What can be the reason for yellow and dark marks on leaves?
Yellow leaf spots have various shapes, depending on the cause. Downy mildew starts with angular pale yellow spots. In cucumbers (Cucumis sativus) and other cucurbits, powdery mildew causes pale yellow spots followed by powdery white patches.

The location of the yellow spots may help you determine the cause of the symptom. Some diseases, such as powdery mildew, cause spots on older leaves first and then spread to younger leaves. Yellow spots from powdery mildew can occur on upper and lower leaf surfaces. Downy mildew spots occur on the upper sides of leaves between leaf veins that give the spots their angular shape. 

Now lets jump into the causes :- 


  1 -Moisture Stress


Over-watering or under-watering are the most common culprits when a plant's leaves turn yellow.
If plants don't receive enough water, they drop leaves to prevent transpiration (essentially, a plant's way of sweating) and conserve water. Before they drop, though, the leaves will typically turn yellow.
Too much water can be just as damaging to leaves. When the soil doesn't drain well, an overdose of water leaves the soil waterlogged and root systems can literally drown. Without oxygen, roots start to die. 


  2-Normal Aging


As many plants age, the lower leaves will turn yellow and drop off. 

However, to solve this case, don't worry. If the plant becomes too leggy, consider trimming back the main stem to promote new growth and bushiness.


  3-Cold Draft


Now let's first understand what a cold draft is. A cold draft is a current of cold air being drawn indoors. 

A plant sitting near it is giving off more heat than it receives from the room. Remember, heat travels from places of higher temperature to places of lower temperature.Cold drafts on tropical plants will often cause the leaves to turn yellow and drop. 

To solve this problem, particularly If your plant is near an air-conditioner vent in summer or a drafty window in winter, move it to a less turbulent place.It's also a good idea to mist tropicals that you're overwintering to increase the humidity.


  4- Lack of Light


A plant that is yellowing from a lack of light will typically yellow on the side that is away from the light source.

To solve this, move the plant to a sunnier location and see how it does. If window light is tough to come by in your home—especially in winter—you might need to rig up an artificial plant light or two.


  5- Nutrient Deficiency


This can be caused by too much calcium in the water if you're using hard water, or by a nitrogen deficiency.If this is the problem, the plant's top leaves may be the first to go yellow.

However to solve this issue, since it is species specific, its important to diagnose the problem firstly and them take some remedies. 


  6-Viral Infection


If your plant has a viral infection, it might show up as blotchy, spreading yellow patches on leaves throughout the plant. This may be accompanied by deformed leaves and stems, as well as discolored flowers.

To solve this , discard any plants that you suspect are infected since viral infections can not be treated. 


  7- Fungal infection 


If caused by a fungus, there is nearly always fungus growth of some type in the spot, particularly in damp weather. This fungus growth may be tiny pimple-like structures, often black in color, or a moldy growth of spores.

To solve this, you can use sulphur(in dust form) and milk.

Sources - https://www.thespruce.com/reasons-for-yellow-leaves-on-houseplants-4150680
https://www.hunker.com/12140067/how-to-diagnose-cause-of-yellow-spots-on-leaves
www.uky.edu/Ag/PAT/cat1/leafdis.htm