Grower Solutions Magazine
Lefroy Valley Australia
June 2007
Disorders in Lettuce

Other factors that reduce the uptake of calcium such as high salt concentrations and high humidity can intensify the problem. Good cultural & management practices which reduce rapid and excessive plant growth, limit the incidence of Tipburn. Additional applications of calcium fertilizer are generally ineffective on crisphead lettuce.

Bolting Bolting is the premature elongation of the stem, due to premature initiation of the reproductive (flowering) process. Head formation will be prevented and the plant will not be harvested. Resistance to bolting is inherited, and it is therefore possible to select appropriate varieties for climate zones and seasons. A variety grown in the season and location for which it was bred usually does not bolt. However, if temperatures are higher than usual for that region, bolting can occur.

Downy mildew Cause: Caused by the fungus Bremia lactucae.: Bl. Symptoms: Light green patches develop on leaves, which turn yellow then brown. A fluffy growth can often be seen on the underside of the leaves below the discoloured patches. Source of infection: The primary source of infection is from spores in paddock debris, spores from wild lettuce weeds and from nearby fields. Control: Use resistant varieties. When growing a variety that is susceptible to downy mildew, the strategic use of fungicides is the best option. Both protective and curative (systemic) fungicides are available.
 

Sclerotinia drop Cause: Caused by the soil borne fungi Sclerotinia minor and S. sclerotiorum. Symptoms: The early signs of the disease are wilting and browning of leaves followed by eventual death. Source of infection: Both species produce hard, black reproductive survival structures called sclerotia.  These can be seen on the lower leaves and at the base of lettuce infected by S. minor and higher in the head when infected by S. sclerotiorum. Excess rainfall or irrigation which results in soil moisture saturation predispose lettuce crops to drop. Control: Rotation with non-host crops helps in controlling both types of fungi. Drip irrigation has also been shown to reduce levels of drop.

Big Vein Cause: Big Vein or Mirafiori lettuce virus: MiLV. Symptoms: The lettuce develops leaves with large veins that appear stiff with ruffled margins. Depending on the severity of infection, some hearts will make it through to harvest, whereas other hearts are stunted and not harvestable. Cooler weather and low light levels favour lettuce Big Vein development. Source of infection: The soil- borne fungus, Olpidium brassicae spreads this virus. The fungus itself has no effect on the lettuce crop and can survive in the soil for many years. Control: There is no chemical control available for Big Vein. Irrigation management to reduce water logging will help reduce the likelihood of this disease. Where possible, growers should use varieties that show a useful degree of resistance.

Corky Root Cause: Caused by the corky root bacterium, Rhizomonas suberifaciens. Rs. Symptoms: Early symptoms of corky root are yellow bands on tap and lateral roots of lettuce seedlings. These yellow areas gradually expand, taking on a green brown colour and developing cracks and rough areas on the surface of the root. As disease severity increases, the entire tap root may become brown, severely cracked, and non-functional; the feeder root system will also be reduced and damaged. When the root is severely diseased, aboveground symptoms consist of wilting during warm temperatures, stunting of plants, and general poor and uneven growth. Control: Rotate crops out of lettuce; do not grow lettuce consecutively. Avoid over fertilizing with nitrogen fertilizers. Some corky root resistant cultivars are now available. For corky root infected crops, growers may need to add additional fertilizer and water in order to bring the crop to maturity. High, welldraining beds may sometimes reduce corky root severity.