Following a description of symptoms and management practices of some of the most common problems that affect garlic and how to distinguish them, both in your own garlic field and in garlic you may purchase from another farm. Be aware that the following descriptions are meant for general educational purposes only. Please consult your local plant health laboratory for definitive results. The best printed source of detailed, up-to-date information on garlic diseases that we know of is the Compendium of Onion and Garlic Diseases and Pests by H. F. Schwartz and S. K. Mohan now available in its second edition (2007) from the American Phytopathological Society (APS). General Preventative PracticesTake measures to prevent the onset of allium specific diseases and pests right from the first clove you're planting. Ensuring that your seed garlic comes from a reputable source is the first step. Don't use garlic from the grocery store just because it looks good. It may very well be a carrier of disease without showing any symptoms.
Rasa Creek Farm's grower team annually submits samples to plant health labs to ensure the seed garlic we sell is free of devastating diseases such as white rot and bloat nematodes.
Good prevention practices include:
White Rot (caused by Sclerotium cepivorum)"White rot" is the most devastating of all garlic diseases. It is caused by a fungus that often wipes out entire clusters of garlic plants and produces sclerotia that can remain viable in the soil for decades. When white rot incidence is low, infected plants may be "rogued" (selectively culled) and destroyed but when widespread infection occurs a complete restart in a different location and with sterilized tools is inevitable.
White rot can be difficult to differentiate from other diseases above ground. It usually affects patches of plants, rather than individuals. Growers may first notice stunted plant growth, followed by the early yellowing and death first of the outer leaves, then the rest of the leaves and the central stem. If allowed to progress, there will also be an obvious rotting of the stem above the bulb.
The disease is much more apparent on the bulb itself where white rot first appears as a white, fluffy mycelial growth around the basal plate that moves upwards and quickly develops small, black, poppy-seed sized sclerotia in and on decaying tissue:
This newsletter published by the California Garlic & Onion Research Advisory Board contains information about how to control white rot once you have it in your fields.
By the time you're buying seed bulbs in the fall any white rot mycelium will most likely have converted into black sclerotium. If at this point you spot white mycelium on your bulbs it is therefore more likely that you're dealing with the more common and less threatening:
Vegetable rot only occurs on garlic when bulbs have been compromised by mechanical damage or another disease. The former is often caused by
Vegetable rot almost always sets in sooner or later when there is damage to the cloves, be it caused mechanically or by another disease or organism discussed below. An essential skill in determining diseases on garlic is therefore knowing the symptoms of vegetable rot and being able to tell them apart from what other symptoms are present so as not to draw incorrect conclusions about the presence of the so-called "white rot," for instance.
Fusarium Bulb and Basal RotThere are many soilborne diseases of garlic with different symptoms caused by species of the genus Fusarium, another fungus. The two every garlic farmer should be familiar with are Fusarium bulb rot, caused by F. proliferatum, and Fusarium basal rot, caused by F. culmorum & F. oxysporum.
Fusarium basal rot attacks the bulb from the bottom up, similar to white rot, but proceeds more slowly and does not develop the characteristic poppy-seed sclerotia present on white rot. Another frequent (but not exclusive!) symptom of basal rot is a more intense reddish pigment margin of lesions. On the other hand, Fusarium bulb rot does not usually affect the basal plate but displays a speckling of water-soaked, tan lesions all over the clove. Neither disease develops the sclerotia so characteristic of white rot or Botrytis neck rot (treated in the next section).
Fusaria live in practically all soils and should be managed by roguing of diseased plants and planting of vigorous cloves. Botrytis Neck Rot of Garlic (caused by Botrytis porri)Botrytis neck rot causes frequent and significant losses to garlic. First symptoms appear in spring or early summer in the form of water-soaked neck rot near the soil line. After infecting the pseudostem, the fungus grows downward towards the bulb attacking the inner axis and leaving the exterior asymptomatic at first. After curing, the outer wrappers of affected bulbs often display an intense purplish discoloration and deterioration. The inner wrappers may at this point become hardened brown or black.
Neck rot may persist through storage developing grey mold and large black sclerotia, as opposed to the poppy-seed-sized sclerotia of white rot.
Botrytis porri lives in virtually all soils and particular weather patterns determine its impact from year to year. Cool and wet conditions in late spring or early summer favor the disease which naturally dies at temperatures above 30 degrees C. Management strategies include
Recently, a preventative cure for Botrytis has been proposed to us by BioFert Manufacturing Inc., a subsidiary of TerraLink. The following recipe treats 7,500 square feet:
To 4 gallons of water add:
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| Mild symptoms Courtesy Rasa Creek Farm |
Severe symptoms Courtesy Rasa Creek Farm |
Rust (caused by Puccinia allii)
Initial symptoms may include small, circular to elongate, white flecks on leaves and stems. As the disease progresses, the flecks develop into orange oblong lesions. Heavy infection results in leaves turning yellow, wilting, and drying and bulbs may be significantly reduced in size and quality. |
| Rust on young garlic plants Courtesy Wikimedia Commons |
Rust is an airborne pathogen and spreads from infected plant material, such as crop residue and infected volunteer plants. High humidity conditions such as coastal climates or heavy rainfalls are favorable to the onset of the disease. Collection of all plant residues after harvest, crop rotation, and weed management are the primary defenses against this fungus. In wet and warmer climates, space the planted cloves farther apart to allow more room for the top growth to breathe. Finally, don't overdo your Nitrogen since it make the leaves soft, lush and vulnerable.
We've also learnt of a preventative from Linky Smith, a fellow garlic farmer in South Africa. It uses Neem oil (which is not approved for organic certification in Canada). Here's the recipe:
- 10 milliliters Neem oil
- 20 liters water
- 10 milliliters liquid soap or equivalent (as a solving agent)
Apply by spraying onto the plant after a heavy rain event. If you live in an area where rain events are a daily occurrence, then apply every few days when it seems the heaviest rainfall has passed. Prevention is easier than cure, spray after each rain if reasonable to do so.
Stem and Bulb (or Bloat) Nematode (caused by Ditylenchus dipsaci)
Nematodes, also known as roundworms or eelworms, are microscopic, harmful or beneficial, parasitic organisms that live in the soil and on plant tissue and may feed on fungi, bacteria, mites, or the plant tissue itself.
A nematode feeding on the flesh of the garlic bulb with devastating results is Ditylenchus dipsaci causing stem and bulb rot mostly from secondary infection due to fungi and bacteria. Symptom response is related to nematode population density, meaning that infected bulbs may not show problems until favorable conditions arise (generally warm and wet weather).
A nematode feeding on the flesh of the garlic bulb with devastating results is Ditylenchus dipsaci causing stem and bulb rot mostly from secondary infection due to fungi and bacteria. Symptom response is related to nematode population density, meaning that infected bulbs may not show problems until favorable conditions arise (generally warm and wet weather).
Although only a laboratory analysis can bring certainty of the type of nematodes present, you may wish to establish their presence or absence inside the bulb using the technique described in this video. Note that many nematodes exist in the soil but only very few migrate into the bulb itself. For instance, root lesion nematodes (Pratylenchus sp.) attack the roots of garlic thus potentially causing stunted growth but do not enter the flesh of the bulb. They will thus neither cause flesh wounds nor travel to other fields via host cloves.
The bloat nematode can cause garlic leaves to twist and malform and garlic bulbs to turn brown, split, and swell (or bloat). Wrappers often crack and become detached from the basal plate of the bulb.
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| Twisted leaves and split wrappers Courtesy OMAFRA |
Cracked wrappers, missing roots, and bloating Courtesy OMAFRA |
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| Splitting at the basal plate and deformation typical of Ditylenchus dipsaci infestation Courtesy Rasa Creek Farm |
As already mentioned, secondary invaders causing decay following a nematode infection make it difficult to draw conclusions about nematode presence from symptoms alone.
Bloat nematode can be managed by planting disease-free seed, crop rotation with non-susceptible crops allowing three years before replanting allium sp., planting bio-fumigants such as brown mustard and sudangrass as cover crops, and carefully removing all garlic plant debris from the field after harvest—practices that should be followed in regards to managing all other problems as well.
Wireworms
Wireworms are the larvae of the click beetle living in the soil for 2 to 6 years before becoming a beetle. They are phytophagous, i.e., feeding on plants such as—but not exclusively—garlic, but also carnivorous and even cannibalistic. Extensive root damage will cause a plant to wither or at least stunt its growth. Frequently, wireworms bore their way into the outer layers of a bulb leaving an unattractive but not contagious mark that can often be removed by peeling off a few layers.
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| Wireworm in the field (ca. half an inch) Courtesy University of Massachusetts Extension |
Wireworm bite marks Courtesy Rasa Creek Farm |
Cloves affected by wireworm bites are usually still viable seed garlic unless the flesh has been compromised allowing other diseases such as fungi to take a hold. You will not usually find the wireworm itself in mature garlic as it returns back deeper into the soil before harvest time.
Most farmers don't experience high enough populations of wireworm to see significant damage to the crop, but things can get out of control without your realizing it. Basic practices to prevent wireworm build-up in the soil:
- Cover Crops: Brown mustard produces a bio-fumigant in its roots that deters wireworms and perhaps even kills them. Including brown mustard in your regular crop rotation is a good practice. Buckwheat also seems to drive populations of wireworm down in the soil. Click here to learn how to incorporate these rotation crops into your field.
- Trap Crops: If wireworms are present but not in huge numbers, you can plant a trap crop in the aisle between your garlic beds to lure them away from the garlic. Radish works well, or even wheat. Plant your bait crop in a straight line right down the middle of the aisle with seeds very close together. Once the bait crop is mature, check it for wireworms and then harvest the entire crop, including the wireworms! Potatoes also make great wireworm traps. Cut a potato in half and run a stick through the middle. Bury the spud about one inch deep so that the stick stands vertically as a handle. Pull the traps out after a day or two and discard wireworms.
The PEI Potato website contains frequent updates about and summaries of cutting edge research into wireworm management strategies.
A specialty sulfur amendment called HumiSul-90 produced by BioFert Manufacturing Inc., a subsidiary of TerraLink, has been reported to eradicate wireworm in the fields of several farms, including ours. Sulfur is also a great booster for garlic crops in general.
Leek Moth
As the name suggests, leek is the preferred host of the leek moth; however, garlic is also very attractive to the pest. Leek moth larvae can cause extensive damage by tunneling mines and feeding on leaf tissue and occasionally on bulbs.
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| Garlic leaves damaged by leek moth larvae Courtesy Agriculture and Agri-Food Canada |
Leek moth larvae feeding on garlic clove Courtesy Rasa Creek Farm |
Besides the aforementioned general preventative practices, leek moth presence and activity can be controlled using commercially available pheromone trapping systems. More information available here.
Bulb Mites
Bulb mites are a problem of garlic that can go unrecognized unless you know what you're looking for or have your bulbs inspected by a plant health laboratory. They can reduce stands, slow plant vigor, and increase post-harvest diseases. Bulb mites have a very wide host range, but cause most of their damage to onions and garlic. Species from the genera Rhizoglyphus and Aceria are the most common garlic parasitic mites, with the latter being known as the dry bulb mite.
Bulb mites can overwinter in soil and also
survive in stored garlic. They can damage garlic
in the field, but are particularly troublesome in
storage. Their feeding can cause desiccation and
creates wounds that provide an ingress for bacteria and pathogenic fungi such as
Fusarium and Penicillium. In the field, mites are usually not seen on the bulb but feed mainly on the roots and
basal plate. In storage, mites move into the garlic bulb.
Rhizoglyphus spp. are 0.3-1.0 mm long, have two or four pairs of legs, and are cigar- or bulb-shaped while Aceria spp. are banana-shaped. The mites' bodies are off-white to pale yellow in color and may be
visible with a hand lens.
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| Damage caused by Aceria tulipae with some secondary infection (white) Courtesy L. Du Toit, Washington State Univ. |
Banana-shaped Aceria tulipae on the surface of a garlic clove Courtesy M. Putnam, Oregon State Univ. Ext. |
Bulb mites survive in the soil on organic matter left behind from the previous crop. As long as there is decaying allium vegetable matter in the soil, bulb mites can survive in the field. The best way to control bulb mites is to allow the vegetation from the previous crop to break down before any new crop, especially garlic or onions are planted again. Some other management options:
- Flood irrigation or heavy winter rain will reduce mite populations.
- Hot water treatment of seed garlic is effective, but can decrease germination. Put seed in water heated to 130°F for 10-20 minutes, or 140°F for 10-15 minutes.
- Soak seed for 24 hours in 2% soap (not detergent) and 2% mineral oil prior to planting.
- Dust bulbs with sulfur.
Blister Beetle
Not a serious menace, but the blister beetle has a voracious appetite for a few weeks in spring. We've watched as a single beetle has destroyed an entire young garlic plant (with four leaves) in less than half an hour. They grow very large and are soft-shelled and black. The best management is just to walk the fields and inspect for the little guys, removing and squashing them as you find them. They're only a danger during a few weeks in the plant's tender youth, so once your garlic is well established there's no need to continue the surveillance.
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| Blister beetle attacking garlic leaf Courtesy Rasa Creek Farm |
Viruses
It is accepted that all garlic contains viruses. Leek Yellow Stripe Virus and Onion Yellow Dwarf Virus, belonging to the potyvirus group and both causing the so-called "Garlic Mosaic," are the most common. If present in combination, heavy viral loads can reduce bulb size and thus crop yield. When alone, viruses may not cause any noticeable consequence in the crop unless plants are heavily infected.Growers should monitor their fields on a regular basis, identifying and culling symptomatic plants and destroying the infected plant material.
Viruses are passed between plants primarily through aphids and possibly through mites and other insects that suck on a plant's leaves and then move on to a new plant nearby. Ladybugs prey on aphids and should be encouraged near the garlic fields through planting flowers and plants that attract ladybugs.
Left: Symptoms of heavy infestation of the Onion Yellow Dwarf Virus. Plants will appear stunted and twisted. Bulbs will never form properly and the entire plant should be removed and destroyed.
References
(1) Schwartz, H. F., Mohan, S. K. "Compendium of onion and garlic diseases and pests." The American Phytopathological Society, Ed. 2 (2007).
(2) Tamburini, E., et al. "Quantitative Determination of Fusarium proliferatum Concentration in Intact Garlic Cloves Using Near-Infrared Spectroscopy." Sensors 16.7 (2016): 1099.
(7) Johnson, S. B. "White Rot of Garlic and Onions." University of Maine Extension (2015).
(9) Noronha, C. "Procedure For Using Rotation Crops As A Wireworm Management Strategy." Agriculture and Agri-Food Canada, Charlottetown.
(12) Madeiras, A. "Bulb Mites in Garlic." University of Massachusetts Extension (2013).
(13) Jepson, S. B., Putnam, M. L. "Eriophyid Mites on Stored Garlic." Oregon State University Extension (2008).
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