MOFGA's Pest Report - July 25, 2011
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist
Garlic: Growers have either harvested their garlic, or will be harvesting soon. There are two diseases that are not common here in New England and we want to keep it that way. Now is the time to look and see if you have either of them. As you harvest, please inspect your bulbs carefully. Both of these diseases are devastating and long lasting in the field, preventing future garlic production. And, both are spread with the garlic seed pieces so it is of the utmost importance that anyone with this problem not sell seed.
White Rot of Garlic (Sclerotium cepivorum) - White rot is a very serious problem because it may spread fast, and once in a field it can persist for many years. Luckily it is a spotty disease and at this time is only present in a small number of fields around the northeast. Those farms can no longer grow Allium in infested fields.
|White rote of garlic. Photo by Eric Sideman.
White rot is one of the most destructive fungal diseases affecting the onion family. It is only a problem on crops in the onion family. [It is not the same pathogen as white mold, which attacks many other crops such as beans, carrots, lettuce, tomatos, peppers and more]. Symptoms of white rot on the leaves include premature yellowing and dying of the older leaves and then death of the plant. That is not much different than many other garlic problems so is not a very useful diagnostic symptom, but the white, fluffy fungal growth (mycelia) on the root end of the bulb is the give away. Eventually this fungal growth moves around the bulb and inward between the storage leaves of onion and cloves of garlic. Small, black sclerotia (tiny hard, black bodies of dormant mycelia) form in the decaying tissue and throughout the white fluffy mycelia. Secondary infections by other fungi may occur.
The pathogen is not known to produce spores. This fungus reproduces only by the sclerotia, and it also spreads by direct contact, i.e., when the mycelium growing on one plant reaches the roots of the neighbor plant in the row. The sclerotia can lie dormant in the soil for many years until roots of a host plant grows nearby and the sclerotia are stimulated to germinate (see below). Dragging the pathogen around on boots, tillage or other equipment, or moving with soil in heavy rains are other ways the disease spreads. Also, animals feeding on diseased bulbs can defecate viable sclerotia.
The best control is good sanitation. Use clean seed cloves for garlic and clean onion sets and transplants. If only a small number of plants are infected, which is usually the case the first year it is found on a farm, pull the infected plants and destroy before sclerotia are formed.
An interesting idea for speeding up the eradication of white rot sclerotia from the soil is to stimulate them to germinate but not have a crop of Alliums for them. Sclerotia will sit waiting in the soil for 20 or more years until a signal is received that onions or garlic are growing nearby. The growing Allium releases a chemical that the sclerotia can sense. Over the past decade researchers have been studying methods that stimulate the sclerotia to germinate but, of course, with no Allium crop growing for the pathogen to complete its life cycle. This "biostimulation" reduces the number of sclerotia in the soil. There is not a specific recommendation yet, but a few things that may reduce sclerotia in the soil include:
- growing scallions which stimulate the sclerotia to germinate, but the scallions are harvested before the disease completes its life cycle
- making compost out of onion or garlic waste and spreading that in the spring or fall repeated for a bunch of years before trying to grow an Allium again
- making a sprayable concoction from ground up onion or garlic waste, or using garlic powder as a soil amendment for a few years before trying to go back to an Allium
If the disease is known to be present, or if onions from other farms are being stored and packed, equipment, storage bins, etc. should be thoroughly pressure-washed and then disinfested for ten minutes with a 0.5% solution of sodium or calcium hypochlorite, (for example, 1:10 dilution of a household bleach). Then rinse with potable water.
Seed producers should execute extra diligence and may want to regularly disinfect any surface in contact with garlic.
Since chlorine materials will be inactivated by organic matter stuck on boots, quaternary ammonium compounds may be used as boot dips inside storage areas and packing sheds, and before and after leaving fields. Quaternary should not be used on any apparatus that is in direct contact with the garlic or onions or any other crop. Disposal of the dip solution must be in a manner that does not contaminate the soil, water or crop. Note: not all quaternary ammonium products are labeled for boot washes so read the label.
At this time, no material has shown efficacy.
Garlic Bulb Nematode (Ditylenchus dipsaci) - The garlic bulb nematode, also called the garlic bloat nematode, is a new garlic pest that first appeared during the wet year of 2009, with confirmed cases in New York and Vermont. It is now in New Hampshire and Maine too, brought in with garlic seed purchases. The microscopic worms feed by piercing root and leaf cells with their stylet. Leaves of severely infected plants turn yellow and dry prematurely. Plants may be stunted. The roots may be missing and the basal plate may appear to have a dry rot similar to Fusarium basal plate rot.
|Garlic buld nematode damage. Photograph Ontario Ministry of Agriculture, Food and Rural Affairs. Copyright Queen's Printer for Ontario, 2010. Reprinted with permission.
The pest is favored by wet, cool conditions. Although the pest is not active in hot dry weather, such weather may exacerbate symptoms. The nematode survives freezing and hot weather in soil and plant debris.
The best way to avoid garlic bulb nematode is to use your own garlic for seed, if it is not yet infested. Monitor for symptoms of infestation during the growing season and submit suspect plants to a diagnostic lab for confirmation. Both UNH and UMO can ID this pest for you. Contact the lab to get instructions how to take and where to send the sample.
If it is determined that you do have the problem, do not use your own garlic for seed, or sell the seed. Even bulbs that show no symptoms may have low leves of infestation. Obviously, do not sell any garlic for seed from a potentially infested lot. Do not replant garlic in an infested field for at least four years. Other hosts include all Alliums, celery, parsley, and salsify. Mustards, Sorghum-sudan grass, and other bio-fumigant cover crops have been shown to reduce nematode populations. These nematodes can survive in dry debris so carefully clean equipment and storage areas.
There are no materials approved for organic production.
Purple Blotch (Alternaria porri) - Purple blotch is a very common and sometime destructive disease that affects onions, garlic, shallots and leeks. It is not as devastating as the other two, but much more common. Lesions begin as whitish sunken areas that elongate and develop purplish centers. Under favorable conditions (warm with wet leaves) the lesions become large and oval with concentric rings of spores. The lesions may merge and take down whole leaves, which may become covered with brownish spores. The older leaves are more susceptible than the younger leaves. This pathogen may also result in a watery rot at the neck of onions or garlic and lead to poor storage life.
|Purple blotch on garlic leaves. Photo by Eric Sideman.
Onion residue is the source of the disease in the spring. The fungal mycelium and conidia (spores) persist as long as the onion debris remains in the field or in cull piles. New conidia are produced on infected tissue in the spring and wind blown or carried in water to the new crop. The leaves must be wet for the spores to germinate, but germination is very quick, less than an hour. Symptoms may appear in less than a week after germination and new spores are quickly produced.
Sanitation is very important to limiting spread. Infected crop debris should be destroyed or buried after harvest. Cull piles should be eliminated. Grow onions in rotation with non-host crops.
Serenade has been shown to be effective against purple blotch. Copper has demonstrated mixed results in trials, fair at best.