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Root-knot Nematode Diseases of Horticulture Crops Root-knot nematodes, Meloidogyne spp., are the most important plant-parasitic nematodes in our country because of their wide host range and widespread distribution. Root-knot nematodes cause large economic damage. Yield losses is from 26% to 50%， even no Yield. In addition, root-knot nematodes interact with other plant pathogens, resulting in increased damage caused by other diseases. Root-knot nematodes have an extremely wide host range. All major field crops, most vegetable crops, peaches, ornamentals, and many weed or grass plants are susceptible to one or more species of root-knot nematodes. Host vegetables include: carrot, tomato, potato, eggplant, beans, peas, onion, shallot, leeks, celery, cauliflower, cabbage, brussel sprouts, turnip, capsicum, watermelon, cucumber, gherkins, squash, pumpkin, zucchini, lettuce, parsley, coriander, fennel, radish, spinach, and over 2 000 other plant species. 1. symptoms Above-ground symptoms include wilting, loss of vigor, yellowing, and other symptoms similar to a lack of water or nutrients. Infested plants grow more slowly than neighboring healthy plants. Plants often wilt during the hottest part of the day, even with adequate soil moisture, and leaves may turn yellow. Fewer and smaller leaves and fruits are produced, and plants heavily infested early in the season may die. Damage is most serious in warm, irrigated, sandy soils. Below-ground symptoms are the swollen, knotted roots. Root-knot galls can be single , or coalesce with other root-knot galls and form a big one . It should not be confused with nitrogen-fixing nodules that develop on roots of legumes. Nitrogen-fixing nodules are attached to the side of the roots, but root-knot nematode galls are formed by enlarged root tissue .It will not "flick off" as will nitrogen nodules. Nematode galls on leeks, capsicum, shallot and members of the grass family are usually small and even difficult to see. We can see the pear-shaped mature female in the Root-knot galls . Eggplant Cucumber Carrot 2. Pathogen The cause of disease is one or more species of root-knot nematodes (Meloidogyne). The main species are M. incognita(南方根结线虫)， M.arenaria(花生根结线 虫)，M.hapla(北方根结线虫)and M.javanica(爪哇 根结线虫). M. incognita is the dominant type . Meloidogyne(根结线虫） Female nematode Male nematode 3. Disease cycle 3.1 Dispersal Nematodes are usually introduced into new areas with infested soil or plants. 3.2 Survival It is believed that root knot nematode survives from season to season primarily as an egg in the soil. 3.3 Infection and Life cycle After the eggs hatch, the second stage juveniles invade roots near root tips and remain inside the root at one location throughout their life. As larvae feed, the root cells divide rapidly near the nematode's head. This rapid cell division and enlargement cause the swelling or knots on roots. The female nematode undergoes three additional molts and changes shape to become a pear shaped adult. The adult female may live two to three months or longer and lay 500 to 1000 eggs into a jellylike mass that extends out through the root surface and into the soil. The male nematodes eventually leave the roots. Eggs may hatch if temperature and moisture are favorable. In temperate climates, generation time is four to six weeks and there are usually three or four generations per year. Life cycle 4. Management Management of nematodes is difficult. The most reliable practices are preventive, including sanitation and choice of plant varieties. Existing infestations can be reduced through fallowing（休耕）, crop rotation, or soil solarization（曝晒）. Once an area or crop is infested, try to minimize damage by adjusting planting and harvesting dates and irrigation or by the use of soil amendments（改良剂）. 4.1. Sanitation Nematodes are usually introduced into new areas with infested soil or plants. Prevent nematodes from entering pathogen-free garden by using only nematode-free plants purchased from reliable nurseries. To prevent the spread of nematodes, avoid moving plants and soil from infested parts of the garden. Do not allow irrigation water from around infested plants to run off, as this spreads nematodes. Nematodes may be present in soil attached to tools and equipment used elsewhere, so clean tools thoroughly before using them in your garden. Remove the roots of infested plants as through as possible . 4.2 Resistant or Tolerant Varieties and Rootstocks One of the best ways to manage nematodes is to use vegetable varieties and fruit tree rootstocks that are resistant to nematode injury. Consider replacing severely infested plants with plant species and varieties that are more tolerant of the nematodes present. Unfortunately, resistant varieties are not available for many crops and ornamentals. 4.3 Rotation / Fallow Growing a crop on which the nematode pest cannot reproduce is a good way to control some nematodes. Unfortunately, rotation is not as easy for controlling root knot nematodes because so many vegetable crops and weeds are hosts of the pest. Annual crops that are useful in a rotation plan for reducing root knot nematode populations include small grains such as wheat and barley, sudangrass, and resistant tomato and bean varieties. With a nonhost crop such as sweet corn and other grain crops will be effective in controlling the northern rootknot nematode. Fallowing is the practice of leaving the soil bare for a period of time. Fallowing is hardly used in our country. 4.4 Planting and Harvesting Dates Most nematode species are active during the warm summer months and cannot penetrate roots at soil temperatures below 64°F. Therefore, we can reduce nematode injury to fall-planted crops such as carrots, lettuce, spinach, and peas by waiting until soil temperatures have dropped below 64°F. Plant summer vegetables as early as possible in spring before nematodes become active. It is also helpful to remove annual vegetables (including their roots) as soon as harvest is over, to prevent nematodes from feeding and breeding on root systems. Nematode-suppressive Plants Certain marigolds (Tagetes)(万寿菊） suppress root knot and lesion nematodes. French marigolds are most effective. Marigolds do not work well against the northern root knot nematode (Meloidogyne hapla). The effect of marigolds is greatest when they are grown as a solid planting for an entire season. When grown along with annual vegetables or under trees or vines (intercropping), nematode control is usually not very good. Soil amendments and Irrigation Various organic amendments can be added to the soil to reduce the impact of nematodes on crop plants. The amendments, which include peat（泥煤）, manure, and composts（混合肥料）, are useful for increasing the water- and nutrient-holding capacity of the soil, especially sandy soils. Increasing the soil's capacity to hold water can lessen the effects of nematode injury. Likewise, more frequent irrigation can help reduce the damage caused by nematodes. In either case, there will be just as many nematodes in the soil, but they will cause less damage. 4.5 Pesticides Chemical control involves the application of certain volatile and nonvolatile nematicides to the soil to reduce nematode populations. methyl bromide (溴甲烷） Dozomet（棉隆、必速灭） ai 6kg/ 667m2 Ethoprophos （ethoprop，灭克磷)20% ai 3kg/667m2 1.8%Avermectin（阿维菌素）1ml/㎡treat soil，or 3%Miral granules（米乐尔/氯唑磷）60kg/ hm² row replacement （沟施)or hole replacement(穴施） 4.6 Biological control Paecilomyse lilacinus（淡紫拟青霉）， Cylindrocarpon(柱孢属）, heteronema （异丝藻属）， Verticillium chamydosporium（厚垣轮枝菌） preparations have been found to be highly effective against the nematode. Bulb and Stem Nematode in Onions and Carrots The bulb and stem nematode (Ditylenchus dipsaci) occurs in many regions around the world and is a particularly destructive pest on vegetables grown in temperate climates. Although many different crops are susceptible to invasion by this nematode, more than 20 biological races have been identified. Only a few of these races are know to occur in Canada and the federal government regulates the importation of potential hosts of exotic races to prevent their entry. Some biological races are host or crop specific while others are capable of invading more than one host. Crops infected by bulb and stem nematode include onions, garlic, leeks, chives, shallots, peas, beets, carrots, celery, tomato, cucumber, narcissus, hyacinth, tulip, oats, rye, alfalfa, red clover, phlox and strawberry. Onions are susceptible to several of these races of bulb and stem nematode. 1. Symptoms Onions Symptoms of bulb and stem nematode infestations on different crops vary depending on crop and population of nematodes. In onions, the base of infected seedlings appears swollen and leaves appear twisted, malformed and may bear slightly raised pimple like spots (Figure 1). Figure 1. Young onion plants infected with bulb and stem nematode. Initial symptoms include irregular swelling and twisted leaves (A). As the disease progresses, the outer leaves begin to die back from the tips, the outer bulb scales split and the neck may become spongy in texture (B). Severely infected plants eventually turn yellow and die. Scales on older infected bulbs often split and appear swollen or bloated (Figure 2). Symptoms frequently look similar to damage caused by onion maggot. Infected bulbs are also very susceptible to secondary infections by bacteria and fungi. Figure 2. Symptoms of severely infected onions with bulb and stem nematodes (left) and uninfected healthy onions (right). The outer layers of scales of the infected bulbs are split allowing the inner layers to bulge through. When the splits extend into the root region, leaf-like shoots often grow out where roots normally grow. Carrots Although strains of the bulb and stem nematode that infect carrots have not been confirmed in Ontario, growers should become familiar with the damage symptoms. Young carrot seedlings develop distorted leaves and by harvest time, infected crowns become spongy and crumbly making mechanical harvesting difficult. The symptoms often resemble those caused by Aster Yellows. Infected carrots are also more prone to infection from crown rot fungi and soft rot bacteria. 2. Biology Bulb and stem nematode activity begins early in the spring when they leave the soil under conditions of high humidity caused by a recent rain or heavy dew. The nematodes travel in the film of water covering the surface of stem and leaves. In onions the nematodes will move down inside the leaf sheath until they reach the bulb and feed between onion scales. During feeding, the nematode injects saliva-containing a toxin (enzyme) into the cells, which creates the lesions and stimulates distorted growth. They prefer moist muck soil（腐殖质土） and are less likely to cause damage during hot, dry seasons. However, irrigation can encourage the invasion of susceptible crops during drought periods. The nematodes can spread through irrigation water, infected bulbs or seed, and on contaminated equipment. They can survive freezing or extreme dry conditions in a dormant state in dried infested plant residues. 3. Recommendations for Control Vegetable growers should consider having a nematode test done on fields in the fall after harvest or in the spring before cultivation to determine if the bulb and stem nematode is present. Clean Seed, Sets, Bulb Make every effort to insure that seed, sets from onions, garlic, shallots and flower bulbs are clean to prevent contaminating your farm. Sets and bulbs should be examined carefully and only plant if they appear perfectly healthy. Infected sets tend to be soft, shrunken and discolored (dark brown) near the neck of the plant and are lighter in weight. Sanitation Dispose of cull onions and carrots away from the field, where they will not contaminate irrigation canals. Clean off soil adhering to equipment before moving between fields. Remove infested plants and debris from both field and storage, then destroy them. Rotation Rotate all fields, including those with low populations, with a non-host crop for 3 years. Fumigation Field fumigation with a registered fumigant in the fall prior to planting a susceptible crop the following spring has also been shown to effectively control these nematodes.