I have several cucumber plants in my greenhouse as well as outdoors in my raised beds and they all have some degree of the brown/gold spots that you can see in the photos that I have attached. The plants have many blooms, but the fruit never seems to get more than 1/4 of an inch long and doesn't seem to grow any larger. Please help!
Pennington County South Dakota
Ok so we have two problems here - some sort of pathogen and what appears to be poor pollination.
First - I am not certain on the type of pathogen is attacking your leaves. Here is a list from University of Minnesota extension office:
The main take-away is to practice good sanitation. Remove all infected debris at the end of the season so that it can't redevelop. Overhead watering should also be suspended as this will splash infected soil onto the leaves allowing the pathogen to spread. Also, crop rotation will also help stop the spread of whatever disease this is as it is unlikely to infect other plants outside the Cucurbit family.
Next issue is that juvenile fruit yellow and fall off. The issue is that they are not properly pollinated and the plant will abort them. If these are in your greenhouse then we need to introduce some pollinators in there - or do it yourself...
I think that this might be a problem with spider mites, which are extremely common on cucumbers, particularly in a greenhouse. The speckling in the leaves are the first clue, the larger spots can be tissue that just got so much feeding that it died out. Look for minute webs in the leaf axils and you may be able to see the mites themselves, on the undersides of the leaves. They are very difficult to control this time of year because they prefer hot, dry conditions. Generally the best option is to use Predatory Mite Neoseiulus predatory mites that are available from various biological control supply companies. Unfortunately, under heavy infestations, it is often too late to help the plants recover. They need to be present from the beginning of the infestation and re-released on a regular basis, often once a week. If you do a web search for predatory mites you can learn much more about spider mites and using predatory mites for their control.