Mycosphaerellaleaf disease reduces growth of plantation-grownEucalyptus globulus

Summary Species of Mycosphaerella cause significant leaf damage and defoliation in young eucalypt plantations in southern Australia and overseas. The effects on growth, however, have not been adequately quantified, although over 310 000 ha of Eucalyptus globulus had been planted in Australia by the end of 2000. In a replicated field experiment (2-y-old E. globulus family trial) exposed to natural infection by species of Mycosphaerella, we periodically sprayed fungicides (benomyl and chlorothalonil) on half of the trees and not the other half. Quantitative assessments of disease (both severity of leaf spots and defoliation) and measurements of tree height and diameter were conducted periodically over the following 17 mo. The fungicides effectively controlled the disease caused by the species of Mycosphaerella present in the trial (M. cryptica and M. nubilosa). Even at low levels of infection by the Mycosphaerella spp. (less than 10% leaf area affected), the unsprayed trees had significantly poorer growth, with height increment reduced by 13% and diameter increment by 4%. Defoliation of unsprayed trees (61 %) was double that of sprayed trees (32%). Severity of leaf spots was positively correlated with defoliation (r = 0.36, P < 0.001), and defoliation was negatively correlated with both height increment (r = -0.37, P < 0.001) and diameter increment (r = -0.29, P < 0.001) — evidence that disease was reducing growth. While spraying controls disease, it is time-consuming and may not be commercially viable. Breeding for resistance to reduce the impact of Mycosphaerella leaf disease may be the most economic control strategy.