Planting a Healthy Garlic Crop

Planting considerations for a healthy garlic crop

The presence of the stem and bulb, or ‘bloat’ nematode (Ditylenchus dipsaci) in garlic throughout New England requires that garlic growers take new precautions at planting time to ensure a healthy crop next year. Transport on seed is the primary means for spread of the bloat nematode from farm to farm and from garden to garden. Without a seed certification program in place some growers are wondering how they should source and treat new seed introduced onto the farm.  The first step is to find sources of healthy-looking seed from reliable growers whose seed lots that have tested negative for Garlic Bloat Nematode (GNB). However, this result is not a guarantee that every bulb that the grower produced is free of bloat nematode; it is only a guarantee that the garlic used in the test is GBN free! Additionally, new seed may come with Fusarium or surface molds.  To minimize risk of infesting established seed stock, and to promote healthy and vigorous garlic next year, include a few safeguards and best practices in your fall plans.

1. Practice long rotations. The dry, resting stage of the nematodes can survive in the field for long periods. Use your planting maps to help you establish a 4 year rotation (at minimum) out of host crops which include Alliums (garlic, onions, leek, chives), parsley, celery, and salsify as well as weeds Canadian thistle and hairy nightshade. This is a best practice for garlic which will also reduce other diseases.
2. Map it out Create a planting map for the garlic, and separate the new seed from your existing seed stock. Although GBN can move no more than one foot in soil on its own, it is easily spread by water run-off, contaminated equipment, shoes or clothes, and by any other means of moving infested soil. Separate new seed from your established seedstocks to prevent movement with erosion or cultivation. Place your new garlic where you will be able to plant and cultivate it separately, and clean equipment between plots as you would for other soil-borne diseases. Label each garlic seedlot clearly in the field for reference next year.
3. Cull bulbs or cloves with symptoms or damage when cracking: Carefully feel and look at each clove during this process, and remove anything that looks suspect. Discard cloves with unhealthy looking basal plates, with dents or lesions on or under the wrapper leaf, and any cloves that feel unusually light. Do not compost these cloves---either bury them away from the field or throw them away.
4. Test your own seed or untested seed from off the farm: if the symptoms described above are common in any of your own seed lots, send samples to your Extension Disease Diagnostic Lab before planting.  Select 4-6 symptomatic bulbs (not just 1) and have them tested for nematodes and other bulb diseases. For UMass Plant Diagnostic Lab contact Angela Madeiras, (413) 545-3209, mbdicklo[at]umext[dot]umass[dot]edu (ammadeir[at]umass[dot]edu).
5. Treat all seed with a surface sterilizer: Sterilizing the surface of the cloves will NOT control GBN! However, it will reduce issues with surface molds such as aspergillus and will kill surface penicillium. This is a best practice for all garlic. You can either use a 10% commercial bleach solution (1 part bleach and 9 parts water) or you can use an OxiDate dip (32 oz per 25 gallons water). Remember to test bleach and OxiDate dips for activity if treating large amounts of seed, and replace solution when activity decreases. Plant cloves immediately after dipping, not after they have dried back out.
6. Optimize pre-planting soil fertility: See Garlic Fertility Table for New England fertility recommendations (see also New England Vegetable Management Guide p 153 or http://nevegetable.org/). All phosphorus and potassium should be applied at planting. Up to 40 lb available N, preferably from a slow release organic form such as alfalfa and soybean meal, can be applied at planting. Quick release synthetic or soluble forms of N should be reserved for use in the spring. Side dressing in spring when shoots are 6” high and again 3-4 weeks later is recommended. Optimum fertility and soil conditioning will help keep garlic healthy, and healthy garlic will withstand everything from GBN to Fusarium better than stressed, unhealthy garlic. 
7. Next year, watch new seed closely: During the growing season, cull suspicious looking plants and have them tested for GBN. Selecting the most suspicious plants gives you the highest probability of detecting GBN, if present. If a seed certification program is developed, farm inspectors will take this step for you. Until then, you can act as your own informal inspector.
8. If any garlic on your farm turns out to be positive for GBN, you can still sell it as food. Clearly label this garlic as for table use only. Avoid returning any infested bulbs, plant tops, or scales to your fields or compost piles; bag and discard off the farm.

Adapted from Crystal Stewart, Capital District Veg. and Small Fruit Program, Cornell Coop. Extension as published in Veg Edge, October 2012 Vol 8:26.  Adapted for New England by R. Hazzard. Thanks to Steve Johnson’s article and New England Vegetable Management Guide as cited above.