Mineral Nutrition of Leafy Lettuce and the Impact on Verticillium Severity

The Salinas Valley of California has had a serious problem with Verticillium wilt on lettuce since 1995. There is evidence that the supply of mineral nutrients can influence the severity of Verticillium outbreaks. A greenhouse study is examining the effect of N, P and K nutrition on disease severity using a hydroponic growth system. The purpose of evaluating lettuce nutrition and Verticillium in a hydroponic setting is that nutrient concentrations and growth conditions can be carefully and rigorously controlled to minimize confounding and varying effects. Certified disease-free lettuce seed are sprouted and grown in a hydroponic system containing balanced levels of all elements until the plantlets are 2 in. tall. The hydroponic solutions are then changed and varying concentrations of K, Ca, Mg, Mn, and Al are added according deficient, optimum, and excessive levels. This first phase of the experiment has been to identify the optimal concentrations of N, P, and K in the nutrient solutions for lettuce growth. This recently completed research will now allow us to establish treatments ranging from deficient to toxic nutrient concentrations while examining Verticillium infection on lettuce.

In the upcoming phase, Verticillium inoculum will be added one week following the nutrient solution change at a rate of 20 ml (2,000,000 spores/ml) of V. dahliae. Four weeks following the addition of inoculum, eight plants will be removed from solution and the roots separated from above-ground foliage. Randomly selected roots will be freeze-dried and prepared for quantification by PCR. Since the lettuce plants will be around 6 in. tall, randomly selected plants will be air-dried and the roots and foliage will be analyzed for nutrient content through tissue analysis. Measuring the interaction between varying concentrations of N, P and K (ranging from deficient to toxic) and Verticillium infection will provide valuable guidance on using proper plant nutrition to counteract a serious challenge to lettuce production.