Occurrence of Phytophthora macrochlamydospora in New South Wales and its pathogenicity to some legume species
Herbarium material of a Phytophthora species recorded on soybeans in New South Wales during 1985 was re-examined and identified as P. macrochlamydospora Irwin. A range of soybean cultivars and some legumes that occur naturally on the north coast region of New South Wales were tested for their reaction to artificial inoculation with P. macrochlamydospora. On soybeans, variable reactions were recorded indicating resistance in some cultivars. A number of the legumes were susceptible to infection. P. macrochlamydospora appears to be endemic on the north coast of New South Wales but has not been recorded from other areas of New South Wales.
The prevalence and severity of diseases in the coastal soybean crop of New South Wales
Rainfed soybean (Glycine max) crops in the northern coastal region of New South Wales were surveyed from 1985 to 1987 for the prevalence and severity of diseases. Sixteen diseases were recorded over the 3 seasons, but only rust (Phakopsora pachyrhizi), black leaf blight (Arkoola nigra) and bacterial pustule (Xanthomonas campestris pv. glycines) were present each season. Rust was the most significant disease, infecting 50, 20 and 6% of crops in 1985, 1986 and 1987. The severity of rust was related to rainfall and temperature after flowering, with the most severe outbreaks in the higher rainfall area of the lower Richmond Valley. Black leaf blight was also significant and occurred in 39, 10 and 27% of crops in 1985, 1986 and 1987, but in most crops severity was low. The other diseases recorded were downy mildew (Peronospora manschurica), root and stem rot (Phytophthora megasperma f. sp. glycinea), stem canker (Rhizoctonia sp.), pod and stem blight (Phomopsis spp.), anthracnose (Colletotrichum spp.), seedling blight (Rhizoctonia sp.), stem rot (Phytophthora sp.), white mould (Sclerotinia sclerotiorum), flower blight (Botrytis cinerea), leaf spot (Phoma sp.), bacterial blight (Pseudomonas syringae pv. glycinea), mosaic (soybean mosaic virus) and orange bark (not identified). All of these diseases were generally of low prevalence and severity.
Selection of soybean cultivar and sowing date as a strategy for avoidance of rust (Phakopsora pachyrhizi Syd.) losses in coastal New South Wales
Critical outbreaks of rust on soybeans in coastal New South Wales may occur erratically from year to year, and are most likely to cause defoliation in the latter part of the season. Selection of cultivar and sowing date was investigated as a means of reducing rust losses. Seasonal effects on agronomic and phenological performance were determined for five susceptible soybean cultivars (Ruse, Forrest, Bragg, Ransom and Fitzroy), sown early or late in each of three seasons, with fungicide applications to control rust on half the plots. In unsprayed treatments, rust infestation occurred late in two seasons but reduced yield in only one season, and only in the latest maturing cultivar, Fitzroy, for which yield at both sowing dates was reduced by an average of 19.4%. Seed size was smaller in the sprayed plants of all cultivars in the one season when rust was detected. Rusting had no effect on time to pod maturity in any cultivar. In two seasons, cultivar x sowing date yield responses could be related to the severity and timing of moisture stress. Later sowing dates produced yields greater than or equivalent to those of early dates when significant moisture stress occurred before mid-February. Seed size was affected by sowing date but the differences were not consistent with yield response. Averaged over the three seasons, later sowings (late December-early January) led to higher yields in the absence of rust, but late maturing cultivars (later than Bragg) would be required to ensure that mature plant heights were adequate to reduce harvesting losses. The merits and problems of resistant cultivars are discussed and it is concluded that, in an environment where rust and moisture stress occur intermittently, selection of adapted later maturing cultivars and later sowing dates, appropriate to the locality, is the best short term strategy.