A review of Florida citrus blight and its association with soil edaphic factors, nutrition andFusarium solani

Blight is an important disease of citrus and is the most serious disease of this crop in Florida, a major citrus producer. The disease affects the xylem of the tree and symptoms are those associated with water stress. Interveinal zinc‐deficiency occurs in young leaves on a branch and eventually spreads until the whole tree is affected. Wilt occurs in areas on the tree. Fruits are usually normal in appearance but reduced in size, and sometimes irregular in shape. Blighted trees also have fibrous dry root rot symptoms and Fusarium solani appears to be associated with those symptoms. In Florida there appears to be a relationship between blight occurrence and shallow soils. Studies indicate that soil moisture stress, unfavourable drainage and poor rooting volume unfavourably affect root growth and development in shallow soils. It is thought that the cause of blight is a complex of factors involving soils and their management and Fusarium solani. Ammonium nitrate predisposes roots to infection by F. solani and it is thought that this might happen in the field when ammonium nitrate fertilizers are commonly used. Ammonium forms of nitrogen are required by the fungus for toxin production. Chemical control methods have not proved very successful. It is suggested that effective control might be achieved by planting tolerant rootstocks in well‐drained soils with timely applications of lower rates of fertilizers. Also irrigation measures that reduce soil moisture stress around the roots should be practised.