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. Areas of the Salinas and Pajaro Valleys cannot grow lettuce because Verticillium levels within the soil are too high and attempts to grow lettuce result in complete crop failure. Verticillium populations within the soil take a minimum of four years without a host to get to a level where lettuce can be replanted, but in some cases lettuce cannot be replanted because soil populations do not decrease because Verticillium can survive on over 300 woody and herbaceous plant species, and over 100 weedy species are alternate hosts for V. dahliae.

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.


Justification

The Salinas Valley of California has had a serious problem with Verticillium wilt on lettuce since 1995. Areas of the Salinas and Pajaro Valleys cannot grow lettuce because Verticillium levels within the soil are too high and attempts to grow lettuce result in complete crop failure. Verticillium populations within the soil take a minimum of four years without a host to get to a level where lettuce can be replanted, but in some cases lettuce cannot be replanted because soil populations do not decrease because Verticillium can survive on over 300 woody and herbaceous plant species, and over 100 weedy species are alternate hosts for V. dahliae.

Control options include fumigant chemicals Methyl Bromide and Chloropicrin, however, both fumigants are being phased out of use because of human health issues and risks to the environment. Mineral nutrition of the plant and condition of the plant rhizospere could keep V. dahliae populations low so that lettuce could be grown without significant losses. Reports have indicated that where acidic soils are limed Verticillium wilt severity usually increases. This exacerbating effect may be because of a reduction in Mn and Al ions as pH increases. Mn and Al ions have been shown to have inhibitory effects on V. dahliae in cotton where data suggests that a pH of 5.5 or below will limit growth and survival of V. dahliae because of the accumulations of Mn and Al ions within the plant under acidic conditions.

Concentrations of Ca and Mg also need to be studied since the soils are most likely acidic due to relatively high rates of N fertilizer and low carbonate irrigation water on very sandy soils with low cation exchange capacity. The apparent correlation of decreased Verticillium severity in acidic soils needs to be studied to identify which mineral, or combination of minerals, results in decreased disease severity. The interaction of K with Verticillium has also been reported for several crops, where adequate K is associated with reduced infection.


Methodology

All studies will be performed in a hydroponic growth system. The purpose of evaluating lettuce nutrition and Verticillium in a hydroponic setting is that the nutrient concentrations and growth conditions can be carefully and rigorously controlled to minimize confounding and varying effects. The chelate-buffered hydroponic solutions to be used in this study will allow strict control over the solution/media in which the lettuce are being grown. Certified disease free lettuce seed will be sprouted and grown in a hydroponic system containing balanced levels of all elements until the plantlets are two inches tall, then the hydroponic solution will be changed and varying levels of K, Ca, Mg, Mn, and Al will be added according deficient, optimum, and excessive levels. Inoculum will be added one week following the solution change at a rate of 20 mL (2 X 10^6 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 six inches tall, randomly selected plants will be air-dried and the roots and foliage will be analyzed for nutrient content through tissue analysis.