This review is motivated by (i) the magnitude of the threat to world food security and diversity of natural vegetation posed by viral and bacterial pathogens of plants at a time of accelerating climate change; and (ii) the inadequate attention given to this subject by earlier reviews on climate change and plant disease. It starts by providing background information on current climate change predictions, the increasing worldwide importance of viral and bacterial diseases, critical features of their pathosystems and the general influence of environmental factors upon them. It then develops comprehensive climatic and biological frameworks and uses them to determine the likely influences of direct and indirect climate change parameters on the many different host, vector and pathogen parameters that represent the diversity of viral and bacterial pathosystems. This approach proved a powerful way to identify the relevant international research data available and many information gaps where research is needed in the future. The analysis suggested that climate change is likely to modify many critical viral and bacterial epidemic components in different ways, often resulting in epidemic enhancement but sometimes having the opposite effect, depending on the type of pathosystem and circumstances. With vector-borne pathosystems and new encounter scenarios, the complication of having to consider the effects climate change parameters on diverse types of vectors and the emergence of previously unknown pathogens added important additional variables. The increasing difficulties in controlling damaging plant viral and bacterial epidemics predicted to arise from future climate instability warrants considerable research effort to safeguard world food security and biodiversity.
The effects of soil temperature (10, 15, 20 and 25�C) and moisture (45% water holding capacity (WHC), 65% WHC, and flooding) on the pathogenicity of five fungi, both alone and in combinations, were investigated to determine the involvement of these fungi in a severe root rot disorder of subterranean clover in Western Australia. Fusarium avenaceum, Pythium irregulare, and Rhizoctonia solani were highly pathogenic while Fusarium oxysporum and Phoma medicaginis, particularly when used singly, were only weakly pathogenic. Compared with individual fungi, fungal combinations increased the severity of root disease and decreased plant survival and plant fresh weight. While the fungi investigated caused root rot over the range of soil temperatures and moisture conditions of this investigation, the most severe root rot occurred at 10�C, with less at 15 and 25�C, and least at 20�C. Temperature had a marked effect on the disease severity and its effect varied with individual fungi and their combinations, in particular, combinations involving P. irregulare (severest root rot at 10 and 15�C). The most severe root rotting, compared with the control, occurred at 65% WHC, with less at 45% WHC, and least under flooding conditions. There was often a significant interaction between temperature and moisture for the various fungi and fungal combinations tested.
The development and spread of blackleg leaf, stem and crown infections in rape plants (Brassica napus cv. Zephyr), caused by Leptosphaeria maculans pycnidiospores, was examined under field conditions for plants artificially inoculated at 3, 5 1/2 and 8 1/2 weeks of age. Spread of leaf, stem and crown infections, via pycnidiospores, occurred for inoculations made on 3- and 5 1/2-week-old plants. Inoculation at three weeks oflage resulted in theigreatest disease spread. There was no leaf infection spread and very little stem and crown infection spread for the inoculation made on the 8 1/2-week-old plants.
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