If soil pH is allowed to drop too low or high rates of acidifying fertilizers are applied dropping pH into the low 5's, 
manganese becomes more available to the plant.  Once the plant has accumulated toxic levels symptoms appear.
Veins tend to clear the manganese out to the leaf tissue where it damages leaves.  A pale yellow plant with green 
veins like the picture below are typical of this nutritional disease.  Application of pelleted lime may temporary reduce
symptoms, but a soil test in the fall is needed to access the nature of the problem.  Application of the appropriate
amount of a good agricultural lime should reduce the chance of a reoccurrence.