Controle alternativo da ramulose do algodoeiro via utilização de óleos fixos e essenciais

Abstract

Colletotrichum gossypii South. var. cephalosporioides Costa is the causal agent of one of the most important cotton diseases, the ramulosis. The disease is characterized by necrotic lesions on leaves and petioles mainly on young leaves. As the disease progresses, shortening of the internódios, apical death and overbudding are also observed. The main way of fungal dissemination is through the seeds, but also able to be externally propagated, in the form of conidia, or internally, by dormant mycelia. The pathogen control has been achieved through the use of healthy seeds, use of resistant cultivars and chemical control. However, some of these control practices not only of limited efficiency, but also cause unwanted side in natural environments. The use of the biological activity of secondary compounds obtained from plant crude extracts and their essential oils can be an effective way to control plant diseases. Preliminary tests toevaluate the fungistatic and fungicidal effect of these products are essential for revealing new alternatives of control. The effect of different oil dosages (0, 500, 1000, 1500, 2000 and 2500 ppm) prepared from leaves of Hibiscus cannabinus, Cymbopogus nardus, Eucalptus citriodora, Pipper marginatum, Malpighia glabra, and Lippia gracillis were evaluated in the inhibition of C. gossypii var. cephalosporioides mycelia growth, conidia germination, and apressorium formation. The oils of Pipper marginatum, Lippia gracillis and Cymbopogus nardus presented the greatest mycelia growth inhibition effects. The oils of citronela, lípia, and eucalipto showed significant control when conidial germination and apressorium formation were analyzed. When the mycelia growth inhibition was evaluated, piper oil presented lower DL50, followed by the lípia oil. The best results in the inhibition of apressorium formation were presented bycitronela and eucalipto with lower DL50 in relation to other oils. The kenaf oil did notpresent any effect on the micelial growth, conidial germination and apressorium formation. In a second experiment it was evaluated the curative and preventive effect of citronela oil on the control of the cotton ramulosis based on epidemiological components. The Area Below the Curve of Disease Progress and Rate of Disease Progress, showed significant differences when compared with the control treatment. However, the parameter Index of Disease Final in the curative treatment did not differ significantly from control treatment. When the variables Area Below the Curve of Disease Progress, Index of Disease Final, and Rate of Disease Progress were analyzed, it was observed that the preventive treatments that used citronela oil and fungicide differed significantly from the control, showing that citronela oil had an excellent preventive effect on the cotton ramulosis under greenhouse conditions.