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I've tried growing courgettes (zucchini) for the first time this year and the plants have been delightfully easy to grow. (For reference, I'm in NE England, UK, btw.) However, many of the fruits rarely get thicker than my index finger before starting to rot from their tip. What am I doing wrong?
Should I be putting straw or something else dry underneath them to stop them touching the soil?
This could be Blossom End Rot (BER) to which most of the cucurbit family are susceptible. It is caused by a calcium deficiency - see my answer here.
However, given that some of your courgettes are growing normally, and those that are rotting are very small, I think the problem is almost certainly caused by inadequate pollination:
No fruit, or fruit rotting when very small: This is a physiological problem, caused by the growing conditions, not a pest or disease. It is a problem when the weather in early summer is cool and this causes inadequate pollination. Remedy: This is usually a temporary problem, and once the weather starts to improve, so will pollination. You can try to hand-pollinate plants yourself by removing a male flower (male flowers don’t have a swelling at their base) and brushing the central parts against the centre of a female flower (female flowers have a swelling at the base – this is the beginning of the fruit). But this is a bit of a hassle, and normally the plant will correct this problem itself.
Pollinate the early batch of flowers by hand, rather than relying on insects. Pick an open male flower (which doesn't have a swelling at the base) and strip off the petals to expose the stamens and pollen. Then just rub them thoroughly against the stigma of a female flower (which has a swelling or an immature fruit at its base).
It does, indeed, sound like blossom end rot, but from what I've read, it's actually rarely the case that the soil being calcium deficient is the cause of the problem. Calcium may be implicated in blossom end rot (but that doesn't mean the soil is deficient).
Anyway, from my own experience, I gave plants loads of basalt rockdust (which is high in calcium). I even gave them things like calcium nitrate and eggshells. It was a pretty bad problem with several plants, this year, notwithstanding the calcium they had. If anything, I'm convinced that too much calcium can cause blossom end rot, but I could be wrong.
Really, though, the weather seemed to correlate rather strongly with blossom end rot, this year. It was very, very hot, and dry. When it cooled, the plants didn't have as many problems.
I've read on a forum that one person likes to use straw to prevent it (by burying it a few inches below the soil where the plant is). Straw/hay is actually high in nutrients (such as nitrogen) and it may do similar things for your soil as manure. I hear it can encourage good microbes (especially alfalfa hay). It can also act as ground cover. Anyway, that was only one person, but it really sounded like it worked, and it perked my interest enough to tell you about it.
Considering the fruits are small when they're rotting, that reminds me of a Yellow Straightneck squash with the problem I have this year. The squash appears to have a fungal infection at the roots, actually. I don't know if that's related to the problem, but possibly. It doesn't have enough light, though. A lack of light may be your problem.
