Seedlings of six cauliflower cultivars (Brassica oleracea convar. botrytis var. botrytis) were assessed for resistance to a Danish isolate of Peronospora parasitica, under controlled conditions. Resistance, characterized by restricted sporulation and necrotic dark flecks at the inoculation site on the cotyledons, was expressed in the hybrids 9306 F1, 9311 F1, and the open pollinated cultivar Perfection. Testing of the parent lines and F2 generations of the two resistant hybrids suggested that resistance was a dominantly inherited trait controlled by a single gene. Inoculation of the cultivars with seven isolates, from different geographical origins, showed that the resistance was isolate specific. The two hybrid cultivars expressing cotyledon resistance and two hybrids expressing susceptibility were assessed for adult plant resistance under field conditions. The AUDPC (Area Under the Disease Progress Curve), based on disease incidence and severity, revealed significant differences between the cultivars. At harvest, the cultivars exhibited significantly different levels of defoliation and curd attack. The cultivars 9306 F1 and 9311 F1 showed high levels of resistance in all assessments, whereas the two cultivars exhibiting susceptibility at the seedling stage, 9304 F1 and 9305 F1, also exhibited susceptibility through the adult plant stage. Thus, the resistance exhibited under field conditions resembled that identified at the seedling stage under controlled conditions. The results suggest that cotyledon resistance similar to that described could provide resistance throughout the adult plant stage, including curds.

The potential impact of herbicide-tolerant winter oilseed rape (Brassica napus L.) on future herbicide use was investigated with a simulation model. The model uses a sigmoid function to simulate the growth of crops and weeds that compete for a maximum yield potential. Thresholds for weed control are based upon critical levels of weed biomass. The dynamics of the weed population are determined by the ecacy of representative herbicides on individual weed species and by seedbank parameters. Herbicide ecacy is determined by a log-logistic dose±response curve for each species. Simulation of a rotation with winter oilseed rape/wheat/wheat/barley showed contradictory predictions of herbicide use, because herbicide use in a rotation with either glyphosate-or glufosinate-tolerant oilseed rape was not reduced in the amount of kg a.i. ha ±1 compared with a traditional treatment, whereas the treatment frequency (number of standard recommended doses per unit area) decreased. PrerequisitesThe model (Fig. 1) is empirical and based on alleged relationships, comparative studies between transgenic and non-transgenic crops, and practical experience from current crop rotations. The model requires few growth-related parameters and does not take climatic and soil conditions into 96 K. H. Madsen et al.

The effect of temperature and soil available water (AW) on the emergence of Ranunculus repens␣L. was examined in artificial media in growth chambers. Two seed populations, `arable type' and `wild type', were investigated. Moisture levels were 20%, 40%, 60% and 80% AW, and temperatures were 10/5 °C, 15/7 °C, 20/10 °C and 25/15 °C. Time to onset of emergence varied from 11 to 28 days for both seed populations, primarily as a result of mean temperature. Mean thermal time (TT) required for the onset of emergence of both populations was 251 ± 45 TT. Total emergence of the `arable type' R. repens varied from 3% at 10/5 °C and 20% AW to 30% at 25/15 °C and 60% AW. Total emergence of the `wild type' was greater, ranging from 4% at 10/5 °C and 20% AW to 57% at 25/15 °C and 80% AW. Temperature and AW significantly affected total emergence. A logistic response was displayed for the cumulative emergence of both populations. Seedling emergence models based on TT were developed for both populations at each of the four levels of AW.