zucchini rot at end

Asked July 17, 2019, 9:01 PM EDT

Some of my zucchini grow to about 5 inches long, then turn yellow or brown or mushy at the end. Others on the same bush develop normally and some even become huge (just right for zucchini bread). The plants are growing well, no yellow or dead leaves. I use a soaker hose to water about once a week unless it's very hot. What causes this? Thanks.

Yamhill County Oregon

1 Response

Bummer! It sounds like you've got a case of blossom end rot.

Blossom end rot is the result of calcium not getting to where it needs to go for fruit (zucchini) development in your squash plants. It is characterized by the end part of a fruit, the end opposite the stem, not filling out and developing a brown-black lesion. Not every fruit will be affected. The good news is that it is not a disease that will spread to your other plants!

The calcium deficiency in your plant could be because a.) there is not enough calcium in your soil to feed the plant, b.) there is enough calcium in the soil, but the plants are not receiving enough water to take it up and distribute it to the fruits, or c.) there is too much nitrogen in the soil and the plants are growing quicker than they can move the calcium around.

A. To determine if you have enough calcium in your soil, you can take a soil test. The results will tell you how much calcium is in your soil and should give you a fertilizer recommendation.

Of note, adding calcium to your soil now, with bone meal, oyster shell, lime, etc., will not provide calcium that is immediately available to your plants. This will help prevent next year's crop from having the same problem. That said, many gardens have adequate amounts of calcium, especially if you have been applying compost or fertilizers. So, if you're not keen on taking a soil test, I recommend turning a critical eye to your watering.

B. It is very likely that irrigation is to blame. If you are using a soaker hose, it may be that water is concentrated in one area and is not getting to enough of the roots or it may not be enough water overall. An easy test is to stick your finger in the soil about 2-3 inches down. The soil should feel moist, like a wrung out sponge. If the soil is dry, you need more water! Water plants once or twice a week and provide 1" to 1.5" inches of water. You can gauge how long it takes to provide this much water via your soaker hose by putting a shallow dish, like a tuna can, under the hose and noting how long it takes to fill 1"deep of water. Water your plants for that length of time each week, or half that amount of time twice a week.

If you're interested in learning more, there is a thorough explanation of why lack of water causes calcium deficiency by Michigan State Extension: https://www.canr.msu.edu/news/blossom_end_rot_causes_and_cures_in_garden_vegetables

C. Lastly, it may be the case that you are using high-nitrogen fertilizers on your garden. Nitrogen promotes rapid leaf development. If this is the case, your plants may be taking up nitrogen, putting all their energy into developing leaves, and are growing too quickly to move calcium into the developing fruit.

Best of luck!