Bremia lactucae (downy mildew of lettuce)
Identity
- Preferred Scientific Name
- Bremia lactucae Regel (1843)
- Preferred Common Name
- downy mildew of lettuce
- Other Scientific Names
- Botrytis ganglioniformis Berk. (1846)
- Peronospora gangliformis Berk. ex de Bary (1863)
- Peronospora ganglioniformis (Berk.) Tul. (1854)
- International Common Names
- Englishlettuce downy mildew
- Spanishmildiu de la lechuga
- Frenchblanc de la laituemeunier de la laituemildiou de la laituemildiou de l'artichaut
- Local Common Names
- GermanyFalscher Mehltau: Salat
- EPPO code
- BREMLA (Bremia lactucae)
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Distribution
Host Plants and Other Plants Affected
Symptoms
In lettuce (Lactuca sativa) infection can occur on any part of the plant except the capitulum (Verhoeff, 1960) and the fungus may colonize the plant systemically even as far as the roots (Marlatt et al., 1962). In infected seedlings, the cotyledons stop growing, leading to stunting or death of the plant. Sporulation occurs on both sides of the cotyledon, which becomes chlorotic. As seedlings age, they become less susceptible, systemic infection becomes progressively less and fewer sporangiophores are produced (Dickinson and Crute, 1974). Newly formed true leaves are less susceptible than cotyledons (Dickinson and Crute, 1974). On mature leaves, profuse sporulation on the lower surface is sometimes preceded by a slight chlorosis; in widespread infections of wrapper leaves conidiophores are often the first sign of infection. On the upper surface chlorosis becomes severe and the lesion, bounded by the main veins, is frequently angular in appearance. Browning may occur later, probably due to secondary infections.On globe artichoke (Cynara scolymus) infection of the peduncle and basal bracts allows access to secondary infections (Moreau and Moreau, 1962).
List of Symptoms/Signs
Symptom or sign | Life stages | Sign or diagnosis |
---|---|---|
Plants/Leaves/abnormal colours | ||
Plants/Leaves/fungal growth | ||
Plants/Leaves/necrotic areas | ||
Plants/Stems/discoloration | ||
Plants/Whole plant/discoloration | ||
Plants/Whole plant/plant dead; dieback |
Prevention and Control
Chemical Control
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.
Cultural Control
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.
Disease Forecasting
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.
Impact
Wicks et al. (1994) reported that 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.
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History
Published online: 17 April 2023
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