Farmer's Weekly (South Africa)
Mildew: a major threat to lettuce
In general, the most common lettuce disease that growers have to contend with is downy mildew. This appears as a whitish-grey fungus under the leaves, and yellowish blotches on the corresponding areas on the upper surfaces.
The disease starts on the lower, older leaves of the plant, as these are more susceptible. In addition, being on or just above the ground, they are in a moist micro-climate that is ideal for development of the disease.
In many cases, older leaves become infected when the plant is close to maturity. This usually causes little damage, and as long as the wrapper leaves are not infected, there is no need to spray.
When downy mildew is very severe, it can become systemic
Low air temperature and high humidity favour development of downy mildew. The pathogen also requires free water on the leaves to enter the tissues of the plant. Heavy dew may be all that is necessary, and the longer it is present, the greater the possibility of infection.
Under cool, moist conditions, you should therefore avoid irrigation until the leaves are dry, as this extends the period that free moisture remains on the leaves.
prevention
Disease-resistant varieties of lettuce are available, but there are a number of downy mildew races, and the disease in any case evolves rapidly to overcome plant resistance. For these reasons, controlling downy mildew is always a battle.
Your best weapon is to acquire a feel for the conditions that favour downy mildew, and this comes from observation and experience. When you suspect that the conditions are right for the disease, start spraying before symptoms develop. Use a dithiocarbamate fungicide or copper-containing fungicide, and apply thoroughly to all surfaces. Ideally, you should alternate with a product that has a different mode of action.
Severe downy mildew can become systemic and cause damage in the head of the lettuce. The mildew releases spores in the early morning that can survive for 15 hours, giving it ample opportunity to infect the plant. If free moisture is present, the plant can become infected within three hours. The spores are dispersed by rain, irrigation splash and light wind.
Apply systemic products only when necessary. Ask a pest control representative about the best product for your area.
Powdery mildew
A second type of mildew that can attack lettuce is powdery mildew. Growers sometimes get confused between the two types. Downy mildew attacks
or product information, such as a couple of lines about the farm, the care taken by the farmer, or the nutrient value of the product.
Yet another trick, especially if using a netting bag, is to include a tag with a recipe or two to add value to the product.
My point is that even a modest, everyday product such as cabbage can be elevated to new heights through good product differentiation.
The value of cabbage does not, of course, justify expensive, ‘glamorous’ packaging, as is sometimes used for apples, avocadoes or asparagus. But ordinary fruit and veggies have more potential than we appreciate, as long as the quality is right.
• Michael Cordes is an agricultural journalist, consultant, trainer and former farmer. Email him at farmersweekly@caxton.co.za. Subject line: Market floor.