In greenhouse experiments at Davis, and field experiments at Five Points, California, Western muskmelon varieties and breeding lines were subjected to excess boron in various amounts. External symptoms were noted, and leaf content (dry-weight basis) of boron was determined. Possible relationships were sought between varietal sensitivity to boron and resistance to powdery mildew and to crown blight in Imperial Valley and at Five Points as determined in earlier experiments.

Varieties and breeding lines did not differ in ability to absorb and accumulate boron; none was able to limit the amount absorbed. Some breeding lines, however, were more tolerant, others less so, than standard varieties.

Selection for resistance to crown blight in Imperial Valley and to powdery mildew did not influence boron response in the greenhouse tests nor resistance to crown blight in the field tests.

Crown blight severity at Five Points was closely correlated with an unknown environmental factor that produced intercalary necrotic flecks (freckles) in leaves. Large phenotypic effects of this factor masked boron-toxicity effects, and prevented their measurement.

Long-established commercial varieties, such as ‘PMR 45’ and ‘Honeydew,’ are surprisingly tolerant, in the field, to high boron concentration in their leaves and in the soil, and to factors that cause freckles in some muskmelons. Such tolerance may explain, in part, their comparatively wide range of adaptation.

The data suggest that simultaneous or alternate selection in different environments would be more effective than selection in a single environment, in breeding programs designed to produce a widely adapted commercial variety.