease control in situations where the regional climate contributes to a high susceptibility of the plant, the objective of the experiments was to investigate the influence of calcium and potassium fertilization on the control of C. pteridis in eucalyptus. The experiments consisted of subjecting eucalyptus seedlings to different combinations of Ca+ and K+ doses in nutrient solution. In the first study, a 5 x 5 factorial scheme was used, totaling 25 treatments. The seedlings were inoculated with fungal spore suspension. The following variables were evaluated: severity, defoliation, height, chlorophyll content, area weight and dry root. It was observed in combinations with high K + concentration a high disease severity, but with low defoliation. In treatment with high concentrations of both nutrients, high

severity and high defoliation were observed (82% and 64%, respectively). Non-disease- related defoliation was also found. In this case, defoliation was attributed to the

imbalance of one or both nutrients. In the treatment 6.0 mmol L-1 K × 8.0 mmol L-1 Ca it was observed low severity and defoliation, besides conferring with high chlorophyll content, area and root weight. Among the 25 treatments, three were selected based on these results to conduct the second experiment. Treatments were chosen based on the standard dose of Clark's solution (6 mmol L-1 K × 4mmol L-1 Ca) at one dose with the highest calcium concentration and the other with both high calcium and potassium concentrations. The second experiment aimed to verify the distribution and concentration of Ca and K in the leaf tissue region injured by the pathogen. Leaves were detached at 24, 48 and 72 hours after inoculation (h.a.i.) and evaluated by X-ray Microanalysis (MAX). Calcium and potassium content increased over time in the treatment 6.0 mmol L-1 K × 8.0 mmol L-1 Ca, reaching the highest averages at 72 h.a.i., in symptomatic and asymptomatic tissue, respectively. In the other treatments, at the same time, the content of both nutrients decreased, with higher averages observed at 48 h.a.i.. The high calcium and potassium content at 72 h.a.i. around the lesion indicates persistence in the signaling of fungal defense mechanisms. Both studies have shown that balanced calcium and potassium supply directly influence disease severity, defoliation, morphological variables, and define early and late eucalyptus defense responses to C. pteridis.