Bremia lactucae (downy mildew of lettuce)

Davies (1994) found that fosetyl-Al was successful when applied either as a drench during propagation at the two-three leaf stage, rinsed with water after application, or as a compost-incorporated treatment. Downy mildew was also controlled by compost-incorporated propamocarb hydrochloride and by foliar sprays of metalaxyl + thiram, zineb and thiram. Foliar sprays of zineb metalaxyl + thiram, propamocarb hydrochloride, and thiram and mancozeb also gave control. Meier (1994) reported that a formulated mixture of metalaxyl zineb, applied to the plant bed and then sprayed no later than 2 weeks after planting, was found to protect the crop. More satisfactory in iceberg lettuce (Lactuca sativa), with its longer crop cycle, was plant bed treatment with fosetyl-Al followed by spraying with propamocarb. Metalaxyl-tolerant B. lactucae strains were controlled with two sprays of fosetyl-Al or three of propamocarb. Wicks et al. (1994) found strains of B. lactucae tolerant of metalaxyl in all the lettuce-growing districts of South Australia. Despite this, applications of a formulated mixture of metalaxyl + mancozeb controlled B. lactucae in field experiments, where applications of either metalaxyl or mancozeb alone were ineffective. Protectant schedules of either dimethomorph or phosphonic acid also controlled B. lactucae, but were less effective than the metalaxyl + mancozeb formulation. Mixtures of either dimethomorph or phosphonic acid with mancozeb enabled the time between the sprays to be increased to 14 days without significantly reducing control. Fungicide applications increased marketable yield by 60% compared with that of the unsprayed plants. Drenching seedlings with phosphonic acid controlled B. lactucae for at least 14 days and it was suggested that this method should prevent the spread of downy mildew from plant production areas.

Anon. (1994) reported that cv. Marksman produced a spherical firm head of 16 cm diameter, weighing 1191 g, possessed the genes Dm10 and Dm11 for resistance to B. lactucae isolates SF3 and TV, respectively, and was expected to be resistant to turnip mosaic potyvirus. Anon. (1994) reported cv. 'Diamond' as resistant to downy mildew (B. lactucae) Californian pathotypes I, IIA and III.Scherm and Bruggen (1995b) determined that disease intensity was always lower under drip irrigation than under furrow irrigation. However, the magnitude of the differences in disease was small. It was concluded that on most days in coastal California, mesoclimatic variations outweigh microclimatic modifications that could potentially influence disease development.

Scherm et al. (1995) evaluated fungicide spray advice based on measured or forecast of morning leaf wetness. Based on measurements of morning leaf wetness, the total number of sprays in the seven trials was reduced by 67% relative to the calendar-based schedule, with no difference in disease intensity. Based on forecasts of morning leaf wetness, which were generated using a physical dew simulation model with numerical weather forecasts from the National Meteorological Center as input, ca 90% of the days were correctly classified as days with or without prolonged morning wet period. However, the occurrence of important wet periods due to fog drizzle was not predicted and the forecasts for the exact time of onset and end of leaf wetness were inaccurate. It was concluded that the number of fungicide applications against lettuce downy mildew could be reduced with sprays scheduled according to a morning leaf wetness threshold of 1000 h and that fog drizzle should be included in leaf wetness forecasts for coastal California, USA.