Mineral Nutrition of Leafy Lettuce and the Impact on Verticillium Severity

Lettuce production in the US is worth more than US$1 billion annually. Until the mid-1990s lettuce was thought to be resistant to Verticillium wilt, but in 1995 it was first identified in the central coast of California. Some field infections have been so severe the entire field was lost. Adjustments in mineral nutrition can reduce Verticillium wilt severity. Mineral nutrients have marked effects on plant health by providing building blocks for plant growth and for mitigating stress factors such as disease development. Despite these known interactions, studies with N, P and K are typically traditional rate response fertility trials for agronomic analyses rather than finely tuned nutrient studies for biotic stress suppression. Uncontaminated root tissue for disease assessment is also difficult to obtain in field studies. The use of a hydroponic solution offers control of the root and shoot environments to allow detection of subtle differences in disease severity that are lost in field studies. Initial studies were conducted to determine the optimum concentration of N, P and K of dry matter production for the lettuce (var. Salinas).

Increasing concentrations of N, P and K produced significantly more shoot tissue growth for all three minerals. Shoot growth peaked at 80 mg N/L, 4 mg P/L and 10 mg K/L. Expectedly, as N increased in shoot tissues, C declined as plants became more succulent, perhaps making the shoots a more likely target for disease development. This effect was less pronounced for the roots. Overall the C to N ratio decreased (plants becoming more succulent) for both shoots and roots as the available N increased. Phosphorus deficiency limits lettuce yield and will decrease overall plant health, but reports indicate that P does not have a great influence on Verticillium wilt. Potassium is important as a regulator of lettuce growth and increased availability of K to plants corresponds to a decrease in Verticillium wilt rates. Studies will now be conducted to determine the influence of K on the infection levels of Verticillium.

Mitigation of Verticillium wilt in lettuce through mineral nutrition is a promising avenue. The ability of N and K to act through multiple pathways to prevent infection implies a possible role for these elements to limit or prevent Verticillium. Studies with Verticillium added to the hydroponic growth system will begin in 2015.