Fungal diseases such as verticillium wilt are a common cause of stem rot and are often exacerbated by over watering, high humidity and poor ventilation. Always let the compost dry out slightly before between watering and try to water from below rather than above to prevent the stems sitting wet. Also make sure that your plants are not crowded together as warm, wet conditions are perfect for the spread of fungal diseases. If they are in the greenhouse you can improve conditions by watering in the morning and leaving the greenhouse door open during the day to ensure adequate ventilation. Unfortunately you will need to remove and destroy the infected plants to prevent this problem spreading.
It sounds like a fungal infection but it is hard to say what the disease is without a little more information. Often fungal spores spread under quite specific conditions e.g. wet and warm or cool and dry. Try to make sure that the soil is kept moist and that there is adequate air flow around each plant. If only a few of the leaves are infected then you could remove and destroy them. This may be sufficient to prevent the spread of fungal spores. If you are able to provide a more detailed description and a picture, I can refer your question to our vegetable specialist for his opinion.
It is possible that your maple has a fungal infection. If so, then the best course of action is to clear up the fallen leaves this autumn and dispose of them in your normal household waste or burn them to destroy overwintering spores.
If the speckles are bumpy and appear on the underside of the leaves and stems, this could be scale insects. The insects cocoon themselves in small hard protective scales. There are chemical controls available readily in most garden centres - each product will list if it is effective against scale insects and the types of plants it can be used on. The best time to spray is from June to July as this is when new insects hatch.
Many fungal problems can be reduced by practising the following cultivation tips. Water the plants from beneath the foliage rather than over the top, as this may cause the plant to rot and also improves conditions for the spread of fungal spores. Dont crowd the plants too closely together - the better the ventilation, the less the risk of fungal infection. Remove and destroy any infected plant material as soon as you spot it. After harvesting your crop, remove any mulch from around the plant and cut back the foliage to reduce the build up of pests and diseases.