Delaware scientist gets to the heart of watermelon malady

     An example of hollow heart disorder in watermelons. (Image courtesy of University of Delaware)

    An example of hollow heart disorder in watermelons. (Image courtesy of University of Delaware)

    Have you ever cut open a watermelon, only to find stunning psychedelic designs seemingly borrowed from decades past? The patterns can be downright magical: Symmetrical swirls and curlicues amid gaps in the sweet, red flesh.

    To farmers, though, this kind of natural watermelon art—which goes by the name hollow heart—hurts. The fruit is perfectly edible, but a field with too many affected melons can be difficult to sell. Until a few years ago, it wasn’t clear what caused the condition.

    “The explanation you saw — in fact you’ll still see it in some guides — is that it could be overfertilization of nitrogen or a change in the environment,” said Gordon Johnson, a fruit and vegetable specialist with the University of Delaware’s Cooperative Extension. Some growers noticed hollow heart was more common in melons cultivated at the edge of fields. Others pointed to over- or under-watering.

    Johnson decided to investigate, planting seedless watermelons — the preferred type in most supermarkets, and the bulk of Delaware’s crop — at varying distances from seeded, pollinating melons.

    “We saw that as we got further away from the pollen source — in other words, as we were limiting pollen more — we were seeing a higher incidence of hollow heart,” Johnson said.

    Bees do the hard work of pollination, so adding more hives to fields can cut down on the disorder. Especially for the first crops of the season, when problems are often most acute, Johnson said farmers should make sure their melons are getting enough pollen.

    “Maybe in your earliest fields putting in extra bees,” Johnson suggested. “Maybe thinking about bumblebees, which fly at colder temperatures.”

    If pollen transfer isn’t the problem, growers might need to evaluate the quality or quantity of pollen.

    This time of year, though, is still peak watermelon season, so everyone can focus on simply eating summertime’s iconic fruit.

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