Epidemiology of diseases affecting cassava shoot in Alagoa Nova City, Paraíba

The cassava is used in human and animal food, as well as in the industry. In the state of Paraíba, it is commercially explored on the Coast, in the microregion of "Brejo Paraibano" and as subsistence crop in inner regions of the state. In view of the lack of epidemiological studies of this crop in Paraiba State, the aim of this study was to evaluate the severity progress and the incidence of brown leaf spot (Cercosporidium henningsii), anthracnose (Colletotrichum gloeosporioides) and white leaf spot (Phaeoramularia manihotis) under conditions of natural infection in the field over one year at the small farm Geraldo, in the city of Alagoa Nova-PB, "Brejo Paraibano". The data used for the epidemiological studies were recorded on the 18th day of every month, from September 2012 t oAugust 2013. Toassess the incidence, the percentage of plant sex hibiting symptoms was calculated. Disease severity was quantified based on observations of lesions in leaf tissues by using a diagrammatic scale, descriptive scale and the Mackinneyindex. The disease progresscurve data were subjected to Pearson correlation analysis relative to the monthly meteorological data. The area under the disease progresscurve (AUDPC) was also calculated. Of the selected plants, 95%, 17% and 8.5% showed symptoms of brown leaf spot, white leaf spot and anthracnose, respectively. The lesions of these diseases were observed in the order of 13.3%, 4.7% and 3.3% of extension in shoot tissues. The values of incidence and severity of the three diseases gradually increased over the evaluation period, reaching the final results of 61.02, 32.45 and 15.28 for AUDPC. The highest monthlyrates of severity progress were 0.441 for brown leaf spotin September, 0.197 for white leaf spotand 0.192 for anthracnose in November and December. There was a positive correlation between the pluviometric indexand the disease progressand a negative correlation for the decline in the monthly average temperatures for the three diseases.

Spot; Cercosporidium henningsii; severity; AUDPC