Cause, Prevention and Treatment of Problems that Afflict Garlic Plants
1. A 3 to 4 year crop rotation with non related plants is one strategy that improves the odds in your favor.
2. Another tactic is to allow wide spaces between the garlic plants which will make it less likely that any pathogens will transfer from one plant to the next. It also allows for better air circulation between plants which is not conducive to fungal growth.
3. Practice proper garden sanitation. Dead leaves, dead and decaying plant tissues harbor fungal spores - get rid of them.
Symptoms of Fungal infestation
1. Premature Yellowing
2. Off Color foliage usually various shades of whitish gray.
3. Visible white fungal growth at the bulb base
4. Powdery fungal growth on the stem that extends around the bulb base.
5. Whitish, furry growth on leaves
At times the fungus does not manifest itself till the cloves are harvested and put in storage. 'A hot water treatment of the garlic cloves can reduce infection up to 50%.' 
Serenade Fungicides are perhaps the best control of most Garlic fungal diseases, it will not cure them, but it will slow it down and prevent it from spreading to other plants. The active ingredient is bacillus subtilis. Bacillus subtilis will not harm beneficial insects such as bees and is harmless to people and other mammals.
The more common mites, such as the dreaded spider mites are not a problem with garlic as they are repelled by it. However other mites such that also feast of related Onions and alliums can be a nuisance.
Bulb mites will feed on all parts of the plant including the roots. Infected plants are generally stunted and not as sturdily anchored into the ground as healthy plants. These mites are pale white with reddish brown legs, they are visible to the naked eye under close examination. They tend to cluster at the base of roots and at times under foliage. These mites are sap suckers and reduce the plants vigor. Their feeding allows for other opportunistic parasites to enter the plant and wreak further havoc.
Nematodes are microscopic soil dwelling roundworms that feed on roots and bulbs. If your plants are not as vigorous as they should be. Perhaps weakened by nutrient issues, insects or fungal issues the nematode is there to finish the job.
A hot water treatment for suspect bulbs that have already been harvested is helpful
'Once a field is infested, rotation away from garlic is possibly the only way to effectively reduce nematode populations, although this may not be helpful with general having broad host ranges.' 
Beneficial nematodes are helpful in controlling the malevolent ones. Beneficial nematodes will attack other nematodes feeding on you garlic as well as soil dwelling insects. They are carnivores and won't bother plants.
Cornell University Plant Disease Diagnostic Clinic