Squash variety trial for pollination issues and blossom end rot

Golden Glory zucchini in our hoophouse in late May.
Photo Pam Dawling

I wrote about our problems last year with unpollinated squash and our decision to try squash varieties that were less dependent on pollination. So this year we planted some Golden Glory zucchini along with Gentry yellow squash that we had troubles with for several years. The trial is not over, but the early results are promising.

We planted 15 Golden Glory and 25 Gentry and I recorded the number of small rotting squash we removed on 5 dates so far.

Gentry yellow squash in our hoophouse in late May.
Photo Pam Dawling
Date 15 Golden Glory rotted fruit Per plant 25 Gentry rotted fruit Per plant
5/13 2 0.13 12 0.48
5/14 2 0.13 5 0.2
5/17 0 0 32 1.28
5/21 15 1 54 2.16
5/27 9 0.6 39 1.56

The unscientific parts of this trial are

  • we are also removing rotten fruits on other day but not counting them (hey, we’re busy!). But we make sure not to leave rotted squash near the plants and confound the next count.
  • It’s early days still, and we may get different results over the whole of the (short-lived!) hoophouse squash season.
  • Productivity is also important. We won’t be as impressed if we get low yields from the (beautiful) Golden Glories

I’ll keep you posted.Later I learned  that summer squash can get blossom end rot, and wondered if we had that problem rather than a pollination issue.

Is this an unpollinated squash or one with Blossom End Rot? It is shrunken, so I suspect poor pollination.
Photo Pam Dawling

See this excellent article Why Are My Squash Rotting? by Charlotte Glen. Here are some of the points she makes:

  • Blossom end rot is not a disease, but a physiological disorder caused by a shortage of calcium. Many factors can slow the absorption or movement of calcium in the plant, leaving it prone to blossom end rot.
  • Depending on what is stopping calcium from reaching the developing fruits, blossom end rot can be temporary or persistent.
  • Low soil calcium levels are rarely a cause of blossom end rot.
  • Blossom end rot is most usually caused by acidic soil (low pH). In soils with pH below 5.5, nutrients (including calcium) can get chemically locked up and unavailable to the plants (even if abundant in the soil). If low soil pH is the cause of blossom end rot, the problem usually lasts the whole growing season.
  • Too much high-nitrogen fertilizer can also cause blossom end rot. Fast-growing plants often cannot move enough calcium into fruits to support healthy development.
  • Stressors such as unusually cool or hot weather, low nighttime temperatures, drought, or over-wet soil can also trigger BER.
  • Any conditions that cause root damage can lead to poor nutrient absorption and blossom end rot. The most common causes of root damage are wet soils following heavy rain or over-irrigation.
  • Another leading cause of blossom end rot is drought – roots cannot absorb nutrients from dry soil, they need a film of water to convey them.
  • To prevent blossom end rot, the most important thing you can do is to keep the soil evenly moist.
Rotting unpollinated squash. Or some other problem?
Photo Pam Dawling

Here’s how I think our squash stacks up on those factors:

  • Our soil pH and our soil calcium level are both fine, we test the soil every year.
  • Home-made compost is our only source of nutrients in our hoophouse apart from occasional cowpea cover crops. I don’t think we overdid the nitrogen this way.
  • Yes, cool night-time temperatures when we first planted the squash in early April could have been a factor then, but now it’s reliably warm and the problem persists (at least with the Gentry)
  • Root damage? We did transplant these, but carefully. Maybe.
  • Uneven irrigation? We try to run the irrigation every day once it warms up. I don’t think that’s the issue. I don’t think we over-irrigated.
Healthy Gentry squash plants in our hoophouse.
Photo Pam Dawling

Then to help my deliberations, I found this
Squash and Cucurbit Problem Solver with many photos of fruit disorders (also leaf, root, stem, seedling and insect problems)Choanephora RotChoanephora Cucurbitarum is a soft rot of the blossom end, but rapidly developing grey-black fungal spores. I have seen that other times, but it’s not what we have in our hoophouse.Gray MoldBotrytis cinerea is another fungus that enters through the blossom end, leading to yellowed ends with grey furry mold. Bothe these fungal diseases are soil-borne, so more common in fruits touching the soil.Their photo of Blossom End Rot shows margins, and their description says: “The blossom end of the fruit fails to develop normally, turning black-dark brown and eventually shriveling and becoming hard.”Their description of poor pollination is this: “Fruit fail to expand normally, quickly turning brown at the blossom end and falling off the plant.” I’ve seen them fall off sometimes.For further help distinguishing between poor pollination and blossom end rot, I turned to the Garden Mentors (Robin Haglund).  She has helpful photos of these the two cause of problems, and these pointers:

  • Shriveling is due to poor pollination, not blossom end rot. Poorly pollinated fruits become obvious much sooner than fruits with blossom end rot. The fruits start to shrivel and yellow (harder to spot on yellow squash!)
  • If the squash was well pollinated, but has blossom end rot, it will grow plump with a sturdy stem, but rot at the end.

Poor pollination can be exacerbated if you use daytime overhead irrigation when the  female flowers are open. Squash flowers open flowers early in the day and close by early afternoon (and are open for only one day). Daytime overhead watering can discourage bees , leaving  your flowers unpollinated.So, more study needed! We definitely have been having poor pollination (shriveled rotten-ended squash that drop off.) maybe we have some Blossom End Rot too. I’m going to pay more attention! And eat lots of squash!

Date 15 Golden Glory rotted fruit Per plant 25 Gentry rotted fruit Per plant
5/13 2 0.13 12 0.48
5/14 2 0.13 5 0.2
5/17 0 0 32 1.28
5/21 15 1 54 2.16
5/27 9 0.6 39 1.56

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