Contribution to the knowledge of Cercospora leaf spot on olive trees in Uruguay

Doctoral thesis abstract

Abstract

Cercospora leaf spot (CLS) is one of the most important fungal diseases of olive trees in Uruguay, causing severe defoliation and affecting oil quality. This work presents a series of studies aimed at generating local knowledge on this disease, covering its aetiology, epidemiological aspects and control methods. Isolates of Pseudocercospora cladosporioides were collected from olive leaves with CLS symptoms in different olive-growing areas of the country. Identification by phenotypic characteristics and phylogenetic analysis confirmed that this species is the only one causing CLS in Uruguay. Three colony morphotypes were identified on potato dextrose agar, with conidia of mean length between 65.7 and 101.8 µm and width between 4.3 and 5.0 µm. The mean optimum growth temperature was 21.5 °C, with a range of 19.2 to 24.8 °C. In field trials, the annual dynamics of conidial production were determined by monthly leaf sampling, showing two sporulation peaks: late spring to early summer and autumn, with lower production in winter. Also in the field, shoots were exposed sequentially throughout the year to identify times of infection. Infections were detected throughout the year, with a significant decline in early spring. The correlation between meteorological factors and spore production and infections was evaluated, finding that temperatures between 20 and 25°C and a relative humidity of 80-95% were the best correlating variables. Disease development was studied by artificial inoculations in the field, with the first symptoms of CLS appearing after 5 months. Susceptibility of P. cladosporioides to fungicides was also assessed by in vitro assays. Cupric fungicides showed moderate inhibitory efficacy with no significant differences between them. Among the contact fungicides, mancozeb was the most effective, outperforming ziram and captan. The strobilurins (trifloxystrobin, pyraclostrobin and azoxystrobin) and the triazoles (difenoconazole, tebuconazole and propiconazole) were highly effective with no significant differences between them.