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While garlic is a pretty hardy plant, it can be affected by a range of pest species (nearly 100) and the first warning signs once planted is irregular 'roguing' growth. A garlic grower should be regularly inspecting the garlic growth to find the first signs of unusual growth early. The three main categories of garlic problems are diseases, unusual growth or and pests.
Garlic is one of the most susceptible annuals to disease for three main reasons. Firstly the cloves and bulbils are clones of the plant so unlike seed it can carry forward and accumulates disease to future generations. Secondly bulbs grow in the damp soil and for a long time where they are exposed and vulnerable to a whole range of soil borne diseases. Thirdly there are few infield treatments to cure many garlic diseases. For bacterial and viral issues there is almost no treatment options, while fungal issues only has limited options. The best method is crop rotation, raising the soil, planting good stock and removing rogue shoots early.
The first sign is the plant having smaller, yellowing or shriveled leaves compared to others in the bed. It is worth inspecting plants regularly to catch these rogue sick plants as they will never grow into healthy ones. Also, be careful to remove them including the surrounding soil before disposing. To leave sick plants in the soil increases the disease and affects soil health and nearby garlic.
Most fungal issues can be reduced by practicing crop rotation (recommended 3+ years), choosing good planting stock, having good watering practices and planting a variety of garlic groups known as polyculture. Once shooting begins look out for the warning signs of disease or other non-uniform growth.
While it is handy to have early season garlic like turbans and asiatics, the mid to late harvesting garlics are generally are less prone to seasonal pest and diseases as the season warms up.
Garlic produces roots first, before sending out shoots. In mild-winter climates where cloves are planted shallower, shoots are likely to appear 1-2 weeks after planting. In such climates where the daytime temperature is above 12°C then leaf growth will continue through winter. In colder climates it can take up to six weeks for leaves to appear.
In colder climates only 3-6 leaves will initially grow before the plant goes into dormancy. Once spring arrives and temperatures exceed 14°C, then leaf growth resumes. Note that standard purple stripe and sometimes the porcelain garlic group has an 'alien' early growth form where they lie flat with the ground. Porcelain is also known to be the most susceptible to having viruses.
Garlic Rust (Puccina porri)
There are 7,000 species of rust, but garlic rust (Puccinia porri) is a real threat to garlic crops. This pathogenic fungi is prevalent, and seems to have infested every corner of our country. This fungal disease generally spreads from spores in the wind (it can travel long distances), animals, people and their machinery. Puccinia allii also infects other garden vegetables too such as leeks, onions, spring onions and chives, so it's best not to plant these in the same soil each year.
More detailed information on garlic rust and how to prevent or treat it can be found on our garlic rust page.
This generally charcoal coloured covering normally over the stalk and sometimes in some of the bulb wrappers (skin) with dark blotches.
It is normally a cosmetic injury caused by pathogens Embellisia allii, and Aspergillus niger. These diseases sit in the soil on dead plant material and can infect the plant due to a bulb injury, or gain entry through the basal plate. It often occurs in warm dry climates such as Central Otago.
The infection seems to occur more commonly on white wrapper garlic groups than red and from our experience the softneck garlics in particular the artichoke garlic group seems to get the worst infestations.
Black mould typically visibly appears during the curing process if the bulbs have not dried properly. It occurs when the humidity levels are too high, curing spacing is tight and there is not sufficient ventilation flow.
Fortunately, black mould can be prevented if bulbs can dry in well ventilated dry environments and can be reduced by removing some bulb wrappers before storing.
Penicillium corymbiferum is carried in the garlic bulb. The mould makes the clove soft and it will shrivel and reveal white to green or blue spores on the clove. The first signs if a clove with it is planted is the leaf yellowing and it looking different than other garlic plants.
Moulds can occur more frequently when cloves are damaged or poorly stored. These moulds normally happen when cloves have been left too late for planting and their roots grow in cool and moist air conditions.
Pull out the infected plant including the soil around the clove and you will notice the spores. While healthy bulbs can carry spores, only use clean planting stock to prevent this growth using a pre-treatment.
Basal Rot (Fusarium root rot)
Basal rot first appears in the field with young shoot yellowing. The fungus can continue to spread post-harvest with the thin basal plate rotting with a white mold appearing. The rot is caused by the fungus Fusarium oxysporum and thrives in warm and wet conditions. Remove and dispose of these plants at the first sign of disease.
To reduce the chances in getting basal rot use disease-free bulbs, ensure crop rotation, proper spacing between plants, avoid root disturbance, overwatering, and overfertilising. Also cure bulbs as quickly and thoroughly as possible before storing.
Neck rot also known as bacterial centre rot, is caused by several types of botrytis and also pantoea complex bacteria. It makes the pseudostem and cloves have a water-soaked coffee colour appearance.
The infection can move into the bulb during storage and has no simple cure. To reduce the chances of neck rot consider pre-treatment before planting using a bleach soak for a few minutes. Ensure good air circulation for the crop and during storing. Also avoid bulb injury, excess soil nitrogen, too much watering or mulch around the plant.
Soft Pink Rot
Neck rot is a type of bacterial rot which is often not found until the garlic is cured and the garlic is cut for storage. Sometimes it can be spotted in the field when the stalk has a brown rot the centres of some of the leaves centres at the vein are water soaked, swollen and yellow-brown in appearance.
It is caused by the a rod shaped bacterium called Erwina persicina and causes a pink appearance in the the stalk which sometimes travels down into the clove where rot can form. There is no solution to rid the rot but bactericides that contain copper can slow the spread of this disease.
Fusarum Dry Rot
Dry rot also known as 'storage rot' is a is a wet seaon or poor storage rot caused by Fusarium proliferatum fungi. All Fusarium diseases are latent and all garlic has some level of infection with most unseen. The first signs of the disease show up on the basal plate and roots and spread to form small brown lesions covering the clove. Avoid planting cloves with this fungi are their is no cure, so check while cracking garlic for soft tissue, and cloves with unusually pale, brown spots and loose skins.
The pathogen overwinters in the soil and can be carried on the bulb. The best way to avoid the occurrence is to plant healthy cloves from quality bulbs, pre-treatment, crop rotation and avoid bulb injury during harvest and handling.
White rot (Stromatinia cepivorum or Schlerotium cepivorum) is a devastating fungal disease for the garlic grower. It can spell the end of garlic growing in a bed. The disease favours cool and moist soil conditions. Infection in soil temperature ranges from 10-23°C, with 15-18°C optimal for the fungus. Soils temperatures greater than 25°C have a low risk of developing this rot.
Early signs are dark brown pigmentation which stain plant's neck or inner bulb skins or slimy bulb skin which soil sticks to the mycelium (white fungal threads). After a few seasons the disease in plants gets worse. The fungus creates black reproductive sclerotia which form small black poppy seed-like features in the white threads. At this stage clusters of plants dying back early and are easily pulled out due to their decayed roots and basal plate.
White rot affects other allium species, and there is no effective organic treatment. The only option is to remove the soil or never grow alliums in the diseased soil. The disease can be still viable in the soil for 20+ years and can lay dormant for 50+ years before a allium species is planted. If the infected soil remains, the disease is likely to spread via animals and wind movement. Ensure that tools and gloves are sterilized as to not affect other areas.
Plant clean stock. Use preventative pre-treatment measures if obtaining garlic from an unknown source such as briefly soaking the garlic clove at no more than 49°C to reduce the fungal spores. Also practice crop rotation.
There is no common name for this plant bacteria. The pathogen can be present in many vegetables including garlic. In garlic, the disease affects clove and scape development. This results in single clove bulbs or fewer mis-shaped cloves, enlarged bulbils or neck bulbils on types that do not normally have them. The stalks can be also mis-shapen or have multiple stalks, can be pinkish in colour and is soft and spongy at the base. The leaves of the plant often have strong yellow edges all the way down leaves.
The disease can arrive with new seed stock and there is no cure. The bacteria can be more common with overly wet soils. It is best to remove the plants from stock and avoid planting any cloves with symptoms.
Garlic Mosaic is caused by a range of viruses that is present in garlic, it's commonly caused by those of the potyvirus group. It causes angular striping and discolouration (yellow to light green) of garlic leaves particularly in younger plants creating a mosaic pattern amongst the healthier darker green plant tissue. If severe, plants are often stunted and bulb size is reduced.
The disease is transmitted from garlic stock (clonally propagated) and aphids. Most plants only show mild symptoms with only one type of virus, and are severely affected by several types. To reduce reoccurrence, cull affected plants to reduce the chances of the disease in next years crop.
There are many other less common diseases which cause unusual growth habits of garlic plants. When the plants are shooting keep an eye on any rouge growth habits. Compare the shoots and early leaf growth to other nearby plants. If they are stunted, have unusual colour (ie. pink/red could be rhizoctonia or stemphylium) or discoloured or have wobbly growth then it is best to remove and disposed of these plants. If these plants are left to grow, the disease might spread to other plants and extend further into the soil affecting the health of plants in future years.
Side Sprouting (Witches Broom)
Side sprouting also known as secondary shooting. It occurs when the bulb is still growing but the clove skins begin to sprout into leaves coming out of the pseudostem or false stem and into the top of the plant. Often this condition is called 'witches broom' and is caused by fluctuating weather extremes of hot and cold (typically cold), higher than usual rainfall or planting too early for the clove.
It can be formed by planting store bought garlic that has been kept in cold storage, also vernalising for more than two months can have the same result. If planting in a cold winter area try planting a little later to avoid changeable weather at the plants vulnerable growing stage.
This condition also occurs during bulb development and clove formation with excess nitrogen in the soil contributing to bulbs and cloves being bigger than usual. After harvesting such bulbs do not store them rather try to consume them within a month or two.
Waxy breakdown is a condition that normally develops after harvest (4-6 weeks after). It is not discovered until the bulb wrapper is removed revealing cloves with a soft waxy translucent appearance. The cloves initially appear yellow and in time transform into a orange colour and are normally become sticky. This physiological condition occurs as a result of very hot conditions prior to harvest, either too much sunlight during curing (avoid sunlight when curing), mild not hold curing conditions or poor ventilation during storage. Avoid planting cloves with waxy breakdown.
This where one or a few cloves begin to appear on the outside of the bulb wrapper which is often seen in the artichoke garlic group. These external cloves occur as a result of an early spring hot snap. The unusually hot weather will initiate clove development early in the lower leaves. When these leaves eventually die off and the outer cloves are exposed outside the main bulb wrapper. Providing they are not part of the fertile leaves (the last leaves to appear) these cloves will be viable for planting the following year.
This condition is where sprouting seems to send out several shoots from the ground not just from one clove. This is as a result of not identifying joined cloves during cracking them out of the bulb pre-planting. When dividing them what appeared to be one clove was in fact several cloves. It is a good idea while cracking the bulb at the pre-planting stage to check the basal plate of extra large cloves to see whether they are doubles.
This condition is easy to resolve if caught early by running your fingers into the soil between the shoots and pulling one away from the other gently to separate the roots. It should be possible to replant the extra shoot if caught early (less than 10cm high), and if there is room in the garden.
The softneck types (silverskin and artichoke) of garlic which have multiple layers of cloves that are very tightly bundled can make separating the cloves challenging. Sometimes, only a hairline fracture is barely visible between cloves. Generally, the single layer hardneck types are more uniform and easier to spot joined cloves.
Sometimes this condition is outside our control. Bulbs divide into cloves well before bulbing. Clove division is actually is a result of tiny clove buds that swell into large cloves that make up the bulb. This can be impossible for any gardener to spot.
Leaf colour can also indicate a deficiency in the soil. This often occurs a month after sprouting when the clove no longer supplies nutrients for growth. If the leaf tip yellows around this time, it is likely frost damage, minor nutrient deficiency or nutrient imbalance. A little yellowing is normal unless you have very fertile soils where the leaves are a standard green.
If the yellowing continues and advances into the season (and it's not a mould or rot) then there is a likelyhood it is either a serious disease, lack of water or a soil nutrient deficiency. If it's a disease, remove and dispose of the sick plant or plants. Note that leaf yellowing is not to be confused with maturity where leaves turn yellow from the leaf tip down beginning with the leaves at the bottom of the plant.
Nutrient imbalances in garlic can generally be determined by the effect on their leaves. A lack of:
Nitrogen - yellow leaf tips affecting oldest leaves first, each new leaf smaller, folded, stunted leading to purple veins at base of leaf
Phosphorus - same as nitrogen except young leaves do not fold
Potassium - deep yellowing of older leaf tips then leads to complete leaf yellowing
Magnesium - mottling (chlorosis) affecting base first with lower leaves yellow
Calcium - spotting on all leaves particularly the upper third which increase in size
The occurrence of the bulb not dividing into cloves (called rounds) is difficult to determine pre-harvesting. This condition is a result of the cloves planted being too small, being planted too late in the season and not wintering over or if the weather conditions have been either too dry or wet. They can be still eaten (some imported garlics are rounds) or replanted the following year. It is typical to get rounds in the first year of growing bulbils before replanting over the next 1-2 years to get bulbs with cloves.
Nematodes or roundworms are microscopic to tiny creatures which there is thought to be over a million types. They have successfully adapted to nearly every ecosystem including the 35 species in humans and thousands of types in soils. Only a couple of types affect garlic; Pratylinchus species attack the garlics roots while the Ditylynchus species which are 1.5mm long and attack the stems and bulbs.
The damage to garlic is known as bulb and stem nematode or bloat nematode. They can be identified on a clove as tiny pimple-like spots or the brownish desiccated discoloration just above the basal plate where they have entered the plant. Once way to kill them from gloves is to soak the cloves in hot water (43°C) for ten minutes before planting.
For the soil, reduce populations by growing mustard seed family such as brassica plants then tilling the green plants into the soil as brassica are toxic to nematodes. It will take 3-4 years for the mustard seed family crops to release the compound that reduces nematode populations Otherwise avoid growing allium species in the same soil.
Aphids, particularly black aphids can quickly infect garlic plants. These tiny oval insects which are 2-3mm long can be undetected at first before rapidly hatching to cover plants. They expand rapidly because female aphids give birth to other females, who are already pregnant when born.
If there is only a few then you can pick them off. However it is difficult to get every one so they are best treated with neem oil which an organic and biodegradable treatment. A soil using a mustard family crop as mulch a month before planting will reduced the chances to getting aphids.
Bulb mites (typically Rhizoglyphus echinopus) is a tiny (0.5-1mm) type of mite. The mite burrows into the garlic basal plate and then migrate into the stem and cloves if there is sufficient moisture. They leave a hollowed out honeycombed soft void of rot to bulbs.
In some cases the mites leave very small brown spots on the garlic cloves visible once the bulb wrapper and clove skins are removed. They may look like small fungal lesions starting, but by looking closely bulb mites might be spotted living under the skin of the clove. The spots create scars left behind from the bulb mite feeding.
Pre-treat garlic before planting, use free draining soil in raised beds or mounded soil. Ensure that crops are rotated. Mites do not survive dry conditions and rapidly changing humidity. Bulb mites are more common in high ambient relative humidity around harvest time. Thus the most effective way to control bulb mite infestations is to cure the crop quickly after harvest (1-2 days via fans) rather than passive drying over a few weeks. Allow plenty of airflow around curing bulbs.
Other pests which might affect your NZ garlic include rats (pre-shooting), rabbits, slugs and thrips. It is possible to protect against some of these larger pest by installing fine netting and using neem soil or pyrethrum for smaller pests.
It is best to quarantine new garlic for your garden into a separate bed if possible. This reduces the possible spread of any disease or pest from the source garlic in the first year or two.
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