Vol 120 (2007): Proceedings of the Florida State Horticultural Society

Effect of water management and soil application of nitrogen fertilizers, petroleum oils, and lime on inoculum production by mycosphaerella citri , the cause of Citrus Greasy Spot

L. W. (Pete) Timmer
University of Florida
Published December 1, 2007
  • Mycosphaerella citri,
  • grapefruit,
  • irrigation,
  • dolomite


Greasy spot, caused by Mycosphaerella citri, produces leaf spots and defoliation of citrus trees, which reduces tree vigor and yield. The fungus produces airborne ascospores from pseudothecia in decomposing leaf litter on the grove floor. Factors affecting production of inoculum on decomposing leaves were evaluated. Pseudothecial formation and ascospore production increased as greasy spot severity on the leaves increased. Applications of urea, dolomite, or increased irrigation frequency on leaf litter reduced inoculum production by 70% to 90%. Of the N fertilizer materials evaluated, urea and ammonium sulfate were very effective in reducing inoculum, whereas nitrate fertilizers were ineffective. The effects of ammonium fertilizer are thought to be due to the toxicity of ammonia gas to the fungus. Soil surface applications of petroleum oils also reduced inoculum, but had to be applied in large volumes of water for maximum effectiveness. These measures may provide practical methods of lowering inoculum levels of M. citri and may reduce fungicide applications to foliage. However, inoculum levels must be reduced greatly to delay symptom development and reduce disease severity.