JoAnne Skelly: Blossom end rot in zucchinis

Finally, I’m not writing about pests this week, but I am writing about a different plant problem in my garden. I’ve been waiting to harvest zucchinis for weeks. Everyone knows that zucchinis grow easily and produce so many fruits that the neighbors get tired of receiving them as gifts.

Now I discover I can’t even grow zucchinis to size. They start to develop, get to about two inches to three inches long, brown on the end and wither. I finally remembered that zucchinis suffer from blossom end rot, just as tomatoes do.

It’s a common problem in tomatoes. In tomatoes, the blossom end (the bottom) often browns and gets hard. Zucchinis are a bit different. The fruits grow quickly and then the blossom ends brown and shortly after the entire fruit shrivels. I have often read that when this happens in tomatoes, it is due to a calcium deficiency.

What is actually going on is that calcium movement through the plant is inefficient because the roots cannot pull calcium-containing water up quickly enough to meet the demand of developing tissues. This might be due to lack of soil moisture. Or, extreme heat, high nitrogen fertilizers and roots that are too wet can also interfere with calcium delivery. Without calcium, cells can’t develop normally.

However, not all fruits get blossom end rot because temperatures, soil moisture and humidity vary almost daily. I water twice a day by hand, so I thought the soil was moist. The plant is growing in a container though and its roots may be crowded. Perhaps there isn’t enough soil to hold an adequate amount of water through the heat of the day.

Rarely blossom end rot can be caused by poor pollination. I can’t verify pollination issues. While there are plenty of bumblebees, I haven’t seen many honeybees. And more to the point, I haven’t seen either type inside the protective mesh cages that keep the ground squirrels out.

I had thought and hoped the mesh was big enough to let bees in. This physiological disorder is not a disease. Besides zucchinis and tomatoes, it affects peppers, eggplants, summer squashes and melons.

According to the University of Nebraska Extension, https://extension.unl.edu/statewide/dodge/blossom-end-rot-of-vegetables/, “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.”

They suggest maintaining even moisture throughout the root zone, avoiding over-fertilizing and mulching to keep the soil cooler. “No other treatment is necessary.” For additional information, visit https://byf.unl.edu/blossom-end-rot.

JoAnne Skelly is Associate Professor & Extension Educator Emerita at University of Nevada Cooperative Extension. Email skellyj@unr.edu.