The rapid development of herbicide resistance in weeds, and environmental imperatives, have forced the consideration of non-chemical tactics such as crop competition for weed management. This review of wheat–weed competition examines the plant traits associated with wheat competitiveness, and the opportunities for plant breeding or manipulating crop agronomy to differentially favour the growth of the crop. Many studies have proven that enhancing crop competitive ability can reduce weed seed production and crop yield loss, although a number of difficulties in conducting this research are identified and suggestions are made for improvement. It remains to be seen whether crop competitiveness will be considered as a priority by farmers and plant breeders. Farmers require precise information on the reliability of agronomic factors such as increased crop seeding rate or choice of variety for enhancing crop competitive ability in different environments. Plant breeders need to know which plant traits to incorporate in varieties to increase competitive ability. A thorough analysis of the benefits and costs of enhancing wheat competitiveness is needed. Competitive wheat crops should be available as part of reliable and economical integrated weed management packages for farmers.

SU MMARYIncreasing crop competitiveness using higher seeding rates is a possible technique for weed management in low input and organic farming systems or when herbicide resistance develops in weeds. A range of wheat seeding rates were sown and resulted in crop densities between 50-400 plants/m 2 (current recommendations are 100-150 plants/m 2 ) in the presence and absence of annual ryegrass (Lolium rigidum Gaud.) in three wheat cultivars at nine experiments in southern Australia. Wheat densities of at least 200 plants/m 2 were required to suppress L. rigidum and to a lesser extent increase crop yield across a wide range of environments (seasonal rainfall between 200-420 mm) and weed densities (50-450 L. rigidum plants/m 2 ). Doubling crop density of all cultivars from 100 to 200 plants/m 2 halved L. rigidum dry weight (averaged over all experiments) from 100 g/m 2 to about 50 g/m 2 . Higher crop densities gave diminishing marginal reductions in weed biomass, while cultivar differences in weed suppression were small. Grain yields ranged from 0 . 5 t/ha to over 5 t/ha depending on site and season. Maximum yields in the weed-free plots (averaged over environments and cultivars) were at 200 crop plants/m 2 , and yield declined only slightly by 4-5 % at densities up to 425 plants/m 2 . In the weedy plots grain yield continued to increase up to the highest density but at a slower rate. The percentage yield loss from weed competition was of a smaller magnitude than the suppression of L. rigidum by wheat. For example, 100 wheat plants/m 2 led to an average 23 % yield loss compared with 17 % at 200 plants/m 2 , and the probability of reduced crop grain size and increased proportion of small seeds was negligible at these densities. Cultivar differences in yield loss from weed competition were small compared with differences due to crop density. Adoption of higher wheat seed rates as part of integrated weed management is now strongly promoted to farmers.

The effect of timing of annual ryegrass (Lolium rigidum) emergence on the uptake and utilisation of N by wheat was investigated in a field trial on a duplex soil at Katanning, Western Australia, and in a glasshouse study in which 15N-fertiliser was applied. Three treatments were used to investigate the effect of timing of annual ryegrass emergence on the uptake and utilisation of N by wheat: simultaneous sowing of wheat and annual ryegrass, sowing of annual ryegrass 1 week before wheat, and sowing of the annual ryegrass 1 week after wheat. A control treatment, consisting of wheat sown alone, was also included. Plant densities during the field trial were 105 and 140 plants/m2 for wheat and annual ryegrass, respectively, whereas in the glasshouse they were 105 plants/m2 for wheat and 155 plants/m2 for annual ryegrass. Fertiliser-N was applied at seeding of wheat at 50 kg N/ha in the field trial and 60 kg N/ha in the glasshouse. The introduction of annual ryegrass into the wheat system reduced the production of biomass and the grain yield of wheat. The earlier the annual ryegrass was introduced into the system, the greater the reduction in the biomass and grain yield of wheat. Poor tillering and slow rates of growth were accountable for the reduction in biomass, whilst the reduction in wheat grain yield was caused by the reductions in ear number, kernels per ear, and kernel size. Grain N content and hence grain protein was also reduced by the introduction of annual ryegrass into the wheat system. Irrespective of the timing of introduction of annual ryegrass, the low N uptake of wheat resulted from a reduction in the uptake of both soil and fertiliser-N. This indicates that annual ryegrass competed with wheat not only for the fertiliser-N that was applied at seeding of wheat, but also for mineralised soil N. The competition for N reduced the total recoveries of fertiliser-N in the wheat plant. Total recoveries of fertiliser-N in the wheat plant suggest that 59% of the fertiliser-N was not taken up by wheat when annual ryegrass was sown 1 week earlier than wheat or at the same time as wheat, whereas only 32% was not taken up by the wheat when annual ryegrass was sown 1 week later than wheat. More competitive wheat genotypes would be those with better efficiency in the uptake of N and its utilisation in maintaining yield and grain protein under infestations of annual ryegrass.

Soft-rot erwinias were isolated from irrigation water, soil, tubers, and stems from spring (1986) and summer (1987) crops of potato at a field in Manjimup, Western Australia. Most of the isolates from all environments in both seasons belonged to Erwinia carotovora pv. carotovora (Ecc), with the exception of those from stems of the summer crop which were predominantly (68%) Erwinia carotovora pv. atroseptica (Eca) and belonged to sero-group SGI. In spring, SGm was recorded from all environments tested, while SGI was found only in stems and tubers. SGVI was recorded from stems (8%) and soil (15%) only in summer and in water (6.4%) only in spring. In spring, SGV, which was the predominant (28.2%) sero-group isolated from stems, was also found in water, soil, and tubers. In summer, of the SGs isolated from stems, SGm occurred in soil and tubers and SGVI in soil. Although SGXXIX was the most common in water in both seasons, it was isolated from stems only in summer. In a pathogenicity test, representative isolates of all serogroups isolated from stems, caused lesions on stems. Significant proportions of isolates from all environment in both seasons belonged to none of the known sero-groups.

Isolates of Erwinia carotovora pv. atroseptica, E. carotovora pv. carotovora and E. chrysanthemi from Western Australia, all produce stem rots in potato when provided with their specific environmental requirements. The pathogenicity of E. chrysanthemi was highly variable. A high inoculum level (3x 107 colony forming units/ml) was required for rapid rotting by E. chrysanthemi pv. zeae.