Are fungi and viruses the cause of tree decline?

Trees altered by stress factors and subsequently invaded by opportunistic fungi or insects or both may exhibit essentially similar dieback-decline syndromes. In the wheat belt it is likely that clearing and subsequent changes in water tables, fertiliser usage, stocking, salinity, exposure to weather, aging and so on predispose trees to invasion. It is likely that attack by the opportunistic pathogens determines whether these stressed trees survive or succumb. These organisms of secondary action are probably and should be considered as significant components of host-stress-opportunist dieback-decline we are seeing in the lower south-west of Western Australia. All plants actively invest nutrients and energy into protecting themselves from insects and pathogens. As resources decline due to continued stresses such as raised water tables, salinity, and herbicides their defenses will be weakened. Increased herbivory and invasion by pathogens often appear to be the cause of plant disease, when really they are a secondary (albeit final) consequence of stressful conditions