A laboratory procedure for determining relative levels of field resistance in soybeans to Phytophthora megasperma f.sp. Glycinea

Abstract

A reliable laboratory assay is described which allows rapid determination of relative levels of field resistance in soybeans to Phytophthora megasperma fsp, glycinea. The expression of field resistance depended on both the inoculum level and the age of seedlings at time of saturation of the potting mix with water. At inoculum levels in the range 0.018-0.576 g dry weight of mycelium per kg dry weight of potting mix, ready differentiation in the resistance levels of the cultivars Davis (field resistant) and Semstar (field susceptible) could be made after 7 days of continuous saturation of the mix, which commenced 3 days after sowing. When inoculated by the conventional hypocotyl puncture technique, both cultivars gave susceptible reactions. The seedling assay was used to screen a range of soybean cultivars with known levels of field resistance to the fungus. Disease assessments made in the laboratory assay were similar to those expected under natural conditions of infection in the field. Possible use of the method in future soybean breeding programs is discussed.

Influence of pyroxyfur seed treatment, inoculum density, and low level cultivar resistance onPhytophthora megaspermaf. sp.glycinearot of soybean

“…With Altona the reductions were smaller and occurred at higher inoculum concentrations. Such results are in accord with previous reports that plant growth is an excellent indicator of the sensitivity of soybean cultivars to Pmg (Meyer & Sinclair 1972, Jiminez & Lockwood 1980, Irwin & Langdon 1982. All rates of fungicide treatment increased plant growth to a similar extent but generally the increases were small.…”

Phytophthora macrochlamydospora, a New Species from Australia

“…Hampton and cv. Semstar, respectively, following hypocotyl inoculation in the glasshouse using methods outlined by Irwin and Langdon (1982). The symptoms were identical to those obtained following hypocotyl inoculation with P. megasperma f.sp.…”

“…However, P. megasperma f.sp. glycinea has been isolated frequently from soybean in Eastern Australia (Irwin and Langdon, 1982;Ryley, 1988) and can be easily distinguished from P. macrochlamydospora on the basis of its not producing chlamydospores, its non-papillate sporangia and its ready production of sex organs in V-8 me-dium. Also, within the species of Phytophthora that produce semi-papillate sporangia, only P. colocasiae Raciborski and P. melonis Katsura produce chlamydospores (Newhook et al, 1978).…”

Changes in the Racial Composition ofPhytophthora sojaein Australia Between 1979 and 1996

“…Assessments of the resistance of commercial cultivars to P. sojae revealed that most were susceptible, with only the cultivars Davis and Hill having acceptable levels of field resistance. The high levels of field resistance of these two cultivars to race 1 were confirmed in laboratory root inoculation experiments (12) and in yield loss trials (20). Fortunately, Davis was a cultivar that was well adapted to southern Queensland, and crosses with cv.…”