The culinary attributes of Cinnamon are well noted, it's a great flavoring for a multitude of recipes. What is not well noted is it's usefulness in the garden.
The Cinnamon that we know from our kitchens is derived from a tree bark - you guessed it, the cinnamon tree. To the more scientifically astute cinnamon trees are actually a genus or class of trees known as "Cinnamomum". When you buy cinnamon spice you are usually buying "cassia" which is derived from another member of the same genus.
They evolved a natural protection for themselves against many fungi via an organic compound they produce known as Cinnamaldehyde.
This same compound that gives cinnamon spice it's distinct flavor is actually a potent fungicide, a deterrent for some pests and can also be used to propagate root cuttings.
Cinnamon as a Fungicide
Black rot fungal diseases from Pythium and and Phytophthora fungi are common in many crops. There are multiple strains that are at times devastating. Cinnamon has proven to not only be a preventative measure against Black rots - but in many cases a cure.
Damping off is a fungal problem that effects primarily small tender seedlings shortly after they sprout. Cinnamon will work as a preventative measure for damping off by killing the fungus before it kills the plants. It works against many common types of mold as well.
Cinnamon has proven more effective than many commercial products currently available.  Its active ingredient Cinnamaldehyde is registered for use against Verticillium spot, Dry Bubble from Verticillium, Rhizoctonia, Pythium, Dollar spot and Pitch canker disease.
Making a cinnamon spray for plants is relatively easy. Mix 4 tablespoons to a half gallon of water- shake it vigourously and let it sit for a few hours. Strain the liquid through a sieve or coffee filter and put the remaining liquid into a spray bottle.
Spray the stems and foliage of any affected plants or any plants you suspect might become affected. Any residue left over when you strained it is still good to spread around the base of the plants, the only reason for straining it was so that it wouldn't clog up the spray bottle.
Adding some garlic oil gives it an added oomf, the sulfur in the garlic also serves as a natural fungicide against some fungi that the cinnamon doesn't address. When you mix in the garlic oil, it will usually separate in solution from the cinnamon water, so has to be shaken vigorously before each use.
Cinnamon for Pest Control
Cinnamon is also effective in controlling nematodes 
Cinnamon is not an alternative insecticide, but it is an inexpensive remedy for Ants - cinnamon is a good deterrent. It also repels Moths as effectively as moth balls. It has also been suggested against slugs and some beetles but none of this is documented.
Cinnamon as a Rooting Agent
Cinnamon works well as a rooting agent. Cinnamon powder applied to the stem when planting a cutting will stimulate root growth in most plants. Roll the dampened ends of plant cuttings in cinnamon and it will encourage the growth of additional shoots. At the same time it will keep the cutting disease free while it is developing.
Basic raw honey placed on the plant cutting over the Cinnamon will not only help seal the cinnamon in but serves as a rooting agent in its own right. See - Honey As A Root Hormone
Cinnamaldehyde, cinnamon's primary active ingredient, promotes root branching by regulating hydrogen sulfide originating in the cutting itself. 
1. IMPACT OF PLANT OILS AS ANTIFUNGAL ACTIVITY AGAINST FUNGALPATHOGENS OF CINNAMOMUM ZEYLANICUM
2. Nematicidal Activity of Cassia and Cinnamon Oil Compounds and Related Compounds toward Bursaphelenchus xylophilus (Nematoda: Parasitaphelenchidae)
3. EPA - Cinnamaldehyde (040506) Fact Sheet
4. Cinnamaldehyde promotes root branching by regulating endogenous hydrogen sulfide
5. Evaluation of antifungal properties of octyl gallate and its synergy with cinnamaldehyde