Cucumber diseases can put paid to your bountiful harvest. Considering how prolific the growth of other cucurbits like pumpkins and squash, and how few fruit were yielded, it was a delight to harvest the first large cucumber on such a small plant. However it turned out to be our last good cucumber…
I will explain in the next page how we will treat this form of cucumber disease naturally, and the results of that attempt, but first we had to identify it. We ran through a number of cucumber diseases online, and we began by thinking it was a kind of blight or mildew, as the cucumbers appear to have a dry spot or two and then to discolour, go soft and shrivel. However, after cutting one cucumber open we saw a maze of tunnels inside the fruit with some kind of larvae nestled in them. We thought about wasps and moths and googled and came up with nothing. We found a narrow waisted wasp like looking (to an amateur) winged insect crawling on one leaf a day later and caught it.
It was not going to be easy and we had to up the intensity of our research into cucumber diseases a notch, so we took the winged insect and some affected cucumbers to my mother. There was a wide choice of affected fruit, they all looked poorly. My Mother is 86 and qualified with an MA in Entomology at the University of Cape Town in the 1950’s, for which she studied the egg laying behaviour of wasps on caterpillar larvae and suspected the use of a chemical marking device used by the female wasp after laying her eggs, that stopped other wasps from laying their eggs in the same caterpillar. This was before such things as pheromones had even been discovered. She went on to be the beetle inspector for the British Merchant Marines in Simonstown in the days when the training yachts were built of solid teak. To this day she has a really observant eye and keen curiosity about the biology and behaviour of all living things. So we gave her a try and had a fascinating afternoon. Now you can breathe a sigh of relief. Bragging about my senior integrated pest management consultant is done, and we'll proceed with the findings.
My mother pointed out that some of the grub like things, so small we had to look at them with magnifying lenses, were actually pupae. They were the same colour and general shape as the grubs, which appears to distinguish them from some other suspects I'll mention below. The pupae had large bumps on the front showing the adult insect had very large eyes, and another area showing wing casing with a segmented body below. The small flying insect in the jar she immediately identified as not being a wasp but a fly, as it had only one pair of wings. We looked in a comprehensive textbook on local South African insect families and another on the insects harmful to food growing in general. We found nothing, not even an overlap between the books. Eventually in frustration I googled cucumber fly and there came across the cucumber fruit fly, a very thin waisted fragile looking fly which an amateur like myself would mistake for a wasp (as can be seen in pictures you can access in the link below), and to make excuses for myself, it is a mere centimetre in length and I do need new glasses. The creature was not mentioned in either text book, but its scientific generic name is Bactrocera, and our fly may be Bactrocera cucumis, though there are many hundreds of Bactrocera species, and many genera of fruit flies and identification is very speculative at this stage. B. cucumis is one of the most infectious cucumber diseases and is such a problem that there are import controls which legislate on this little fly and the presence of its larvae in produce in many countries.
The small insect which was found on the cucumber leaf seems to be a fruit fly, and it may be unrelated to the larvae and pupae found in the cucumber. There is also very little resemblance between this fly we found, shown below and above and any of the major suspects such as Rhagoletis or Ceratisis species, or Bactrocera cucurbitae, cucumis and invadens or Dacus ciliatus. My mother told me they use the fruit fly Drosophila for genetic experiments as its so genetically malleable, so perhaps this creates great regional diversity in small fruit flies in general and is a barrier to identification. After two weeks of keeping the pupae in a jar some flies hatched out, they may be some kind of cucurbit fruit fly like Bactrocera cucumis. As I've failed to ID the type of fruit fly with certainty, I will have to go for general fruit fly control measures.
In the next article I will outline the various ways proposed to control fruit flies, and describe our experiments. We will cover other plant diseases as they arise in our experience, so as to deal with the common ones first and to help you find natural chemical free ways of ensuring a good crop.
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