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.