Protected (glasshouse and tunnel) and field-grown lettuce crops.
The microbe Bremia lactucae belongs to the oomycete group of water moulds. It invades and parasitises cells inside lettuce leaves and subsequently white reproductive structures re-emerge through the stomata, producing vast numbers of asexual conidia that are spread rapidly to uninfected leaves within infected plants, and by water splash and in air currents to uninfected ones.
Sexual resting bodies (oospores) form when two distinct mating forms of B. lactucae come together. These spherical, thick-walled sclerotia can remain dormant in the soil for months and possibly years. They germinate and invade subsequent lettuce crops.
This pathogen is capable of surviving on wild lettuce (Lactuca spp.) plants from where crops may be infected by conidia spread on air currents. The populations of B. lactucae consist of many physiological races that are compatible with the resistance genes present in lettuce cultivars.
These are distinguished solely by their capacities for causing disease on differing cultivars. The naming of races is complicated by the use of varying systems of nomenclature by breeders in Europe, North America and Israel.
The Bremia disease is first seen as light-yellow (chorotic) zones on the upper leaf surfaces, which are mirrored on the under surfaces with areas of white, fluffy, pin-head sporulation that carries the conidia. The yellowing zones on diseased leaves increase rapidly in size on the upper surfaces as the intensity of sporulation intensifies.
Older outer leaves are normally infected first and the pathogen spreads inwards towards the heart of the lettuce head. All stages of lettuce growth, from seedlings to maturing heads, are susceptible. Badly diseased leaves turn papery brown, while the lower ones nearest the soil rot away as they become infected by secondary soft-rotting bacteria and Botrytis cinerea (grey mould).
Lettuce downy mildew disease is encouraged by moist (relative humidity 90-95 per cent), cool (10-22 degsC) conditions. Normally, asexual sporulation takes place at night and the conidia are released the following morning, especially when relative humidity is high. Invasion of healthy wet leaves is rapid (three-to-four hours) and is encouraged when the atmosphere is heavy with moisture and temperatures are optimal.
Excessively wet soil conditions and the overuse of nitrogenous fertilisers will predispose lettuce crops to Bremia infection.
Integrated disease management
Resistant cultivars Plant breeders are constantly developing resistant lettuce cultivars. As these become popular and more widely adopted by growers, the pathogen population segregates variants that are capable of overcoming specific resistance genes. This is the classic "boom and bust" cycle, whereby resistant cultivars are overcome by newly dominant, compatible, physiological races of a pathogen as they become popular and widely grown.
The emergence and expansion of compatible physiological races of B. lactucae may be slowed but not stopped by managing the resistance. This requires using cultivars containing differing spectra of resistance genes, as supplied by seed company representatives, integrated with husbandry measures and fungicide applications.
Husbandry measures Lettuce crops require irrigation for consistent, high-quality yields. Irrigation should be applied carefully such that moisture is not left standing on the foliage overnight, where it will provide a means for the spread of Bremia disease. Maintaining foliage in a dry state at night delays the spread of this pathogen.
Crop spacing should be widened so that there is ample air movement through the crop, especially in vulnerable periods such as spring and autumn. Crop walking that identifies this disease in the early stages permits rapid responses with fungicide applications.
Crop rotation is essential because the pathogen remains as oospores in soil deposited from harvested crops. Consequently, land should be "rested" from lettuce by sequential rotations.
Crops should be sited such that deposits of early-morning dew are avoided because they produce ideal conditions for the germination and invasion phases of the pathogen.
Deliveries of lettuce seedlings should be carefully inspected for signs of incipient infection before acceptance for transplanting. Wherever possible, new plantings should be placed upwind of older, maturing crops. This practice helps to limit the spread of disease from old to new crops.
Fungicides Physiological races of B. lactucae will develop tolerance to fungicides in a similar manner to that for resistant cultivars. Consequently, fungicide programmes should, wherever possible, include molecules with differing modes of action following information provided by the agrochemical companies.
This is becoming ever-more difficult as limitations are placed on the spectrum of chemicals available.
Timing spray applications is critical because the latent period of this fungus is only a few days, so frequent applications are needed to protect new growth. Normally, applications are made every 10-to-14 days.
Currently allowable chemicals
Dimethomorph (off label)
Dimethomorph + mancozeb (off label)
Fenamidone + fosetyl aluminium (off label)
Fluopicolide + propamocarb hydrochloride (off label)
Fosetyl aluminium + propamocarb hydrochloride
Mancozeb (off label)
Mancozeb + metalaxyl M (off label)
Potassium bicarbonate (commodity substance) (off label)
For use on all fresh produce. Growers of vegetable crops must, in advance of use, ensure that a particular commercial agrochemical formulation is legally acceptable for their particular crop/husbandry regime and also accepted by the intended purchaser's crop-quality standards specification, as agreed with the relevant crop technologist.