Question

what could be the reason for melons and pumpkins cracking open while on the plant?.....

Answer 1

 The physiological and environmental factors causing cracking are poorly understood. Although generally considered a physiological disorder caused by fluctuating environmental conditions, current evidence indicates that this disorder also has a genetic, as well as a genotype x environment component. Certain cultivars are more susceptible than others, but wide fluctuations in irrigation, temperature, and nutrition during late fruit maturation stages appear to predispose fruit to cracking.