Epidemiology and Control of Pseudocercospora Fruit and Leaf Spot Disease of Sweet Orange (Citrus sinensis (L) Osbeck)

Abstract

Citrus plants are regarded as the world’s second fruit crop consumed by volume next to banana, providing food nutrients, foreign currency, raw materials for agro-industries and source of employment. The production and productivity of citrus in tropical Africa including Ghana is threatened by a number of diseases. Pseudocercospora leaf and fruit spot PFLS of citrus caused by a fungus P. angolensis is the most destructive disease of citrus in most citrus growing areas. The disease was observed for the first time in Kwaebibirem District of the Eastern Region of Ghana in 2013. A study was conducted on the epidemiology of Pseudocercospora Fruit and Leaf spot (PFLS) disease of citrus in the humid semi-deciduous tropical rainforest of Ghana where citrus is produced to assess the fungistatic potential of some medicinal plant extracts to be used for control of the disease. A Randomized Control Block Design (RCBD) was established to determine the critical infection period of citrus. Selected Late Valencia fruits from 20 trees under natural infection were subjected to 18 different treatments. Some were exposed throughout the season, others covered throughout the season and the rest exposed fortnightly to the local environmental conditions. The roles of some climatic factors such as rainfall, temperature, humidity and aero-mycoflora in the infection and development of the disease were also studied. Data were analysed by ANOVA and LSD statistically using it to perform mean separation test at (P≤0.5%) probability level. Field inspections revealed that fruits and leaves symptoms were surrounded by prominent yellow halos. High premature defoliation and fruit drops were common. Young fruits leaves were more susceptible compared to the mature ones. Field survey data after harvest indicated that incidence of the disease was very high (93.9%) in July compared with remaining months of the year (57.75%). The trend of disease incidence and severity decreased beyond July. July recorded the highest disease incidence and severity (94.9% and 67.75%) respectively. In vitro studies on the effects of ethanolic extracts of five medicinal plants with antifungal properties namely, Zingiber officinale, Allium sativum (L.), Moringa oleifera, Azadirachta indica, and Carica papaya were also carried out with the view to their replacing the use of chemicals (Mancozeb plus Carbendazim). Allium sativum recorded the highest percentage inhibition (90.45%) followed by Zingiber officinale (52.46%), Azardirachta indica (45.99%), Moringa oleifera (37.35%) and Carica papaya (36.67 %). For humanhealth and eco-friendly reasons, the use of botanicals, such as ethanolic extracts of Allium sativum, is recommended for field trials within the periods of June and August in a semi-deciduous area to see if the active compounds in these extracts withstand environmental conditions