Thank you for your question about garlic.  My research never turned up the term 'root maggot', but nematodes are designated as an issue.  The first article I have included discusses several problems that affect garlic.
https://garlicstore.com/why-is-my-garlic-rotting/

Why is My Garlic Rotting?

July 22, 2022 by Franklin

THIS POST MAY CONTAIN AFFILIATE LINKS. PLEASE READ MY DISCLOSURE FOR MORE INFO.

Reading Time: 8 minutes

In the right conditions, garlic plants are relatively easy to grow. Like other allium plants, garlic thrives the best in sunny climates. That’s because it requires abundant warmth to maintain healthy growth and ripen the bulbs properly.

However, growing garlic at home is daunting, especially if you have no idea how to handle any possible problems. Like many garlic growers, you may face issues like white rot, soil deficiency, bulb mites, and bloat nematodes.

This article discusses the problems of growing garlic in greater detail and offers possible solutions to help you quickly cope. Read on!

The following are the reasons why your garlic is rotting:

1. White Rot

White rot is a disease that affects garlic and other allium crops line the onion. The condition is very aggressive, surviving in the soil for a long time. Therefore, white rot can spread quickly and carry from one crop to another.

It is caused by the white-rot fungus or Stromatinia cepivora (berk). The fungus can remain dormant in the soil as a sclerotium, surviving over the winter until you introduce a susceptible crop into the ground.

A single sclerotium in two pounds of soil is enough to cause severe white-rot infection. White rot transmission happens quickly in cold, wet conditions.

The pathogen doesn’t have to be present in the soil. Shoes and farming tools that have come into contact with the infected ground can quickly spread white rot.

White Rot Symptoms

• Yellowing plants
• Withering plants
• Dead garlic plants
• Rotting garlic bulbs in storage

Plants Affected by White Rot

• Garlic
• Onions
• Shallots

Effects on the Soil

White rot affects much more than allium crops. It has long-term effects on the soil, rendering it unsuitable for onion and garlic production. The ground may remain unusable for up to 20 years.

Given the lack of effective cultural or chemical control methods, there is little you can do when the soil gets infected.

Scientists have recommended practical ways to stimulate then starve the white-rot fungus. However, these methods are ineffective, reducing rather than eliminating the fungus.

How Does White Rot Spread?

• Contaminated shoes
• Infected farm implements
• Infected plant material

The primary way the disease spreads is through infected plant material. Therefore, it’s critical to purchase planting stock only from authorized sellers. Also, you should be careful not to move infected soil, water, or diseased plants into your farm.

How to Identify White Rot?

In the initial stages, initial plants have signs of yellowing leaves. This may start from an older leave and look normal, but it’s not. Furthermore, diseased plants may have stunted growth.

Due to the progression of rot in the bulbs, garlic plants keep weakening until they are easy to pull out.

Then, there’s likely to appear the white fungal mycelium, a fluffy growth at the base of the plant.

Even though the disease has initial symptoms similar to Fusarium basal rot, it has tiny black sclerotia, a unique sign.

Soil Conditions that Support the Growth of White Rot

• Temperature: 60°F to 65°F, but infection is also possible at 50°F to 75°F.
• Cool and wet soil

Thus, it’s easier to arrest the early detection of the disease by stopping irrigation mid-season. Then, depending on your location, the disease appears in cool and wet weather.

White Rot Management Before and After Planting

There are several ways to manage white rot in your garlic and onion farm, from selecting seed plants to in-farm management. Here are suggestions of what you should do:

Choose Disease-Free Sets

Avoid choosing diseased seed garlic. That would mean buying garlic sets that are certified disease-free.

Pulling Out Infected Garlic Plants

When you notice stunted growth throughout the season, inspect the pants for signs of white rot. Once you confirm white rot, pull all diseased plants and all others near them.

Dispose of the soil-infected plant material carefully to avoid inadvertently spreading the disease. Put the materials in a plastic bag, seal it off, and take it to a landfill.

Avoid Allium Plants

If white-rot persists, avoid planting allium plants (onion, shallots, and onions). Instead, use soap and water to sanitize the garden tools that remove the rotting garlic.

Crop Rotation

If white rot is widespread on your farm, avoid planting root crops. Instead, go for other crops not prone to white rot infection.

If you must plant allium species, move them to a separate section of your garden that doesn’t come into contact with soil and water in the infected area.

2. Nematodes

Garlic plants may also rot due to infestation by nematodes. Garlic nematodes burrow into the bulbs and destroy plants. Nematodes live inside the garlic and keep reproducing. They feed on various parts of the garlic plant, including stems, leaves, and bulbs.

Garlic nematodes can survive for a prolonged period in the soil, even during a drought. This is because they are microscopic and live and reproduce inside the garlic stem and bulb.

Even though it may have no significant effect on the growth of garlic plants, nematodes reduce the quality of the bulbs.

Unlike white rot, which has apparent symptoms, nematodes may build up in the soil for several years without damaging the plants.

So, healthy garlic plants can resist the effects of large nematode infestation without getting damaged. But then, the nematode population becomes too large out of the blue, destroying garlic plants in a single season.

Choose clean planting material to avoid carrying nematodes into your garlic garden. Buy seed garlic from sellers with stringent screening practices. Where possible, use the right equipment to screen garlic plants for nematodes.

3. Thrips

Insect infestation of garlic plants in thrips may make garlic rot while still in the ground. Even though there are many insect garlic pests, thrips are the most common.

They infect garlic leaves and feed on sap, making it impossible for the plants to grow. One of the immediate effects of thrips is curtailed garlic growth.

By destroying the leaves, they prevent garlic leaves from attaining their potential. Garlic plants tend to wilt, rot, or die when infected with insect pests like thrips.

Checking for signs of these insets on the farm is the best way of stopping them from attacking the plants. With the help of sticky straps, it’s possible to isolate and identify thrips. After identifying thrips, they can use pesticides to destroy thrips and other insect pests.

4. Fusarium Basal Rot

After harvesting a garlic crop and hanging it to cure, you may discover that the entire crop is rotting only a few weeks later. In most cases, the cause of the rot is basal rot, a dreaded disease that causes rotting in the basal plate.

It prevents the transportation of water and nutrients to the leaves. With time, the fungi destroy the roots, making it impossible for the garlic bulbs to grow. Thus, they remain small, immature, and easy to rot.

Initially, basal rot has similar symptoms to those of white rot. However, the distinguishing factor is signs of black sclerotia, which indicates white rot. Therefore, it causes garlic rot and basal rot.

Fixing Fusarium Basal Rot

Unlike other allium plants, garlic is low-maintenance and requires minimal watering. You only need to water the plants once in two weeks. Once the foliage starts yellowing due to age, stop watering. At that stage, the bulbs start maturing.

Watering garlic bulbs late into their maturity, they will likely start rotting. It can also reduce the ability of the bulbs to store food.

When the plants are still young, keep checking the cloves regularly to ensure there’s no disturbance from birds. If you notice birds attacking the plants, cover them with netting.

5. Garlic Black Bugs

There are other tiny pests like garlic bugs that feast on allium plants. They appear in the form of black spots on top of the plants.

Even though they do not lay eggs, they produce young ones on top of the plants every two days. Therefore, an infestation of garlic plants with bugs can never deteriorate.

The following are the symptoms of garlic black bug infestation:

• Droopy plants
• Stunted plant growth
• Drying garlic leaves

How to Remove Garlic Black Bugs?

Remove black garlic bugs by spraying the plants with an insecticide from a local plant health store.

However, that’s a temporary measure, and the black backs will likely return. In winter, frozen eggs remain dormant in the plants or soil, leaving for reinfestation in warmer weather.

Growing garlic in a different area in exchange for other crops is the best way to stop the black bug infestation.

Avoid panting an allium crop since that would only worsen the problem. After two seasons, you would have destroyed or driven the black bugs somewhere else.

6. Soil Deficiencies and Other Issues Affecting Garlic Production

Soil deficiencies like the lack of nitrogen, potassium, and magnesium can affect the growth of garlic plants.

Apart from stunted growth, it can lead to leaf discoloration and the development of spots. The following are the common problems resulting from soil deficiencies:

Punky Bulbs

Low soil pH can result in manganese toxicity, leading to a punky bulb. In addition, the disease makes the garlic cloves appear loose and discolored. Finally, when infected, the interior of the garlic bulb looks whiter than usual.

Here is how to control punky bulb disease in simple ways:

• First, apply lime to the soil to reduce the pH to below 6. Then, put the lime up to 3 inches into the soil of a raised bed before planting garlic. If you put it too deeply, it wouldn’t have the desired effect.
• Plants garlic on well-drained soil
• Carry out soil tests before applying fertilizer to prevent an increase in the soil ph. Also, limit the application of potassium chloride, which increases manganese uptake.
• Reduce the application of nitrogen in the spring to about 100 lb/A. use calcium nitrate and other non-acid forming nitrogen sources.

Waxy Breakdown

The waxy breakdown is one of the more serious issues that may arise due to soil deficiencies. The disease causes garlic cloves to become translucent and decay.

It is common in garlic plants that grow in extremely high temperatures, incredibly close to harvest. Its initial symptoms include garlic that’s waxy or sticky to the touch. Other causes include low oxygen and ventilation in storage.

Characteristically, the waxy breakdown does not affect the outer protective scales. Therefore, it’s hard to tell which cloves are affected until they shrink and produce an amber-brown color through the outer shells. It may start with one garlic clove and spread to the whole bulb before striking other bulbs.

Excessive Side Shoots

Instead of nutrient deficiency, some growers may apply too much fertilizer leading to an overload. For example, soil with too much nitrogen may make the bulbs develop too many side shoots.

Before planting garlic, you should test the soil and ensure a perfect balance of nutrients. You may use compost to create balance in the soil before using artificial fertilizer.

Preventative Care for Garlic Plants

Growing garlic is an elaborate process that involves careful choice of seed and proper care on the farm and after harvest. The following are some of the general ways to prevent the onset of the issues discussed in this article:

1. Buy Garlic Seed from Reputable Sources

To avoid having garlic plants infected with pests and diseases, buy your seed from reputable sources. Instead of going to the local grocery store, approach a certified seed seller. Look for seed vendors that work with plant health labs to ensure they supply disease- and nematode-free seeds.

2. Use Effective Prevention Practices

• Plant healthy, robust garlic cloves always
• Carry out annual crop rotation
• Cull stunted and diseased plants soon after symptoms appear
• Remove soil and organic matter from the field/garden
• Collect and burn plant material to destroy disease-causing pathogens
• Cover cropping with other crops (such as brown mustard) between garlic crops.

Tips for Caring for Garlic Plants

• Mulch around the plants using aged compost to nourish the plants and cut down the weeds
• Keep weeds out of the planting beds.
• Pinch blossoms from hard-neck garlic to form more giant bulbs
• Heavily mulch using straw or hay to keep garlic plants warm in the winter
• Bend yellowing garlic stems to the ground without breaking them
• Avoid planting garlic in the same field after another garlic or onion crop
• Choose 15-inch wide and 15-inch containers to grow garlic

FAQs

Why Are My Garlic Bulbs Rotting?

Garlic bulbs may rot due to fungal, nematode, or insect infestation. So every time you notice rotting signs, check out its real cause.

What Happens When Garlic Gets Too Much Water?

Too much water in a garlic field can cause rotting. That’s why you should allow the soil to dry in-between watering sessions.

Why is My Garlic Falling?

Your garlic can fall due to planting it in too much water. Healthy garlic plants need a consistent, humid environment. Therefore, watering the garden at least every two weeks is a must.


The Bottom Line

Garlic can rot for many reasons. One of the critical causes of rotten garlic is white rot, a fungal disease that can make garlic growing impossible. Then there is basal rot, which also results from fungal infection.

Finally, infestation by nematodes and certain insect types can also make garlic cloves rot. To grow your garlic, pay attention to the garden’s nutrition levels.

Sources:

http://www.extension.uidaho.edu/publishing/html/BUL955-Beware-Signs-of-White-Rot-in-Garlic-and-Other-Allium-Crops.aspx

https://www.hobbyfarms.com/3-problems-that-could-ruin-your-garlic-crop-3/

https://www.farmhydroponics.com/gardening/why-is-my-garlic-rotting-in-the-ground

https://readytodiy.com/top-problems-growing-garlic-and-how-to-fix-it-0053/

https://www.rasacreekfarm.com/garlic-diseases-and-pests

This next source was very thorough, but would not let me copy the pdf.  I have included the source for your use.

plantclinic.cornell.edu › factsheets › garlicdiseasesDiseases of Garlic: Various Pests - Cornell University

This third source is particular to nematodes:

https://www.gardeningknowhow.com/edible/herbs/garlic/garlic-plant-pests.htm

Nematodes – A particularly insidious garlic bug is the nematode (Ditylenchus dipsaci), which lives and reproduces inside garlic plants. These microscopic worm-like pests eat all parts of the stems, leaves, and bulbs. It can live without water and survive for many years in the soil. Nematode nibbling can result in deformation, discoloration, and tissue collapse of the bulb. The worst thing about garlic nematodes is that their population continues to expand exponentially for years with no visible signs of damage. What this means is that for years, healthy garlic plants will not be significantly affected but then all at once the astronomical nematode population will decimate the garlic crop.

Read more at Gardening Know How: Garlic Bugs In Gardens: Information About Garlic Plant Pests https://www.gardeningknowhow.com/edible/herbs/garlic/garlic-plant-pests.htm
I hope these help you identify your particular issue.  If I can help more, please contact me again.

Thank you. You’ve answered this before I gather.

Good luck with your search.