I have a mandarin tree leaves are turning yellow and brown spots on leaves

https://homeguides.sfgate.com/causes-yellow-spots-meyer-lemon-trees-98519.html

This will happen, - if the Tree needs feeding, you may find it needs an acid type of Feed +Soil - also where you are keeping the potted tree usually better at this time of year under cover eg greenhouse, also check the leaves for bug infections.

Citrus Insects Diseases

olive green bug on mandarin tree Google Search

https://www.gardenclinic.com.au/how-to-grow-article/bugwatch-bronze-orange-bugs

not sure if this link will help you out.
mandarin tree leaves are turning yellow and brown spots on leaves Google...

Please could you give information of your general location, the species of tree and any planting specifics

Scab is the most common and damaging pecan disease. It is caused by a fungus that affects rapidly growing leaves, shoots, and nuts. Symptoms on all plant parts are similar. Velvety olive-brown to black spots occur on the husks. Several spots may develop to form black blotches or may blacken the entire surface of the husks. Severely affected nuts of susceptible varieties may drop prematurely or they may stop growing, die and remain attached to the shoot. Leaf symptoms first appear on the underside as tiny olive-brown lesions on the veins. Later, leaf symptoms appear on the upper surface as small olive-brown to black spots. Severely infected leaves may be shed prematurely and weaken the tree, thus reducing the crop next year. The most practical method of scab control is to plant varieties having some degree of resistance to scab such as: Desirable, Cape Fear, Elliott, Chickasaw, Gloria Grande, and to a lesser extent, Stuart. Varieties severely affected by scab are Schley, Mahan, Success, Van Deman, and Mohawk. Use of "resistant" varieties does not necessarily assure complete absence of the disease as no variety is totally resistant. If possible, removing and destroying all leaves and husks may help reduce the amount of scab the following year.
A spray schedule is available for growers who have spray equipment adequate to cover the trees, or for growers who hire custom spray applicators. The following fungicides are registered on pecans: benomyl (Benlate 50WP), propiconazole (Orbit 3.6EC), fenbuconazole (Enable 2F), and thiophanate methyl (Topsin-M 70WSB). Spray applications should be made every 2-3 weeks beginning when leaves first emerge until shell hardening (early August). Spray at 2-week intervals when the tree is growing rapidly or during wet periods. Check fungicide labels for restrictions on grazing in sprayed orchards and the need to use tractors with enclosed cabs while spraying.

See this... http://homeguides.sfgate.com/brown-spots-fig-leaf-ficus-41712.html

JOKe ...... because of the brown spots .
If you really suffer from brown teeth, at best, you'd be wearing a yellow tie on

You don't say where you are, but from the description the fungal disease Cypress Canker is possible. It is difficult to treat.

http://forestry.about.com/od/forestdiseases/p/dis_com_con.htm