EPIDEMIOLOGICAL ASPECTS OF LEAF BLIGHT OF CARROT (Daucus carota L.) CAUSED BY Alternaria dauci (KÜHN) GROVES & SKOLKO: SURVIVAL OF Alternaria dauci IN VEGETAL RESIDUES OF CARROT (Daucus carota L.) CROP

Abstract

The spore viability of Alternaria dauci (Kühn) Groves & Skolko in carrot (Daucus carota L.) debris which were kept at different levels of temperature and relative humidity, without light, was studied. The temperature levels tested were 8, 16, 24 and 32°C, and the relative humidity levels 3, 18, 51, 73 and 100%. The treatments were different combinations of these levels. Humidity was the more important factor in preserving the fungus spore ability to germinate and its infectivity. Under conditions of high relative humidity and temperatures of 24 and 32°C, the spores rapidly lost their viability. This did not happen when the relative humidity was equal to, or below, 51%. The survival of fungus mycelium on carrot petioles was studied in non-treated organic soil, at different humidity levels, at three different depths, during a period of 120 days. Soil humidity was the factor of major significance in the persistence of the mycelium, followed by depths and time factors. In petioles kept at depths of 10 and 20 cm in humid soil, the survival was greatly reduced, while survival was markedly higher in those kept at the surface of dry soil. The effect of humidity on the viability of mycelium was studied also under soiless condition. Petioles were maintained in relative humidity controlled chamber, at 24°C. In saturated atmosphere (100% humidity), the mycelium viability was lost in less than 20 days. It was concluded that both, the conidial and mycelial form of Alternaria dauci are quite sensitive to high humidity levels.