De bladrandchlorose van Rhododendron catawbiense "Grandiflorum"

Rhododendron catawbiense Grandiflorum, an economically important decorative shrub, suffered from chlorotic leaf symtoms in parts of the Boskoop area.
Two types of symptoms occurred: 1. Yellowish-reddy-green discoloration along the margins and between the primary veins to about-half way along the midrib about July (in transmitted light there were many translucent spots), changing to cadmium-yellow in autumn; 2. Yellow (tissue)-green(veins) marbling of the leaf. Chlorotic leaves had, on average, 44% less chlorophyll than healthy leaves.

Soil acidity was 8 x less for affected than for healthy plants, both soils consisting of clayey peat. The adsorptive complex of bad soils was richer in bases than that of good soils. The higher pH of bad soils resulted from a higher Ca saturation. Above pH 5.1 leaf symptoms arose, abnormal root-hairs formed already at lower pH values. More lime in the soils resulted in higher Ca contents for the plant and lower contents of N, P0 4 , K and Mg. The disease was shown to arise through shortage of N. Despite sufficient acidity too dry a soil also induced the disease. Application of dredgings proved unfavourable (pH about 5.7); a sulphur dressing was favourable (not Al-, Fe-, or Mn-sulphate) for soils with pH>5.1.