So your zucchini has these leathery brown areas on their blossom end? What is this and what can a gardener do about it?
Blossom end rot is confusing because it looks like it’s caused by a fungus. It isn’t a fungus. Instead it is a lack of calcium that is the cause. Before you rush out to purchase lime or gypsum to apply to your plants and soil, it helps to have a better understanding of the mechanism that is causing this deficiency.
First, calcium exists in abundance in our soils, so applying even more won’t help. Further, if we tested the plant for calcium we would find adequate amounts for good plant health. Rather it is the plant’s inability to distribute calcium, in this case to the developing vegetables, which is the cause here.
Some things that interfere with a plant’s ability to move calcium include extreme heat, high nitrogen fertilization, and roots that are soggy wet or bone dry. While we can’t fix extreme heat or Mother Nature’s deluge, the good news is that plants eventually figure out the calcium mobility issue on their own. Initial harvests may have blossom end rot, but later ones will be just fine.
Things gardeners can do to help are to irrigate during dry spells to maintain an evenly moist root zone (about 1 inch/week), refrain from over-fertilizing plants and mulch with shredded newspapers to help keep the soil a little cooler. No other treatment is necessary.
Blossom end rot is prevalent on zucchini, summer squash, cucumber, pepper, and tomato. More information about blossom end rot may be found here: https://byf.unl.edu/blossom-end-rot .