Added phosphorus is associated with reduced severity of Mycosphaerella cryptica in Eucalyptus globulus

Abstract

Summary Leaf disease caused by Mycosphaerella cryptica was assessed on the adult foliage of a 6-year-old Eucalyptus globulus fertiliser trial in south-eastern Australia. The trial consisted of 16 treatments of various combinations of nitrogen, phosphorus, potassium and trace elements, originally established to determine the effects of these additions on tree growth. Disease was assessed as the proportion of the whole crown with leaf spot caused by M. cryptica. The trial had previously been measured for growth parameters (Bennett et al. 1997). The results from our study provide evidence that low levels of phosphorus were correlated with increased susceptibility of E. globulus to infection by M. cryptica compared with trees given high levels of phosphorus fertiliser. Treatments with no phosphorus added were significantly more diseased than those with up to 200 kg ha−1 added phosphorus. Neither the addition of nitrogen, potassium nor a combination of trace elements had a significant effect on disease severity. Disease severity was negatively correlated with height and diameter, providing evidence that smaller trees had more disease than taller trees, but not necessarily proving ‘cause and effect’. Various hypotheses are discussed to explain the results obtained from this study.