Surveys of commercial soybean fields, disease nurseries, and trial plots of soybean were conducted throughout eastern Australia between 1979 and 1996, and 694 isolates of Phytophthora sojae were collected and classified into races. Fourteen races, 1, 2, 4, 10, 15, and 25, and eight new races, 46 to 53, were identified, but only races 1, 4, 15, 25, 46, and 53 were found in commercial fields. Races 1 and 15 were the only races found in commercial fields in the soybean-growing areas of Australia up until 1989, with race 1 being the dominant race. Race 4 was found in central New South Wales in 1989 on cultivars with the Rps1a gene, and it is now the dominant race in central and southern New South Wales. Races 46 and 53 have only been found once, in southern New South Wales, and race 25 was identified in the same region in 1994 on a cultivar with the Rps1k gene. Only races 1 and 15 have been found in the northern soybean-growing regions, with the latter dominating, which coincides with the widespread use of cultivars with the Rps2 gene. Changes in the race structure of the P. sojae population from commercial fields in Australia follow the deployment of specific resistance genes.
The incidence and severity of fusarium wilt of cotton in glasshouse trials
increased when levels of plant residue in the soil were increased by the
incorporation of whole cotton plants (6-week-old seedlings dried out for a
further 6 weeks) into the soil. In non-residue-supplemented potting mix,
disease incidence was <50%, but ranged from 74 to >90% in
residue-supplemented potting mix. The disease was significantly affected by
soil microflora and soil type, but not affected by soil pH in the range
4.0–8.5. Although the same amount of inoculum was added to both
autoclaved and untreated soils, the disease was less severe in autoclaved
soils than in untreated soils regardless of soil type. Among the 4 clay soils
investigated, the disease was less severe in a grey sandy clay and a brown
heavy clay than in a dark grey heavy clay. Compared with the soils collected
at the end of the 1995–96 growing season, the disease became more severe
in the soil of plots planted to the most susceptible cotton cultivar Siokra
1-4, but less severe in the soil of plots planted to the less susceptible
cotton cultivar DP90 at the end of the 1996–97 growing season. However,
no significant differences were observed in the soil of plots planted to
cotton cultivar Siokra L22, sorghum, maize or soybean.
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